Late Quaternary river evolution of floodplain pockets along Mulloon Creek,New South Wales,Australia

Abstract

Valley confinement along upper-middle reaches of Mulloon Creek, in the upper Shoalhaven catchment of southern New South Wales, Australia, restricts floodplain development to a series of distinct pockets. These pockets comprise a downstream-thinning wedge of vertically accreted fine-grained deposits atop basal gravels. Some time before 12 500 years ago a bedload-dominated river was transformed into a suspended load system. In the mid-late Holocene, swamps developed in the middle-lower part of each floodplain pocket. Within a few decades of European settlement of this region (circa AD 1820), discontinuous watercourses in some floodplain pockets had incised to form a low-sinuosity gravel-bed channel. Wedge-shaped units of post-incisional alluvium that thicken downstream overlie the swamp and floodplain deposits in the downstream part of these pockets. In the downstream-most pocket, incision commenced prior to colonization, while upstream swamps retain a continuous swamp across the valley floor. The late Quaternary evolution of this variant of discontinuous watercourse is summarized in a schematic model.