The National Park Service (NPS) is committed to restoring flow and habitat along sections of the Delaware River to protect the federally endangered dwarf wedgemussel (Alasmidonta heterodon). This past year the NPS has partnered with the ERE Department faculty and students to model the likely flow impacts to upstream watershed management decisions. This is a continuation of earlier NPS research on dwarf wedgemussel response to flow and temperature in the Delaware River. The flow regime of the Delaware River through Upper Delaware Scenic and Recreational River and Middle Delaware Scenic and Recreational River has been altered from its precolonial condition by human land use (e.g. deforestation, development), out-of-basin water diversions, and the managed water releases from the Cannonsville (West Branch), Pepacton (East Branch), Neversink, and Wallenpaupack Reservoirs. The flow regime of a river is a “master variable” that strongly affects numerous interacting physical, chemical, and biological characteristics of the river and floodplain.  For example, as summer low flow decreases, the river water depth, width, and velocity typically decrease, river water temperature typically increases, and river water dissolved oxygen typically decreases. The altered flow regime has likely had widespread, systemic impacts on native river biota, including populations of the federally endangered dwarf wedgemussel at the Upper Delaware Scenic and Recreational River.

Dwarf wedgemussel on river bottom. Image from Wikipedia.

Dwarf wedgemussel on river bottom. Image from Wikipedia.

The goal of this research project is to answer a central question to ecological flows:   What is a feasible range natural unregulated flows for the upper Delaware River based on process-based simulation of pristine land use controls on rainfall-runoff processes?  Answering this question will enhance the scientific foundation for managing Delaware River flows in the future.

The National Park Service manages 113 miles of Wild and Scenic River along the upper Delaware River: 73 miles as the Upper Delaware Scenic and Recreational River (UPDE), and 40 miles as the Middle Delaware Scenic and Recreational River (DEWA). New York City operates three major water supply reservoirs in the upper Delaware River Basin: one on the West Branch (Cannonsville), one the East Branch (Pepacton), and one on the Neversink River (Neversink).  As a result of a 1954 Supreme Court Decree, New York City has authority to divert up to 800 million gallons per day (mgd) from these reservoirs and the Delaware River Basin for municipal water supply, provided certain minimum flows are maintained at Montague, New Jersey. The flow regime of the river through UPDE is largely determined by water releases from the Cannonsville Reservoir (West Branch), and to a lesser extent, from the Pepacton Reservoir (East Branch). The flow regime at DEWA is also affected by these releases and the minimum flow requirement.

USGS map of Delaware River Basin and physiographic provinces.

USGS map of Delaware River Basin and physiographic provinces.

The Delaware River Basin Commission initiated the Flexible Flow Management Program (FFMP) in 2007 to identify and adapt for appropriate ecological flows that meet prescribed river management criterion. An underlying goal of this effort, supported by the National Park Service, has been to restore a more “natural” flow regime to the upper Delaware River. Identification of a “natural” flow regime for the Delaware River and major tributaries involves determining how land use change and reservoirs have impacted flows.

In 2009, The Nature Conservancy studied impact of hydrologic alteration (IHA) on the Delaware River caused by water diversion and operation of the New York City reservoirs [Apse et al., 2009]. The IHA analysis compared pre- and post-reservoir flows, with the East Branch Delaware Pepacton Reservoir analysis based on the Downsville gage from 1941-1954 for pre-reservoir conditions and from 1983-2007 for post- reservoir conditions. The West Branch Delaware Cannonsville Reservoir analysis was based on the Stilesville gage from 1953-1963 for pre-reservoir conditions and from 1983-2007 for post- reservoir conditions. The analysis found annual low flows have decreased by 80% below the Pepacton Reservoir and by 43% below the Cannonsville Reservoir, and the timing of the low flows has shifted from late summer to late fall.

The objective of this NPS-ESF research is to apply the proven USGS WATER watershed model for the Delaware River East Branch watershed at Downsville, NY (USGS gage 01417000, 372 mi2), West Branch watershed at Stilesville, NY (USGS gage 01425000, 456 mi2), and larger downstream watershed at Callicoon, NJ (USGS gage 01427510, 1,820 mi2) with updated watershed input data and parameters to compute flows for three distinct land cover types, the 1500s pre-colonial, early-1900s deforested, and mid-1900s pre-reservoir periods. The NPS-ESF team is collaborating with USGS personnel involved in the WaterSMART initiative, where SMART is an acronym for Sustain and Manage America’s Resources for Tomorrow.

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