Sustaining Environmental Flows in Southern New England Rivers: Effects of Watreshed Factors and Land Use

Abstract

Rivers and river systems serve as conduits for nutrients and organisms, function as corridors for fish and wildlife passage, and provide resources for humans. Streamflow has been called the master variable in a river because it affects habitat diversity and availability through its impact on physical factors that influence habitat quality. However, land use changes such as urbanization and irrigation, can have major effects on stream hydrology. Modifications of the land surface due to urbanization alters natural stream hydrographs by increasing flood peaks, decreasing time to peak flows, and causing higher runoff velocities. Irrigation may produce the opposite effects. In order to preserve a spectrum of stream functionality, rivers must maintain seasonally adequate flows. For example, low flows can affect stream connectivity, restrict movement of aquatic organisms, concentrate prey into limited areas, purge invasive species from riparian corridors, and enable recruitment and evolution of floodplain plants. State agencies throughout the Northeast U.S. are considering policies linked to low-flow thresholds that sustain these ecosystem services. Methods that set minimum flow standards often result in conflicting values, due to differing environmental goals and levels of protection they aim to achieve. Two such methods, the USFWS Aquatic Base Flow (ABF) method and the Wetted Perimeter method have been widely used. The USFWS ABF method recommends using the median of August flows and has been refined for Rhode Island (RIABF). The wetted perimeter method uses stream cross-sections at riffle locations to determine critical flow values to maintain flow based on the wetted perimeter of the channel. In addition to setting flow standards, methods to minimize the adverse effects of urbanization have also been proposed. Low impact development (LID) has emerged as a strategy to reduce the hydrologic impacts of urbanization on aquatic ecosystems by combining site planning and design processes with runoff reduction and treatment practices. Within a given climatic region, water resource managers seeking to optimize stream ecosystem services need a clear understanding of the importance of land use, physical/climatic characteristics, and hydrography on different components of stream hydrographs. Within 33 Southern New England watersheds (average area 80 km2), we assessed relationships between watershed variables and a set of low flow parameters: 1-, 7- and 30- day minimum flows. We used an information theoretical approach to develop regression models to identify relationships between landscape attributes and parameters that describe different components of the flow regime. The key variables identified by the AIC weighting