Relative changes in sediment supply and sediment transport capacity in a bedrock‐controlled river

Abstract

Rivers can be affected by multiple natural and human‐induced changes to sediment supply and to sediment transport capacity. Assessment of the relative importance of these changes enables appropriate river management. Here assessments are made of the relative changes in sediment supply and sediment transport capacity of an 83 km section of the bedrock‐controlled Coxs River, New South Wales, Australia. These relative changes are estimated, in turn, for land degradation, historical climate variations, and dam closure. Measurements of gully erosion extent from aerial photographs indicate that since European settlement the average annual sediment supply to the river has increased by a factor of ∼20. Rainfall records and flow‐gauging records show a climate shift in the mid‐1940s. On the basis of hydrologic modeling the increased streamflow in the period since the mid‐1940s is estimated to have increased sediment transport capacity by a factor of almost 3. Closure of Lyell Dam in 1982 stopped sediment supply from the upper catchment. On the basis of hydrologic modeling the flow regulation and abstraction at the dam is estimated to have reduced the long‐term average sediment transport capacity downstream by 15%. The current sediment transport capacity after the climate shift and dam closure is 2.6 times higher that in the first half of this century. In spite of this net increase in sediment transport capacity the huge volume of sediment delivered to the channel from gully erosion has led to widespread sand deposition along much of the river. Much of the river bed that was previously dominated by bedrock is now alluvial in character. Only the steepest reaches remain bedrock‐dominated.