The role of sediment supply in channel instability and stream restoration

Abstract

This paper examines the interaction between sediment supply and channel instability and implications of system-wide instability for stream restoration. Using examples from southwest Ohio, we show that recent channel instability is largely a consequence of reduced sediment supply over several past decades. The widths of the studied channels narrowed by a factor of three to four between the 1930s and 2000s, and channel beds have incised into glacial sediments. Lateral migration of these channels decreased after 1960, signaling the onset of channel incision. Soil loss rates in 2005 were 4 to 20 times lower than in the 1920s, mainly as a result of the shift from conventional to conservation tillage. The construction of impoundments has further reduced sediment delivery downstream, and climate change may exacerbate this imbalance by increasing sediment transport capacity, including peak flows. Thus the decrease in sediment supply occurred during a time when peak stream flows have been increasing, resulting in an imbalance between sediment supply and sediment transporting power in the stream systems. Because sediment supply and transport are governed by watershed-scale conditions, channel instability will persist in this region for some time to come. Reach-scale restoration will neither improve water quality nor prevent continued system-wide channel instability. We advocate an approach that uses setbacks/buffers that allow streams to develop new, dynamic floodplains with which to reestablish connections while selectively protecting significant sites with stabilization measures.