HYDROLOGIC AND MORPHOLOGIC CHANGES OF THE WEST BRANCH DELAWARE RIVER, NEW YORK, DOWNSTREAM OF THE CANNONSVILLE DAM

The Cannonsville Dam and Reservoir are located on the West Branch Delaware River in the Catskill region of New York, about 193 km northwest of New York City. The primary purpose of the dam is to create a reservoir that supplies New York City with potable water. Thus, the West Branch Delaware River has been the subject of several water quality studies, but there has been limited attention to the downstream hydrologic and morphologic effects of the Cannonsville Dam.

The study area contains three USGS gauging stations, including a gauge downstream of the dam but upstream of any confluences and one site upstream of the reservoir. Following impoundment of the West Branch Delaware River, both the minimum and maximum mean annual single- and multiple-day discharges were significantly less than the pre-dam flows at the downstream stations. The post-dam 1.5 year-discharge decreased by 66% compared to the pre-dam condition. Moreover, under the new post-dam hydrologic regime, the pre-dam 1.5-year discharge occurs once every six years. Examination of the upstream gauging data indicates that there have been relatively minor changes in the flow regime of the West Branch Delaware River, suggesting that changes downstream of the dam are in fact related to impoundment. Despite the change in the post-dam hydrologic regime, there was no significant change in the pre-dam river morphology, in contrast to previous studies of the morphologic impacts of dams. While the frequency of small magnitude floods have been decreased, large floods (bankfull and larger) pass through the Cannonsville Dam with little attenuation of their flood peaks. The lack of change in morphology suggests that these large, geomorphically effective discharges occur frequently enough to maintain the morphology of the pre-dam channel.