Influence of four run-of-river dams on channel morphology and sediment characteristics in Illinois, USA

Dams are known to create discontinuities in river flow and sediment transport, with ensuing effects on the fluvial geomorphology of dammed systems. While the effects created by large impoundment dams are well documented, less is known about the influence of small run-of-river dams (common across the eastern United States) on river geomorphology. Recent emphasis on dam removal has focused on run-of-river structures, highlighting the need for improved understanding of the geomorphological effects of these types of dams. This research examines spatial variation in channel morphology and bed sediment character upstream and downstream of four run-of-river dams in Illinois. Results show that the four dams do not create major discontinuities in channel morphology or sediment character. Silt/clay content of bed material at the four sites is higher upstream of the dams than downstream, but this size fraction generally is a minor component by weight of the sediment samples collected. Although percentages of sand are generally higher upstream of the dams and gravel percentages are generally higher downstream, not all of these differences are statistically significant. Longitudinal profiles through the dams and changes in channel depth upstream and downstream of the dams indicate that no major accumulations of sediment have occurred behind the dams. Analysis of ¹³⁷Cs in sediment cores at two sites shows no evidence of long-term fine sediment storage. Apparently, these dams are not acting as major sediment traps, nor do these structures produce substantial downstream channel erosion. Variability in spatial patterns of channel morphology and sediment characteristics among the sites suggests that local site-specific factors have an important influence on geomorphological responses. Because of this variability, the findings of this study indicate that information on site-specific conditions should be an important consideration in the removal planning process for run-of-river dams.