HYDROLOGIC CHANGES AFTER IMPOUNDMENT OF RESERVOIRS, CATSKILL MOUNTAINS, NEW YORK

Impoundment of reservoirs on the West and East Branches of the Delaware River and smaller streams in the Catskill Mountains of New York has had significant effects on discharge magnitude and frequency downstream of the reservoirs. We examine these changes by using the Index of Hydrologic Alteration program, comparing pre- and post-impoundment gaging records from sites upstream of the reservoirs (as “controls”) with downstream stations. We also examine flood-frequency statistics for the gaging stations. Here we report preliminary results for four reservoirs—Rondout Reservoir (Rondout Creek), Neversink Reservoir (Neversink River), Pepacton Reservoir (East Branch Delaware River), and Cannonsville Reservoir (West Branch Delaware River). These reservoirs were impounded between 1950 and 1964 as part of the New York City water-supply system. Gage records from both upstream and downstream of the dams predate reservoir impoundment, allowing us the opportunity to examine changes pre- and post-impoundment, as well as evaluate whether changes downstream of the reservoirs are consistent with the hydrology of unmanaged upstream reaches. Low-flow discharge magnitude and frequency have reduced significantly at all of these reservoirs due to impoundment, reflecting reservoir management as well as regional minimum-flow requirements. Average large-flow discharges have also been reduced downstream of these reservoirs. However, discharge-recurrence relationships have changed little for long-RI events. This indicates that large floods are continuing to pass through these reservoirs, unlike the more common tendency for flood mitigation at reservoirs managed for purposes other than water supply.