Initiation of delta formation and subsequent fluvial sedimentation in the upper delta plain: A case study from the Kiso River delta,central Japan

Abstract

This study investigates the Holocene evolution of the upper Kiso River delta in central Japan, focusing on sedimentary processes and accumulation patterns in the upper delta plain near the upstream limit, an area directly influenced by transgressive and regressive phases driven primarily by eustatic sea-level change and less explored than the lower delta plain. The analysis is based primarily on a 23-m sediment core (UK) retrieved from a floodbasin approximately 40 km inland from the present mouth of the Nagara River. The core was analyzed for sedimentary facies, grain size distribution, colour, electrical conductivity, loss on ignition and AMS radiocarbon dating. Six depositional units were identified, ranging from basal river gravels to marine-influenced sands and overlying floodbasin muds and peats. An upward-coarsening succession from tidal river to distributary channel deposits along with the presence of gravels marks the onset of regression at approximately 8 ka. Radiocarbon data indicate that sediment accumulation initially almost kept pace with eustatic sea-level rise but slowed to approximately 1.6 m/kyr during the highstand phase, exceeding the contemporaneous sea-level rise. Stratigraphic correlation with nearby boreholes along the Kiso and Nagara rivers reveals consistent depositional patterns and a lack of prodelta mud in upstream areas. A turning point in N-values within the sand layer may represent the maximum flooding surface. Floodplain aggradation has persisted since approximately 6 ka, likely accompanied by vertical accretion within the channel belt. Despite the slowdown in sea-level rise and regional crustal uplift associated with glacio-hydro isostatic adjustment, tectonic subsidence appears to have maintained sufficient accommodation. Peat accumulation, which became increasingly prominent after 4 ka, reflects reduced clastic input and associated hydrological changes. Comparative stratigraphic and geomorphological analyses suggest that floodbasin deposits have been less frequently replaced by channel migration and avulsion along the Ibi and Nagara rivers than along the Kiso River.

Keywords

borehole logs delta floodplain grain size peat sea-level change

Introduction

The initiation of Holocene deltas is associated with a decrease in the rate of sea-level rise during the postglacial period (Stanley and Warne, 1994), especially after approximately 8 ka (Hori and Saito, 2007; Tanabe et al., 2022). Numerous studies on delta evolution have primarily focused on the lower delta plain, where typical deltaic sedimentary successions, comprising the prodelta, delta front and delta plain in ascending order, are observed (Amorosi et al., 2008; Frazier, 1967; Hori et al., 2002; Ogami et al., 2015; Scruton, 1960; Ta et al., 2002; Tanabe et al., 2006, 2015). For example, previous research has examined delta evolution processes, including lobe switching (Frazier, 1967; Somoza et al., 1997) and the progradation rate of subaerial and subaqueous deltas (Hori et al., 2001; Ogami et al., 2015; Vespremeanu-Stroe et al., 2017), in relation to both external and internal forcing mechanisms, based on the analysis of radiocarbon-dated borehole cores.

On the other hand, the sedimentary facies and sediment accumulation rates beneath present-day upper delta plains, particularly near the upstream limit where areas are directly influenced by transgressive and regressive phases driven mainly by eustatic sea-level change, have been increasingly studied over the past two decades. Research has focused on deltas such as the Song Hong (Red) (Hori et al., 2004), Mekong (Tamura et al., 2009), Rhine-Meuse (Gouw and Erkens, 2007; Gouw, 2008), Changjiang (Yangtze) (Song et al., 2013), Pearl (Fu et al., 2020), Po (Amorosi et al., 2017, 2019; Bruno et al., 2017) and Tama River (Tanabe et al., 2022). These studies primarily analyzed incised-valley-fill deposits located just below the upper delta plain facies, revealing that prodelta, shelf, or lagoon mud deposits are absent, likely due to the insufficient water depth reached during the Holocene transgression. Additionally, estuarine and/or fluvio-deltaic sediments were deposited at rates approximately equal to the rate of early to middle Holocene sea-level rise (Song et al., 2013; Tamura et al., 2009; Tanabe et al., 2022). However, the characteristics and accumulation of fluvial sediments overlying the marine-influenced deposits during the subsequent sea-level highstand since the middle Holocene, when eustatic sea-level rise had nearly ceased, are less well known. In the Rhine-Meuse delta, the Holocene fluvio-deltaic wedge, corresponding to the upper delta plain, is subdivided into three segments based on the relative influence of eustatic sea-level rise, subsidence and upstream controls including discharge and sediment supply (Gouw and Erkens, 2007). Thus, data obtained from strata beneath the upper delta plain provide critical insights into the architecture and evolution of fluvial and near-coastal depositional systems in response to external forcings.

The longitudinal gradients of deltas formed by steep-gradient rivers, such as those in Japan, are greater than those of mega-deltas formed by large continental rivers such as the Mississippi, Mekong and Changjiang. Consequently, the effects of transgression and regression, primarily driven by eustatic sea-level changes, are confined to relatively narrow areas within Japanese deltas. Additionally, many major cities in Japan are located on fluvial and deltaic plains, providing access to high-density borehole log data (Tanabe et al., 2015), which facilitates comprehensive stratigraphic analyses.

This study aims to clarify the sedimentary characteristics, depositional chronology and evolutionary processes of a delta formed in an upper-incised valley near the upstream limit influenced by sea-level changes during the transition from transgression to regression. The Kiso River delta in central Japan (Figure 1) was selected as the study area owing to the availability of numerous borehole sections along the river, primarily obtained through geotechnical investigations associated with social infrastructure inspections (e.g. artificial levees). Additionally, many radiocarbon-dated borehole cores, most of which were collected at sites where prodelta mud are distributed (Hori et al. 2008, 2011, 2014; Masuda and Iwabuchi, 2003; Ogami et al. 2009; Yamaguchi et al., 2003), provide a robust dataset for stratigraphic and chronological analyses.

Figure 1.

Location map of the Nobi Plain. The upstream limits of tidal rivers are based on topographic maps published approximately 90 years ago. The distribution of the middle mud (MM), which approximately corresponds to prodelta/estuarine deposits, is based on Hasada (2015). NB, YM, KZN, KZ1, KM, MC, AP1, AP2, ST1 and ST2 represent existing borehole core sites (Hori et al., 2008, 2011, 2014; Ogami et al., 2009; Yamaguchi et al., 2003). Elevation data are derived from a 5-metre-resolution DEM provided by the Geospatial Authority of Japan (GSI).

Study area

The Kiso River delta is a Holocene fluvial–coastal lowland formed mainly by the Kiso, Nagara and Ibi rivers (Figure 1). These rivers discharge into the Ise Bay, where the tidal range reaches about 2.5 m during spring tides at Nagoya Port, and the average significant wave height within the bay is approximately 0.3 m. The delta covers an area of about 1300 km² and comprises gently sloping alluvial fans, meander belts and a lower delta plain extending downstream (Iseki, 1983) (Figure 2). The alluvial fans are located at the base of the northern mountains. For instance, the Kiso River has developed an alluvial fan at Inuyama, characterized by an axial length of 13.9 km and a gentle depositional slope of 0.14° (Saito and Oguchi, 2005). Within this section, the river exhibits a braided channel pattern. The meander belt consists of natural levees, abandoned channels and floodbasins. A substantial portion of the lower delta plain lies below mean sea level, and extensive tidal flat reclamation has taken place, particularly since the Edo Period (AD 1603). Land reclamation efforts continued into the 20th century, further modifying the deltaic landscape. Additionally, topographic maps published approximately 90 years ago depict symbols representing the tidal limits of the river channel (Figures 1 and 2). The line connecting these tidal limits likely corresponded to the boundary between the lower and upper delta plains, which consist of the alluvial fans and meander belts. However, at present, the location of the tidal limit may have shifted significantly due to anthropogenic influences, such as the construction of an estuary weir at the mouth of the Nagara River.

Figure 2.

Landform classification map of the Kiso River delta. Channel belts comprising former channels, natural levees and point bars are more extensively distributed along the Kiso River and its eastern side compared to those along the Nagara and Ibi rivers.

The Yoro fault, a major active fault, runs along the western margin of the lowland. The subsidence rate near the fault during the past 1 million years has been estimated to be up to 1.3 m/kyr, and the plain has tilted at a rate of about 1.0 × 10^-4/kyr (Sugai and Sugiyama, 1998). Land subsidence by groundwater pumping has also occurred around the southern area of the lower delta plain mainly during the latter part of the twentieth century. The cumulative subsidence from 1961 to 2023 is ~1.64 m in maximum (Tokai Three-Prefecture Investigation Committee on Land Subsidence, 2024).

The latest Pleistocene–Holocene incised-valley fill beneath the plain is stratigraphically divided, in ascending order, into basal gravel (BG), lower sand (LS), middle mud (MM), upper sand (US) and terrestrial sand and mud layers (TSM) (Hasada and Hori, 2016; Iseki, 1962) (Table 1). This classification is based primarily on lithofacies characteristics and standard penetration test (N-value) results (Iseki, 1962; Yamaguchi et al., 2003). BG consists predominantly of sand and gravel, with N-values commonly close to 50, and is interpreted as fluvial channel deposits. The top of the BG lies about 60 m below the present sea level near the coast and becomes progressively shallower upstream, eventually aligning with the elevation of the modern riverbed. Consequently, distinguishing BG from the contemporary sand and gravel of the riverbed becomes increasingly difficult in upstream sections. Although BG is primarily attributed to sea-level lowstand deposits during the Last Glacial Maximum (LGM), some portions may have been deposited during the subsequent sea-level rise, potentially leading to the amalgamation onto LGM-related deposits (Hori et al., 2019). LS represents floodplain and estuarine deposits formed during the postglacial transgression. MM is characterized by silt or clay containing shell fragments, with very low N-values (<5). US consists of sand, silty sand and sandy silt, with N-values increasing upward. MM and US correspond to the prodelta/estuarine deposits and to the delta front and lower delta plain deposits, respectively. TSM consists mainly of the upper delta plain, including floodplain deposits. MM, US and TSM were primarily formed during the delta progradation since approximately 7.9–7.3 ka (Ogami et al., 2009), which corresponds to a period of decelerated sea-level rise. The distribution of MM is restricted to an area approximately 33–39 km from the present river mouth (Hasada, 2015) (Figure 1).

Table 1.

Characteristics of the lithofacies constituting latest Pleistocene to Holocene sequences beneath the Nobi Plain.

Lithofacies	Lithology	Standard penetration test (N-value)	Sedimentary environment
TSM	Sand, silt, or clay with organic matter	Lower than in US, upward decreasing	Upper delta plain (including floodplain)
US	Sand and silty sand to sandy silt	Generally >5, upward increasing	Delta front and lower delta plain
MM	Silt or cray with shell fragments	Very low (<5)	Prodelta/estuary
LS	Mainly consisting of alternating sand and mud	10–30	Floodplain and estuary
BG	Sand and gravel	~50	Fluvial channel

Modified after Hasada and Hori (2016).

Material and methods

A borehole core was taken at Ushiki (UK; lat. 35°22′45.2″N, long. 136°40′27.2″E, elevation: 5.6 m, length 23 m) (Figure 3) located approximately 40 km landward from the present Nagara River mouth. The site is located on the floodbasin between the Nagara and Ibi rivers (Figure 2). The sediment core was split, photographed, described and subsampled in the laboratory. X-radiographs were taken for all cores using slab samples to clarify sedimentary structures in core sediments. Wet and dry bulk densities were determined for samples collected in polycarbonate containers with a volume of 7 cc at 10 cm intervals. Wet bulk density was determined by weighing the samples immediately after collection, and dry bulk density was determined after drying the samples for 48 h at 60°C. A colorimeter (SPAD-503, Konica Minolta Holdings, Inc.) was used to determine colour parameters L*, a* and b*, where L* is lightness, ranging from 0 (black) to 100 (white), and a* and b* are chromaticity variables. Five-centimetre-thick subsamples were collected at 20-cm intervals, and 0.063 and 2.0 mm sieves were used to determine the mud, sand and gravel contents.

Figure 3.

Sedimentary column of the UK core and profiles of bulk density, grain-size variation, luminosity, electrical conductivity (EC) and loss on ignition (LOI).

Electrical conductivity (EC) measurements were conducted on sediment–water mixtures prepared from samples collected at 50-cm intervals. Each sample was oven-dried, mechanically disaggregated, and then mixed with 10 g of sediment and 120 ml of distilled water. After stirring, the mixture was allowed to stand, and EC was measured after 5 days using a conductivity metre (HORIBA D-54). EC values reflect the concentration of anions such as sulfate (SO₄²⁻) and chloride (Cl⁻) in solution, which are known to adsorb onto fine-grained sediments (Yokoyama and Sato, 1987). Marine sediments typically exhibit EC values of 1.3–3.0 mS/cm, brackish-water sediments range from 0.4 to 1.2 mS/cm, and freshwater sediments are generally below 0.4 mS/cm (Yokoyama and Sato, 1987). Other studies have reported similar values (Niwa et al., 2011).

Loss on ignition (LOI) was determined at 10 cm intervals down to a depth of 10.8 m in the core. Sediment samples were dried at 105°C for 24 h, after which 1.4–2.2 g was weighed, then heated at 750°C for 1 h in an electric muffle furnace. LOI values are commonly used to classify peat (Wüst et al., 2003). In this study, samples with LOI values exceeding 20% were classified as peat (Van Asselen et al., 2010), while those with values between 10% and 20% were defined as organic-rich mud, and those below 10% as mud (Ishii et al., 2016).

Many borehole investigations have been conducted along the Kiso and Nagara rivers by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT). Standard penetration tests (SPT) were typically performed at 1 m intervals during the investigations, and portions of the recovered sediments were preserved in plastic bottles and stored by MLIT. Subsamples of these archived sediments were used for grain-size analysis (Figure 4), following the same procedures applied to the UK core.

Figure 4.

Geological column, grain-size variation and N-value of the existing borehole logs along the Kiso and Nagra rivers.

Accelerator mass spectrometry (AMS) radiocarbon dating of organic materials and molluscan shells retrieved from the UK core and MLIT borehole samples was carried out at Nagoya University (laboratory code: NUTA2), the Institute of Accelerator Analysis Ltd. (IAAA) and DirectAMS (D-AMS) (Table 2). Radiocarbon ages were calibrated to calendar years using OxCal 4.4 software (Bronk Ramsey, 2009) with the IntCal20 (Reimer et al., 2020) and Marine20 (Heaton et al., 2020) calibration curves. A regional marine reservoir correction (ΔR) of −200 was applied, corresponding to a value of 0 in the context of the Marine13 calibration curve.

Table 2.

Radiocarbon ages derived from the UK core and other borehole samples.

Sample ID	Depth (m)	Elevation (m)	Material	¹⁴C age (BP)	Error (1σ)	Calibrated age (cal BP) (2σ range)	Median probability	Laboratory code
UK270	2.70	2.90	Plant fragment	1110	40	1180–920	1010	NUTA2-25819
UK356	3.56	2.04	Plant fragment	1560	40	1530–1360	1450	NUTA2-25820
UK486	4.86	0.74	Plant fragment	3500	60	3970–3580	3770	NUTA2-26293
UK638	6.38	−0.78	Plant fragment	4340	40	5040–4830	4910	NUTA2-25822
UK865	8.65	−3.05	Plant fragment	4990	40	5900–5600	5710	NUTA2-25824
UK961	9.61	−4.01	Plant fragment	5260	50	6190–5920	6050	NUTA2-25825
UK1328	13.28	−7.68	Plant fragment	6570	50	7580–7360	7470	NUTA2-25826
UK1668	16.68	−11.08	Wood piece	7100	50	8020–7790	7930	NUTA2-25827
UK1815	18.15	−12.55	Wood piece	7520	50	8410–8190	8330	NUTA2-25783
K370_1-860	8.60	−2.53	Plant fragment	5080	30	5910–5740	5810	IAAA-171776
K370_2-1500	15.00	−8.24	Plant fragment	6180	30	7170–6980	7070	IAAA-171777
K370_2-1875	18.75	−11.99	Shell fragment (Corbicula sp.)	7650	30	8290–7980	8130	IAAA-171778
K370_2-2255	22.55	−15.79	Plant fragment	8010	30	9010–8720	8880	IAAA-171779
K434-1130	11.3	5.9	Plant fragment	3000	30	3330–3070	3180	D-AMS 026844
K426-2030	20.3	−2.1	Plant fragment	5010	30	5900–5600	5730	D-AMS 026839
K404-1730	17.3	−1.5	Plant fragment	5690	50	6630–6320	6470	D-AMS 026840
K318-830	8.3	−2.8	Wood piece	3780	30	4240–4000	4150	D-AMS 026824
N371-2430	24.3	−10.98	Shell fragment	7300	30	7920–7630	7770	IAAA-182960

The stratigraphy of the upper incised-valley fill was reconstructed using data from the UK core, in combination with previously published analyses of radiocarbon-dated borehole cores (Hori et al., 2008, 2011, 2014; Ogami et al., 2009; Yamaguchi et al., 2003) (Figures 5 and 7; Table S1) and borehole investigation results from the Nobi Plain (Figure 4). The spatial distribution of these cores was projected onto the present river channel for cross-sectional analysis (Figure 7).

Figure 5.

Sedimentary column of the ST1, ST2, AP1 and AP2 cores and profiles of grain-size variation and electrical conductivity (EC). Modified after Hori et al. (2008, 2011).

Results

Sedimentary characteristics of the UK core

Borehole sediments are divided into six depositional units (Units 1–6), arranged from the bottom to the top of the core. The classification is based on lithology, grain size, colour, sedimentary structures, fossil content, electrical conductivity (EC) and loss on ignition (LOI) (Figure 3). The characteristics and interpretations of each depositional unit are described below.

Unit 1 (depth in core: 23.0–21.4 m)

This unit is composed of a gravelly layer dominated by subrounded gravels with a maximum grain size of approximately 5 cm and an overlying fine-grained sandy layer. The matrix of gravelly layers consists of medium- to coarse-grained sand. Based on its depth and facies characteristics, this unit corresponds to the basal gravel (BG) typically found at the base of incised-valley fill deposits, not only in the Nobi Plain (Hori et al., 2011; Iseki, 1962; Ogami et al., 2009) but also in other Japanese coastal plains (Saito, 1995). It is interpreted as fluvial channel deposits (Tanabe et al., 2015). The overlying fine-grained sand may represent sedimentation associated with channel abandonment.

Unit 2 (depth in core: 21.4–19.2 m)

Unit 2 is characterized mainly by greyish silt with charred plant fragments in the upper part. L* slightly decreases upward while EC increases from 0.24 to 2.97 mS/cm. The absence of shells and burrows, along with the dominance of fine-grained silt, suggests deposition in a floodbasin environment. However, the elevated EC values in the upper part of the unit indicate that the site was beginning to experience influence from seawater intrusion.

Unit 3 (depth in core: 19.2–16.4 m)

Unit 3 is composed primarily of very fine- to fine-grained sand, with sand content increasing upward. Molluscan shells, burrows and wood fragments are present. The mollusks are Corbicula sp., and the shell fragments are abraded. The L* values are approximately 30, while a* and b* values show minimal variation. EC values range from 0.48 to 1.19 mS/cm. The presence of shells and burrows, and elevated EC values, indicates that this unit formed under seawater influence. In addition, the high sand content and the occurrence of Corbicula sp. suggest a tidal river environment.

Unit 4 (depth in core: 16.4–11.3 m)

Unit 4 is composed of sand with granules and pebbles reaching up to 2 cm in maximum diameter, exhibiting an overall upward-fining trend. However, mud layers are observed at 13.3–13.2 m and 14.0–13.9 m depths in the core. The boundary between Unit 4 and the underlying Unit 3 is sharp, suggesting a distinct change in depositional pattern. The sand content generally exceeds 80%, and cross-laminated sand is present at approximately 15.7 m depth, as identified in X-ray imagery. Additionally, wood and plant fragments are occasionally found within the unit. EC values are around 0.2 mS/cm at 12.75–12.80 m and 12.25–12.30 m, but exceed 1 mS/cm at four other depths. These elevated EC values suggest a continued influence of seawater, consistent with Unit 3. The coarser grain size indicates an increase in hydraulic energy, while the presence of gravel suggests that coarse-grained sediments were supplied by fluvial processes. These sedimentary characteristics are typical of distributary channel (Bhattacharya and Walker, 1992; Hori et al., 2011; Reading and Collinson, 1996).

Unit 5 (depth in core: 11.3–9.7 m)

This unit consists of alternating layers of very fine- to fine-grained sand and mud, with generally poor sorting. Burrows and plant fragments are present throughout. EC values exceed 0.7 mS/cm but show a decreasing trend upward. The relatively high EC values, together with the presence of burrows, suggest deposition under marine influence. Similar facies have been interpreted as tidal flat deposits in previous studies of the Nobi Plain (Yamaguchi et al., 2006). Stratigraphically, this facies overlies the delta front facies, underlies floodbasin facies and occurs at elevations below 0 m (Yamaguchi et al., 2006). Therefore, Unit 5 is interpreted to represent an intertidal flat deposit.

Unit 6 (depth in core: 9.7–0 m)

Silts, organic mud (LOI: 10%–20%) and peat (LOI: >20%) constitute the unit 6. Organic mud and peat are common between 2.6 and 5.2 m depth in core. Plant fragments and vivianite are locally present in silt layers. L* shows less than 30 in organic mud and peat layers. The values of EC are less than 0.23 mS/cm, though they are around 0.4 mS/cm near the bottom of this unit. The mud-rich texture, dark colour and low EC values indicate that this unit was formed in a floodbasin environment. The presence of vivianite, organic mud and peat further shows that the floodbasin was poorly drained (Aslan and Autin, 1999). The occurrence of peat, in particular, suggests that part of the floodbasin developed into a swamp or marsh environment (Bruno et al. 2019).

Sedimentary characteristics of the existing borehole samples

Existing borehole logs, grain-size variations and N-values from sites near the UK are shown in Figure 4. The sedimentological characteristics of the ST1 and ST2 cores (Hori et al., 2008) and the AP1 and AP2 cores (Hori et al., 2011) are presented in Figure 5. Figure 4 indicates that the MM, characterized by silt or clay containing shell fragments and very low N-values (<5), broadly corresponding to prodelta/estuarine mud, is distributed along the Kiso River up to approximately 32.4 km from the river mouth (cores K305–K324) and along the Nagara River up to approximately 36.2 km from the river mouth (core N362). These cores commonly exhibit upward coarsening from mud to overlying sand, accompanied by increases in N-values. Where continuous sediment samples are available in such settings, grain-size trends similar to those observed from the prodelta to the delta front at AP1 site (Figure 5) are expected. Marine molluscan shell fragments occur in both the prodelta mud and the delta-front sandy–muddy deposits of the AP1 core. The prodelta deposits contain Dosinella angulosa (Philippi), Glossaulax didyma (Röding) and Macoma sp., whereas the delta-front deposits include Mactra veneriformis Deshayes in Reeve (Hori et al., 2011).

In contrast, farther upstream beyond approximately 35.0 km from the Kiso River mouth and 37.1 km from the Nagara River mouth, landward of cores K350 and N371, the MM becomes indistinct, and sand deposits locally containing shell fragments become dominant (e.g. cores K370_1, K370_2 and N380). N-values in these sand deposits frequently exceed 10. Comparable grain-size variations are also observed at the UK, ST1 and ST2 sites. Marine molluscan shell fragments, including Meretrix lusoria (Röding), Crassostrea gigas (Thunberg) and Macoma tokyoensis Makiyama, occur within the delta-front deposits of the ST1 and ST2 cores (Hori et al., 2008). Corbicula sp. occurs at a depth of 18.75 m in core K370_2.

In both rivers, cores obtained from locations more than 42.0 km upstream from the river mouths (e.g. cores K426 and N420) show that sand and gravel corresponding to the BG occur at elevations of approximately 0 m above present sea level or higher. Although sand and mud interpreted as fluvial deposits overlie the BG, such deposits are almost absent at sites located more than 45 km upstream from the river mouths (cores K457, K488 and N450).

Radiocarbon ages

Figure 6 presents age–elevation plots of radiocarbon dates derived from the UK, K370_1 and K370_2 cores. Additionally, age–elevation relationships of previously reported borehole core sediments (ST1, ST2, AP1, AP2 and MC) collected near these sites are also illustrated. Accumulation curves were constructed by connecting the younger ages in cases where age reversals were observed within a core. These curves do not account for sediment compaction.

In the UK core, radiocarbon ages younger than approximately 8.3 ka were obtained from Units 3–6 (Figure 3), with no age reversals identified. Notably, the deposition of Unit 6 (floodbasin deposits) occurred within the past 6000 years, with the accumulation of peat and organic-rich mud becoming prominent after approximately 4 ka. Although the number of radiocarbon ages obtained from the K370_1 and K370_2 cores is limited, their ages and elevations align closely with those from the UK core (Figure 6).

Figure 6.

Plots of calibrated radiocarbon ages and elevation for the UK and existing borehole cores, shown alongside a late Quaternary eustatic sea-level (ESL) curve (Lambeck et al., 2014) and a predicted relative sea-level (RSL) curve for the Ise Bay, which incorporates glacio-hydro isostatic adjustment (Okuno et al., 2014). The median probabilities of the calibrated ages are plotted, with error bars representing the corresponding ±2σ age range.

The average sediment accumulation rate in the UK core between 8.3 and 7.5 ka is estimated at 5.7 m/ka, corresponding to the period of upward-coarsening succession from Units 3–4. After 7.5 ka, the accumulation rate decreases, and during the past 6000 years (Unit 6 formation), the average accumulation rate is approximately 1.6 m/ka. Similar trends in accumulation rate change are observed in the ST1 and ST2 cores, located about 2.5 km downstream of the UK core, where prodelta mud is absent, as in the UK core. In contrast, a significant decrease in accumulation rate is observed in AP1, coinciding with the deposition of prodelta mud around 8 ka, followed by a rapid increase during the passage of the delta front (Hori et al., 2011).

Accumulation rates have increased at many sites since at least 1 ka. A sudden and rapid increase in accumulation rate at AP2 around 400 years ago is attributed to the rapid growth of natural levees (Hori et al., 2011).

Figure 6 also shows the predicted relative sea-level (RSL) curve for Ise Bay, which incorporates glacio-hydro isostatic adjustment (GIA) (Okuno et al., 2014), along with the eustatic sea-level (ESL) curve (Lambeck et al., 2014). The Nobi Plain has undergone crustal deformation, with relatively high subsidence rates observed near the foothills of the Yoro Mountains (Sugai and Sugiyama, 1998). The subsidence rate at the UK site is estimated to be approximately 1 mm/year, based on the elevation difference between Unit 5 (−4.1 m), interpreted as tidal flat deposits formed near sea level around 6 ka, and the relative sea level (RSL), estimated at about 2 m above the present sea-level (Okuno et al., 2014) (Figure 6). Assuming this rate has remained constant, tectonic subsidence and GIA would have effectively offset each other, resulting in a relative sea level that approximates the eustatic sea level (ESL). Therefore, the ESL curve is used in the following discussion.

Stratigraphic cross sections along the Kiso and Nagara rivers

Figure 7 illustrates the stratigraphic cross-sections along the present Kiso and Nagara rivers, incorporating data from existing radiocarbon-dated borehole cores such as KZ1, YM and NB (Hori et al., 2014; Ogami et al., 2009; Yamaguchi et al., 2003), alongside the longitudinal riverbed profiles prior to significant riverbed degradation. Because facies boundaries are difficult to define solely from geotechnical investigations, such as in core K370_1, these boundaries were determined using not only the UK core but also previously studied cores with detailed sedimentological descriptions and interpretations (Ogami et al., 2009). Isochrons were constructed based on calibrated radiocarbon ages (Table 2; Table S1) and estimated ages of widespread tephras, including U-Oki, K-Ah and Kg (Makinouchi et al., 2022; Smith et al., 2013; Tani et al., 2013).

Along the Kiso River (Figure 7a), the elevation of the upper boundary of fluvial channel deposits corresponding to BG is approximately −50 m near the river mouth and rises inland, reaching around 0 m at about 43.4 km from the river mouth. These fluvial channel deposits are thinly overlain by floodplain deposits, which are in turn overlain by estuarine deposits such as tidal river and tidal flat deposits (Hori et al., 2011; Ogami et al., 2009). Prodelta mud corresponding to MM exceeds more than 15 m in thickness near the river mouth, as observed in the NB and YM cores, and can be traced upstream to approximately 32.4 km from the river mouth at core K324. At this location, the MM shows an upward-coarsening transition into delta-front deposits corresponding to US, accompanied by increasing N-values. Farther inland, these characteristics become less distinct, making it difficult to differentiate floodplain to estuarine deposits corresponding to LS from delta-front deposits corresponding to US. However, a thin interbedded mud layer observed at an elevation of approximately −10 m in cores K350, K370_1 and K370_2 may roughly represent the boundary between these deposits.

Figure 7.

Stratigraphic cross sections along the present Kiso (a) and Nagara (b) rivers. The locations of the MC, KM, KZ1, KZN, YM and NB cores were projected onto the cross-sectional profiles along the Kiso River, while the locations of the UK, ST1, ST2, AP1, AP2, KZ1 and KZN cores were projected onto the cross-sectional profiles along the Nagara River. Isochrons based on calibrated radiocarbon ages are presented only for the interval between 10 and 6 ka, the period most relevant to this study. Borehole logs obtained from inspections of artificial levees frequently indicate the presence of thick embankment layers.

Similarly, along the Nagara River (Figure 7b), the upper boundary of the fluvial channel deposits rises inland and reaches approximately 0 m at around 42 km from the river mouth. Prodelta mud can be traced upstream to approximately 36.2 km from the river mouth at core N362. In the AP1 core, which projected onto the Nagara River cross section and is located on an old channel of the Ibi River, prodelta mud exceeds 5 m in thickness (Hori et al., 2011). Although prodelta mud is absent in the ST1 and ST2 cores, as in the UK core, electrical conductivity values (Figure 5) suggest that marine influence was present immediately above the fluvial channel deposits. Within the thick sand layer of cores N380 and N397, a horizon characterized by a transition from decreasing to increasing N-values upward (Figure 4) may correspond to the boundary between floodplain to estuarine deposits corresponding to LS and delta front deposits corresponding to US.

The isochrons indicate that marine influence extended up to approximately 25 km upstream from the present river mouths between 11 and 10 ka. Farther upstream, beyond the locality where prodelta mud becomes indistinct, including the UK, ST1 and ST2 sites, marine influence began to appear after approximately 9 ka. At these locations, sand-dominated deposition appears to have persisted between approximately 9 and 6 ka.

Discussion

Relationship between sedimentary facies near a transgression-to-regression turnaround point and the early Holocene sea-level change

Comparison of the sediment accumulation curve from the UK core with the ESL curve (Lambeck et al., 2014) (Figure 6) indicates that the accumulation rate between Unit 3, interpreted as tidal river deposits, and Unit 4, interpreted as distributary channel deposits, closely matches the rate of sea-level rise. Although no radiocarbon ages are available for Units 1 and 2, the transition from fluvial to marine facies between Unit 1 and Unit 3 suggests the occurrence of a marine transgression. In contrast, the upward-coarsening succession from Unit 3 to Unit 4, along with the presence of gravels likely supplied by fluvial processes in Unit 4, suggest a transition from transgression to regression at approximately 8 ka. Within the framework of sequence stratigraphy, the maximum flooding surface (MFS), which separates the transgressive systems tract (TST) from the highstand systems tract (HST) (van Wagoner et al., 1988), is therefore inferred to lie near the boundary between Unit 3 and Unit 4. A similar stratigraphic configuration and sediment accumulation pattern is observed in the ST1 and ST2 cores (Hori et al., 2008) and in the K370_2 core, all of which lack prodelta mud (Figures 4 –6). These observations suggest that sediment accumulation beneath the present upper delta plain intensified near the end of the transgressive phase, around 8 ka, coinciding with the transition from transgression to regression. In a previous study of the Kiso River delta (Ogami et al., 2009), this transition was estimated to have occurred between 7.9 and 7.3 ka based on changes in sedimentary facies observed in radiocarbon-dated borehole cores at and seaward of the MC site. Considering the results of the present study, the onset of regression likely began closer to 8 ka.

In both the UK and AP1 cores, the thickness of floodplain and point bar deposits, which accumulated during the transgressive phase and overlie the basal fluvial channel deposits corresponding to BG is very thin (less than 2.2 m). In the ST1 core, marine-influenced deposits occur immediately above the fluvial channel deposits. Radiocarbon ages suggest that these deposits formed around 9 ka. In contrast, floodplain deposits formed during the transgressive phase tend to be thicker at more seaward locations. For example, these deposits reach approximately 3.4 m in the YM core (Ogami et al., 2009) and 8.6 m in the NB core (Hori et al., 2014) (Figures 1 and 7), both of which also rest directly on fluvial channel deposits. No paleosols or stiff clay, which are characteristic of interfluves (Amorosi et al., 2003; Lin et al., 2005), are recognized at these five sites. Therefore, it is inferred that these sites were located within an incised valley rather than on interfluves. The longitudinal gradients of the Kiso and Nagara rivers are inferred to have remained steep during the postglacial sea-level rise, similar to or only slightly gentler than during the LGM, when sea level was about 120 m lower than at present. This persistent steep gradient likely limited the longitudinal expansion of floodplains, resulting in progressively thinner floodplain deposits from the seaward to the inland areas. Consequently, marine influence associated with sea-level rise is observed directly or slightly above the fluvial channel deposits at inland sites.

Drilling multiple borehole cores and conducting detailed analyses requires considerable effort. It is therefore important to assess whether the timing of the transition from transgression to regression and the associated facies changes can be inferred from stratigraphic logs and N-values alone. Although this approach is extremely challenging, the stratigraphic logs and N-values from core K370_2, when combined with available radiocarbon ages and grain size variations (Figure 4), suggest that the horizon within the sand layer where N-values shift from a decreasing to an increasing trend, corresponding to a change from fining upward to coarsening upward grain size trends, may represent the turning point from transgression to regression.

In contrast, at the AP1 site (Hori et al., 2011) and the MC site (Ogami et al., 2009), both located near the landward limit of prodelta mud distribution, sediment accumulation rates significantly declined around 8.5 ka (Figure 6). As these rates remained low despite continued sea-level rise, it is estimated that by approximately 8 ka, water depths at these sites had reached around 10 m. This pattern suggests a landward shift of the depocenter in response to rapid sea-level rise. Subsequently, accumulation rates increased at the MC site around 7.5–6.5 ka and at the AP1 site around 7–6.5 ka. These increases are interpreted as reflecting the passage of the delta front across these locations and are consistent with interpretations that place delta initiation at approximately 8 ka.

In the upper delta plain, estuarine and/or fluvial sediments aggraded at rates comparable to sea-level rise during approximately 9–8 ka not only in large river deltas such as the Changjiang (Song et al., 2013), Mekong (Tamura et al., 2009) and Red River (Hori et al., 2004), but also in smaller Japanese deltas such as the Tama River (Tanabe et al., 2022). These systems likewise record a transition from transgression to regression around 8 ka. Bay-head deltas across the northern Gulf of Mexico underwent a backstep between 8.8 and 7.8 ka in response to the abrupt sea-level rise associated with the 8.2 ka climate-change event (Rodriguez et al. 2010). Although transgression and regression are primarily controlled by the balance between sediment supply and sea-level change, sediment supply varies greatly among river systems. As previously proposed by Stanley and Warne (1994), sea-level change therefore likely played a dominant role in delta initiation around 8 ka. Although the timing, duration and magnitude of the sea-level jump associated with the 8.2 ka event remain debated (Hijma and Cohen, 2010, 2019; Li et al., 2012; Törnqvist et al., 2004), the rate of sea-level rise appears to have slowed following this event. A reduction in the rate of eustatic sea-level rise around 8.2 ka has been documented in Tokyo Bay (Tanabe, 2020) as well as a sea-level curve derived from the inversion of ~1000 observations spanning the past 35 ka from sites located far from former ice margins (Lambeck et al., 2014). More recent quantitative analyses along the North Sea coast indicate that the rate of sea-level rise peaked at 8–9 mm/yr around 10.3 and 8.3 ka, followed by a decline (Hijma et al., 2025). This post-8.2 ka deceleration in sea-level rise is therefore considered to have played a key role in delta initiation in areas proximal to river mouths.

Floodplain evolution in the upper delta plain in response to external forcing

Deposition of Unit 6, consisting of floodbasin fine-grained deposits, has continued since approximately 6 ka (Figures 3 and 6) with an average accumulation rate of 1.6 m/kyr. This rate exceeds the contemporaneous rate of ESL rise of approximately 0.5 m/kyr (Figure 6). Based on the age–elevation relationship (Figure 6), accumulation rates at the UK site are broadly estimated to range from 0.6 to 2.8 m/kyr on a millennial timescale (Figure 8). When considering only floodplain deposits at the ST1, ST2, AP1 and AP2 sites (Figure 5), which transitioned to terrestrial environments later than the UK site, average accumulation rates are estimated to be approximately 2.2 m/kyr at ST2 and 3.3 m/kyr at AP2. These values are comparable to those at the UK site (Figure 6) and indicate that floodplain aggradation has persisted for at least the past 6 ka across the upper delta plain.

Figure 8.

Accumulation rates of floodplain deposits at the AP2, ST2 and UK sites compared with the rate of eustatic sea-level rise (Lambeck et al., 2014) on a millennial timescale. Accumulation rates were calculated based on the calibrated radiocarbon ages (Table 2; Table S1) and age–elevation plots (Figure 6).

On a centennial timescale, floodplains typically experience rapid growth during their early stages; however, as the elevation difference between the channel and the floodbasin increases, the rate of vertical accretion declines rapidly (Everitt, 1968). If this trend applies to floodplain evolution over millennial timescales, as examined in this study, a slowdown in aggradation would be expected. However, the observed accumulation rates do not show such a decline. This implies that, in addition to lateral migration of the channel belt including channels and natural levees, vertical aggradation within the channel belt itself also contributed to sustained floodplain growth. The riverbed elevation approximately 50 years ago, prior to major human-induced degradation (River Bureau, MLIT, 2007), was higher than the depositional surface of surrounding floodplain around 4–3 ka at the UK, ST1 and ST2 sites. This relationship further supports the interpretation that vertical aggradation within the channel belt has continued since the middle Holocene.

In addition to glacio-eustatic sea-level changes, crustal uplift due to glacio-hydro-isostatic adjustment and tectonic subsidence in the study area also affect the base-level changes. In a forced regression, during which base level falls (Catuneanu, 2002), sustained channel-belt aggradation on millennial timescales would be unlikely. Moreover, glacio-eustatic sea-level rise had largely ceased by around 4 ka (Lambeck et al., 2014). Therefore, tectonic basin subsidence associated with the activity of the Yoro Fault, located along the western margin of the plain, likely played an important role in creating accommodation by effectively elevating the base level and promoting channel belt aggradation. Basin subsidence has also been recognized as a key factor in mid-Holocene aggradation of the Rhine-Meuse fluvio-deltaic wedge (Gouw and Erkens, 2007). Before 5 ka, eustatic sea-level rise drove rapid aggradation, while from 5 to 3 ka, subsidence dominated, followed by increased sediment supply after 3 ka (Gouw and Erkens, 2007).

At the UK site, accumulation of Unit 6 (floodbasin mud) continued, and peat deposition occurred around 4 ka. Although peat has sometimes been used as an indicator for reconstructing sea-level changes in coastal and fluvial-coastal plains (Hanebuth et al., 2011; Ota et al., 1990), at the UK site, Unit 5 (tidal flat deposits) was formed at elevations closer to sea level than the overlying peat of Unit 6. Shallow sediment cores collected from multiple locations in the Kiso River delta floodplain revealed that peat does not directly overlie tidal flat deposits formed near sea level, but instead occurs within backswamp (floodbasin) deposits of the floodplain (Yamaguchi et al., 2006). Therefore, peat should not be regarded as a direct indicator of sea level, but rather as a signal of environmental change, specifically, a shift to conditions with reduced clastic sediment supply and a rising water table. The formation of peat during this period may reflect not only the near stabilization of sea level but also a decline in flooding due to climatic cooling, as has been pointed out in floodplains such as that of the Ishikari River (Ishii et al., 2016).

The uppermost layer of Unit 6 may have experienced limited compaction, and agricultural cultivation may also have influenced recent sediment accumulation. However, it appears that accumulation rates of floodbasin deposits over the past one thousand years appear to have increased relative to earlier periods (Figure 8). A similar pattern is observed at the ST1, ST2, AP1 and AP2 sites. Several factors may account for this increase. The first possibility is that anthropogenic activities within the drainage basin enhanced sediment production. This interpretation is consistent with the increased volume of sediment deposited in the Kiso River delta (Hasada, 2015) and the accelerated progradation rate of the subaqueous delta (Ogami et al., 2009). The other possibility is that channel migration associated with avulsion may have shifted active channels closer to the UK site, resulting in locally elevated aggradation rates (Törnqvist and Bridge, 2002). The marked increase in accumulation rates at AP2 can be attributed to the shift that occurred approximately 400 years ago, when AP1 became the active channel of the Ibi River and AP2 evolved into its natural levee (Figure 5) (Hori et al., 2011). To evaluate this latter scenario, stratigraphic cross-sectional information across the channel would be necessary.

Floodbasin mud may have been eroded or removed due to channel migration and avulsion in the upper delta plain (Aslan et al., 2005; Bruno et al., 2019; Hori et al., 2023; Slingerland and Smith, 2004). The UK site is situated within a floodbasin. In contrast, the AP1 site lies along a former course of the Ibi River, while ST2 was drilled near the present-day channel of the Nagara River. Unlike at the UK site, channel deposits at AP1 have eroded underlying floodplain sediments, whereas at ST2, channel deposits have eroded and overlie delta plain or delta front deposits (Figures 5 and 7). The thicknesses of these sand and gravel channel deposits are 3.6 m at AP1 and 5.6 m at ST2. Deep fluvial incision of this kind has also been reported during the middle Holocene in the uppermost region of the Changjiang delta (Song et al., 2013).

According to the landform classification map of the Kiso River delta (Figure 2), channel belts (former channels with natural levees and point bars) are widely distributed along the Kiso River, whereas floodbasins dominate the landscape west of the Nagara River. This spatial pattern, together with the preceding discussion, suggests that the replacement of floodbasin deposits due to channel migration and avulsion has been less extensive along the Ibi and Nagara rivers compared to the Kiso River. Therefore, to reveal floodplain aggradation on millennial scales without disturbance from fluvial erosion, it is necessary to sample sediments from the upper delta plain while avoiding channel belts.

Conclusions

This study investigates the Holocene evolution of the upper Kiso River delta through the analysis of borehole sediment cores, radiocarbon dating and borehole stratigraphy with emphasis on allogenic forcing factors such as sea-level change, climate variability and subsidence. The sedimentary succession and radiocarbon chronology indicate that a transition from transgression to regression, marked by the maximum flooding surface (MFS), occurred around 8 ka, similar to other deltas with high sediment supply. This transition corresponds to a deceleration in postglacial sea-level rise after 8.2 ka, when the rate fell below 8–9 mm/yr. These findings suggest that, in the future, if global warming causes sea-level rise to reach approximately 10 mm/year, delta progradation worldwide may come to a halt. Although identifying this transition based solely on stratigraphic logs and N-values is challenging, the horizon within the sand layer where N-values shift from a decreasing to an increasing trend may represent the MFS. Subsequent floodplain aggradation, sustained over the past 6000 years, was with accumulation rates generally exceeding contemporaneous rates of eustatic sea-level rise. Tectonic subsidence appears to have played a significant role in maintaining accommodation despite the cessation of eustatic sea-level rise and crustal uplift from glacio-hydro isostatic adjustment. Peat formation, which became more pronounced around 4 ka, likely reflects environmental changes such as reduced clastic sediment input and hydrological shifts, rather than serving as a direct indicator of sea-level position. Furthermore, the recent increase in sedimentation over the past 1 ka suggests enhanced sediment supply linked to anthropogenic activity within the drainage basin, although autogenic processes such as channel migration associated with avulsion may also have contributed to the increase. The combination of stratigraphic evidence from this study and floodplain landform classification suggests that channel migration and avulsion were less active along the Nagara and Ibi rivers than along the Kiso River, resulting in reduced replacement of floodplain sediments. Because this study focused mainly on the response of the delta to allogenic forcing, our discussion of autogenic processes was very limited. This aspect requires further investigation, and we plan to address it in more detail as additional data become available.

Supplemental Material

sj-xlsx-1-hol-10.1177_09596836261422216 – Supplemental material for Initiation of delta formation and subsequent fluvial sedimentation in the upper delta plain: A case study from the Kiso River delta, central Japan

Supplemental material, sj-xlsx-1-hol-10.1177_09596836261422216 for Initiation of delta formation and subsequent fluvial sedimentation in the upper delta plain: A case study from the Kiso River delta, central Japan by Kazuaki Hori, Yuji Ishii, Takuya Wakasugi, Hiroyuki Kitagawa, Rei Nakashima, Yoshiki Sato and Susumu Tanabe in The Holocene

Footnotes

Acknowledgements

We sincerely appreciate the constructive comments from the anonymous reviewers, which greatly improved the manuscript. We are grateful to the Kiso River Upper Reaches Works Office and the Kiso River Lower Reaches Works Office, Ministry of Land, Infrastructure, Transport and Tourism (MLIT) for providing the sediment samples. We also thank K. Hasada and E. Takahashi for their assistance with sediment sampling.

Author contributions

Kazuaki Hori: Conceptualization; Data curation; Formal analysis; Funding acquisition; Methodology; Visualization; Writing – original draft; Writing – review & editing.

Yuji Ishii: Conceptualization; Data curation; Formal analysis; Methodology; Writing – review & editing.

Takuya Wakasugi: Conceptualization; Data curation; Formal analysis; Methodology; Writing – review & editing.

Hiroyuki Kitagawa: Conceptualization; Formal analysis; Writing – review & editing.

Rei Nakashima: Conceptualization; Writing – review & editing.

Yoshiki Sato: Formal analysis; Writing – review & editing.

Susumu Tanabe: Conceptualization; Writing – review & editing.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This research was funded by the Japan Geographic Data Center and JSPS KAKENHI Grant Numbers 21K18397, 23K25418 and 25K21642.

ORCID iD

Kazuaki Hori

Supplemental material

Supplemental material for this article is available online.

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