THE INFLUENCE OF LAND USE CHANGE AND IN-STREAM MANAGEMENT PRACTICES ON CHANNEL EVOLUTION IN THE GREAT BROOK WATERSHED, CENTRAL VERMONT

The stream channel of Great Brook in Plainfield, Vermont has responded to floodplain encroachment, channelization, and other disturbances by incision, bank failure and widening. The watershed has an area of 14.5 square miles and ranges in elevation from 712 to 3,352 ft. above sea level. The main channel is approximately 9 miles in length. Surficial materials include lodgment till, ablation till, ice-contact sand, lacustrine sand, silt and silty clay and alluvial sand and gravel. Exposures of compact, varved lacustrine silt and silty clay overlain by lodgment till appear to indicate at least a minor late Wisconsinan ice readvance. Weathered lodgment till and lacustrine sand are highly erodible and subject to mass failure, while fine-grained lacustrine silt and silty clay are relatively resistant. The concentration of storm flows due to road ditching, which causes small changes in hydrology, has caused large failures in the weathered basal till. Slope failures are associated with contacts between deposits with markedly different permeabilities. The lower 5 miles of the brook are extremely unstable with no bedrock control and are mostly in Channel Evolution Model (CEM) Stages 2 and 3 of Schumm (1977). The stream is actively incising and widening with over 25 mass failures. Floodplain encroachment and in-stream management practices in the lower part of the watershed, including channelization and removal of armor, contribute to the instability. The forested upper part of the watershed, which is partially controlled by bedrock and has suffered less floodplain encroachment, is stable and is in CEM Stages 1, 4 and 5. Important variables for hazard assessment are CEM stage, channel slope, bank materials and height of bank. The results will be used to create a hazard map to help the town reduce impacts to roads and private property from changes in channel planform, channel incision and widening. Reference: Schumm, S.A., 1977, The fluvial system: John Wiley and Sons, New York, 338p.