The Middle Potomac River basin project demonstrates (1) the determination of environmental flow needs for rivers and streams that are generally more impaired by land use change than by withdrawals or impoundments; (2) engagement of multiple water resource agencies and other stakeholders across jurisdictional boundaries; and (3) a structured, iterative approach for selecting flow and ecology metrics and refining river types to strengthen flow-ecology relationships.
The approximately $1 million ELOHA project budget was funded mainly by the U.S. Army Corps of Engineers (75% Federal cost-share through the Corps’ Section 729 Watershed Assessment program) and The Nature Conservancy (25% non-Federal cost-share), with additional support from the National Park Service, the Interstate Commission on the Potomac River Basin (ICPRB), and other basin jurisdiction agencies. Boundaries of the 11,500-mi2 Middle Potomac project area were determined by Congressional designation of the Corps’ study authority, but the project analyses extended upstream to allow for system connectivity. The project area includes parts of Pennsylvania, Maryland, Virginia, West Virginia, and all of Washington, DC.
ICPRB is the project’s technical lead. The Commission was created by interstate compact in 1940, primarily to provide technical support and expertise to the watershed jurisdictions. ICPRB lacks authority to regulate streamflow. Therefore, the project was designed to support efforts of the five watershed jurisdictions to protect and restore environmental flows.
The project team has informed and involved the watershed jurisdictions throughout the project development and analytical process. A seven-part webinar series, technical advisory group meetings, a technical workshop, agency consultative meetings, and a project website have maintained watershed states’ involvement throughout the project, from inception to completion. Through these interactions, stakeholders have reviewed the technical approach, discussed potential policy applications, and considered how to use the flow-ecology relationships to inform water and land use management decisions.
The project goals are to:
• Estimate current and future water withdrawals, given population, land use, and climate change projections;
• Determine impacts of water withdrawals, discharges, impoundments, land use, and climate change on flow;
• Characterize flows needed to support healthy biotic communities in smaller streams and rivers; and
• Provide data, information, and analyses to support water and land use planning and decision making at the state level.
A modified version of the site-specific “Savannah” process (Richter et al. 2006) was used to determine flow needs for selected segments of the Potomac River mainstem and selected large tributaries (Cummins et al. 2011), while the regional-scale ELOHA framework was used for smaller tributary streams and wadeable rivers.
The project’s hydrologic foundation consists of 21 years of daily flow data at biological monitoring sites under seven scenarios—modelled baseline (or relatively unaltered), modelled current, and five modelled future alternative flow scenarios—simulated by the Virginia Department of Environmental Quality’s decision support system, WOOOMM (Watershed Online Object Oriented Meta-Model), an enhanced version of HSPF (Hydrologic Simulation Program FORTRAN). To identify baseline flow conditions, modelers used a Category and Regression Tree (CART) analysis of 105 gaged watersheds in the Potomac and adjacent Susquehanna River basins. The CART analysis determined thresholds when flows were significantly impacted by anthropogenic land use, withdrawals, discharges, and impoundments. For each anthropogenic factor, thresholds were defined above or below which flows are considered altered. Flow metrics were selected from Indicators of Hydrologic Alteration (The Nature Conservancy 2009) and Hydrologic Integrity Tool (Henriksen et al. 2006) software.
Selected hydrologic metrics were normalized by watershed area and biological metrics were normalized to a comparable scale, accounting for much of the basin’s natural variability and thus obviating the need to classify rivers. Flow-ecology relationships were determined using quantile-regression.
The Potomac project was designed as a holistic, interstate environmental flow needs assessment for the entire watershed, using a shared hydrologic foundation and biological dataset. However, state agencies regulate water withdrawals in the Potomac watershed, and local authorities make land use decisions that affect flows. For this reason, flow recommendations emerging from this regional analysis will need to be implemented at the state or local level. The Potomac project team is sharing flow alteration-ecological response relationships with state-level resource managers to support their technical assessments and recommendations for protecting and restoring environmental flows and stream health throughout the watershed. In doing so, they are facing the challenges of implementing un-mandated recommendations, especially when those recommendations are based on macroinvertebrates, which evoke little sympathy from the public.
