Hydrologic and morphologic changes of the West Branch Delaware River, New York, downstream of the Cannonsville Dam

The Cannonsville Dam, on the West Branch Delaware River, has produced only modest changes in downstream hydrology and morphology. Following impoundment, the minimum and maximum mean annual single- and multiple-day discharges were significantly less than the pre-dam flows. The post-dam 1.5 year-discharge decreased by 66% following impoundment. Under the new post-dam hydrologic regime, the pre-dam 1.5-year discharge occurs once every six years, on average. Upstream gauging data indicates that there have been no significant changes in the natural flow regime, suggesting that downstream changes are related to river impoundment. There was no significant change in the pre-dam river morphology, however. While small magnitude floods have decreased, large floods (bankfull and larger) pass through the Cannonsville Dam with little attenuation of their flood peaks. The lack of change in the post-dam morphology suggests that these large discharges occur frequently enough to maintain the morphology of the pre-dam channel.