Changes in the flux of sediment in the Upper Murrumbidgee catchment, Southeastern Australia, since European settlement

Abstract

Since European settlement began 180 years ago, the sediment flux in the upper Murrumbidgee River, NSW, Australia, has changed as a result of grazing, historical climate variations, and dam closures. Of these, the introduction of grazing stock, which triggered widespread gully erosion, has had the largest effect, increasing the sediment flux by a factor of more than 150. In comparison, multi-decadal variations in rainfall cause less than a twofold increase in the sediment transport capacity of the river, and the effects of dam closures are significant only in some reaches. At the time of settlement, vegetated shallow valley floor depressions were common and many of the creeks, and streams consisted of deep pools linked by shallow vegetated zones. Much of the main channel of the river was cobble and gravel bedded. The sediment flux out of this 10 500 km(2) catchment prior to settlement is estimated to have been very low, similar to2400 t year(-1). Degradation of the vegetated valley bottoms by introduced stock in the 1840s and 1850s triggered a massive phase of gully erosion. Over the next 40 to 50 years the erosion rates in the headwater areas increased by a factor of nearly 245, and similar to43 000 000 t of sediment was generated. We estimate that, during this period, gully erosion increased the sediment flux out of the catchment by a factor of about 200 to similar to480 000 t year(-1). As the gully networks reached maximum extension, sediment yield from the headwater areas declined by a factor of similar to40 and are now estimated to be about six times the pre-European rates. Interestingly, sediment yield from the whole catchment declined only by a factor of two to similar to100 times the pre-European rates (250 000 t year(-1)). We attribute this difference to an increase in the efficiency of sediment delivery through the stream network.