Cyclic perturbation of lowland river channels and ecological response

Certain lowland streams have experienced prehistorical and historical cycle s of aggradation, occlusion, degradation, headward incision, and renewed ag gradation. Historical cycles appear to be related to human activities. A ca se study is presented of the Yalobusha River in Mississippi with emphasis o n the effects of blockage and removal on aquatic habitats and fish. The adj acent Skuna River, which was channelized;and unblocked, was used in space f or time substitution to infer effects of blockage removal on the Yalobusha. Variables describing physical aquatic habitat and fish were sampled from t hree groups of river reaches: unblocked channelized, channelized and blocke d, and naturally sinuous. Fish collections were used to compute six indicat ors of ecological integrity. At baseflow, mean water depths were an order o f 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 m(3)/m for the channelized, blocked channelized, and natural reaches, resp ectively. Mean values for all six ecological indicators were lowest for the channelized group. Species richness was greatest for the channelized block ed 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.