MONITORING EROSION IN A STRAIGHTENED AND DREDGED AGRICULTURAL STREAM

In the 19^th and 20^th centuries, stream straightening and dredging in agricultural landscapes has been a common method of flood control. Given the widespread nature of these modifications, it is critical to understand how streams respond to them. This study analyzes erosion and deposition patterns in Decker’s Creek, a tributary to the Monongahela River in West Virginia, USA, that was dredged extensively for flood control in the 1960’s. Local watershed managers fear that this legacy dredging promotes bank failure, increasing the supply of fine sediment that is harmful to benthic macro-organisms. To assess the associated risks and quantify erosion patterns over time along a reach straightened by dredging, we repeated surveys of four stream cross-sections over the course of approximately one year and installed a pressure transducer and a time-lapse camera to continuously monitor flow conditions. The dataset allows us to understand channel hydrology and patterns of erosion and deposition, as well as the mechanisms of how the channel changes shape over time. Our surveys have shown bank erosion by 0.3 m at one of our four cross-sections, where two shelves (one on each side of the creek) are actively failing. Though we found a similar cross-section form at the other three cross-sections, (shelves between 1-1.5 m above the thalweg on the 2.5 m tall banks) no notable erosion was measured at the other cross-sections. The stream bed showed fluctuations around 0.1 m of deposition and erosion at all cross-section locations throughout the study period. Differences in erosion dynamics between cross-sections could be attributed to the stochasticity of bank failure or to the spatial variability of the dense riparian vegetation on the banks. Our data supports the idea that stream channel straightening can lead to block bank collapse, which increases the fluvial sediment load. However, the magnitude of the contribution of bank collapse to the stream sediment budget--relative to the inputs from agriculture and construction—is unclear. This work informs the long-term (~>60 years) fluvial response to channel straightening. Most critically this study reach does not show patterns of deposition and erosion indicative of a natural return to its unaltered shape.