River Trading: What Happens to Eroded Sediments?

Abstract

Understanding of the sources and in-catchment stores of fluvial sediment, along with a full characterisation of the transport processes operating during the transit of sediment from upstream hillslopes to catchment outlet, is essential to accurately target management actions designed to reduce sediment delivery to receiving waters. An implicit assumption within catchment sediment models is that once sediment has entered the river it is transported to the outlet of the catchment. However, the accumulations of fine and coarse sediments in river channels and on floodplains tell us this is not correct at least not at all time scales. The consequences of this storage on end-of-catchment sediment loads have rarely been examined. To explore this critical knowledge gap, this thesis examines the true path sediment takes, from sites of erosion to the channel, and thence along the channel to the outlet, with apparently numerous periods of storage within alluvium along the way. The study area is Emu Creek, a small headwater catchment in southeastern Queensland, Australia. Prior to determining the contribution of different geological sources to sediments, it is important to select an appropriate mixing model to accurately ascribe the source contributions. Various sediment tracing mixing models are reviewed and techniques compared using data from two different catchments. The results provide clear evidence that the application of different mixing models to the same dataset will produce dramatically different source contributions.