|Shields Jr, Fletcher|
Submitted to: Regulated Rivers Research and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 16, 1999
Publication Date: May 1, 2000
Interpretive Summary: Agricultural watersheds with erosive soils, steep hillslopes and flat valley bottoms are subject to many problems related to erosion of uplands. Severe erosion results in sedimentation in lowland channels known as valley plugging. Initial symptoms include frequent flooding and impaired drainage of downstream floodplain lands as channels fill with sediments. Historical approaches, typically straightening and enlarging mainstem channels, have actually increased upland erosion and downstream sedimentation, leading to cyclic behavior. Response of river ecosystems to this cyclic behavior has not been studied. We compared fish and aquatic habitat in three commonly occurring agricultural situations: a channelized river that was blocked with sediment and woody debris; a nearby unblocked, channelized river; and a natural, unchannelized reach downstream. Our purpose in this study was to better understand what happens to aquatic life long-term under these different common conditions so that we can make recommendations to federal and state agencies that work to restore agricultural streams so that they are a useful resource to rural America. Watershed managers and action agency personnel can use this information to develop master plans for addressing problems in watersheds subject to valley plugging.
Technical Abstract: Certain lowland streams have experienced prehistorical and historical cycles of aggradation, occlusion, degradation, headward incision, and renewed aggradation. Historical cycles appear to be related to human activities. A case study is presented of the Yalobusha River in Mississippi with emphasis on the effects of blockage and removal on aquatic habitats and fish. The adjacent Skuna River, which was channelized and unblocked, was used in space for time substitution to infer effects of blockage removal on the Yalobusha. Variables describing physical aquatic habitat and fish were sampled from three groups of river reaches: unblocked channelized, channelized and blocked, and naturally sinuous. Fish collections were used to compute six indicators of ecological integrity. At baseflow, mean water depths were an order of magnitude lower in the unblocked channelized stream than for the others. In-channel aquatic habitat volume per unit valley length was 5, 85, and 283 m3 /m for the channelized, blocked channelized, and natural reaches, respectively. Mean values for all six ecological indicators were lowest for the channelized group. Species richness was greatest for the channelized blocked reach. The ecological indicators displayed gradients in response to the range of observed physical conditions. Management of corridors susceptible to the cycle described above should involve a blend of measures designed to conserve higher quality habitats.