Changes in stream morphology protected by best management practices under effects of upstream disturbances

Stream channels are usually more stable in forested watersheds. However, intensive land disturbances in forested watersheds may disrupt the balance between flow and sediment supply, and result in variations in stream morphology even in the presence of well-designed best management practices (BMPs). This study evaluated the impacts of upstream land disturbances on downstream stream morphology where streamside management zones (SMZs) were present in two small adjacent watersheds in Auburn, Alabama, USA. Field surveys including the measurements of channel cross sections at bankfull stage were conducted at 12 survey sites over an 18-month period. Both in-stream sediment deposition and erosion were observed downstream of the disturbed areas in the study watersheds. As a result, channel cross sections exhibited very dynamic patterns changing frequently over short time periods. Overall, the dominant process was in-stream deposition in both watersheds due to large amount of sediment delivered from upstream-disturbed areas. In spite of the dynamic patterns of the channel cross sections in response to upstream disturbances, the commonly used parameters of Rosgen stream classification remained stable, suggesting that the upstream disturbances did not lead to changes in stream classification. In general, bank erosion was not observed. Therefore, it is likely that SMZs in each watershed were sufficient to stabilize streambanks by hindering in-stream bank erosion. Moreover, the poorly designed BMPs such as silt fences and straw bales near the disturbed area seemed insufficient to balance the variations in sediment supply under the impacts of upstream disturbances. This study emphasized that proper installation and continuous monitoring of BMPs should play a critical role on watershed management. It also indicated that watershed management should be handled using a holistic approach with well-distributed BMP applications within the entire watershed.