Downstream Channel Responses to Dam Impoundment, Delaware River Basin, Catskill Mountains, New York

Large dams generally reduce downstream sediment loads due to impoundment of sediment in the reservoir. The most common (though not universal) effect of this reduced sediment load is an increase in downstream erosion, often affecting the river over tens of kilometers. Such erosion should be manifested by channel deepening and/or widening. We have investigated changes in channel morphology downstream of three dams operated as part of the New York City water supply system in the upper Delaware River Basin of New York's Catskill Mountains–Neversink Dam on the Neversink River, Pepacton Dam on the East Branch Delaware River, and Cannonsville Dam on the West Branch Delaware River. We compared changes upstream and downstream of the reservoirs to see if impoundment has produced any increase in channel modification downstream of the dams. Examination of repeat aerial photography from all three sites and channel cross-section measurements from gaging stations on the Neversink River reveal: (1) little change in downstream channel planview, particularly as compared with changes upstream of the reservoir; and (2) little change in channel cross sections at gaging sites, again as compared with changes upstream of the reservoir. Thus, these reservoirs seem to have had little downstream impact on river morphology. We speculate that this may be the result of operating these reservoirs as water-supply systems with little historic management for flood control, resulting in the pass-through of most large events and little difference in the hydrology of channel-altering flows between reaches upstream and downstream of the reservoirs.