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Modern Rivers: Geomorphology and Sedimentation
Bank Deposition in Suspended-Load Streams
Many of the western-flowing streams of eastern Australia are suspended-load streams. Most of these streams deposit the majority of their load within the channel as bank benches. Bench is used here to describe a flat surfaced depositional body of sediment on the channel bank. It is distinct from a terrace produced by incision. These benches commonly occur at three levels and form the most notable morphologic feature of these rivers. The three levels form as a result of the interaction of the hydrologic and sedimentologic regimes within the stream.
The Barwon River in northern New South Wales is typical of these Western Rivers and is discussed in detail here. Floods along the Barwon are very slow moving (less than 0.5 ms−1) and the peak sediment concentrations usually preceed the flood wave by up to 14 days along the middle reaches. Sediment concentrations are in general low, rarely exceeding 1500 ppm at peak concentration and the sediment carried is extremely fine grained, 80% being less than 2 μm. These loads are related to the low bed gradients (5 x 10−5). The river is highly sinuous (2.3) but does not actively meander and has a width-depth ratio of about 8.
The benches along the Barwon develop along the banks in straight reaches as well as at the inside and outside of bends. They are narrow and long and have a lensoid sectional shape. The sediments composing the benches vary according to elevation and their situation along the channel. Those benches occuring low in the channel and at point situations contain more sand than those at the outside of bends and those occuring higher up the banks.
The deposits of the lowest bench are dominantly cross-bedded sands, up to 20 cm thick with thin mud interbeds, which drape the underlying sands. The muds protect the sands from erosion during the next rise, hence enabling the bench to develop. The growth of the bench often occurs with limited erosion of the outside bank of the channel, or no erosion of the bench itself.
The deposits which form the two higher benches are thinly interbedded sands and muds which drape the top and front of each deposit. The sand beds attain a maximium thickness of 16 cm and the muds 14 cm. The beds commonly contain parallel or wavy fine lamination, are often graded and rarely cross-bedded. The graded bedding is commonly normal, but may also be reversed or threefold (e.g. sand/mud/sand). No erosional contacts are evident with the exception of very localised scours. Because of the notable lack of erosion and the geometry of the deposits, the benches accrete both laterally and vertically at rates depending on their elevation, and to a lesser extent their situation in the channel.
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