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Potomac Flow-by Studies


Introduction 2004-2005 Potomac Low-Flow Update Workshops 2003 Potomac Instream Flow Methods Workshop 2002 Habitat Assessment Report 1981 Habitat Assessment Report Other Links

Summary of Issues

During the late 1970's, as water withdrawals from the Potomac River began to increase to meet the needs of the watershed’s growing populations, concerns were raised about the potential consequences of such withdrawals on the Potomac River ecosystem. In order to assess what level of flow should be sustained in the river to protect it’s aquatic resources, an assessment study entitled, Potomac River Environmental Flow-By Study, was completed in 1981 by the Maryland Department of Natural Resources (DNR). The findings of this study served as the basis for a multi-agency agreement that established a Potomac River minimum low-flow or flow-by requirement at Little Falls of 100 million gallons per day (mgd) and 300 mgd at Great Falls (see map). To ensure that flows do not drop below these protective levels, natural flows in the river are augmented with water releases from several impoundments in the basin, as needed.

While these low flow requirements have been in place for more than 20 years, the severe drought conditions that occurred in the Potomac River watershed in 1999 raised concerns about the adequacy of these flow-by requirements for protecting the river ecosystem and its resources, particularly given the increasing demand for water within the river basin. At issue was the technical basis for the designation of those flow targets in 1981 and the methods used to develop them. Maryland DNR agreed to coordinate an assessment of natural resource issues and to conduct an evaluation of whether the current requirement adequately protects natural resources from irreversible long-term or significant short-term impacts. PPRP serves as lead agency for this effort, based on its involvement with a preliminary permit for licensing of a hydroelectric project at Jennings-Randolph reservoir and its evaluation of potential power plants with new water intakes on the Potomac River in Frederick and Montgomery Counties. In addition, PPRP served as the Maryland agency in evaluating and establishing minimum flow requirements below Conowingo Dam on the Susquehanna River, and thus has experience in dealing with low flow issues. As part of the low-flow assessment and evaluation, several meetings were held in 2000 among interested stakeholders (The Potomac Flow-by Committee) to discuss potential impacts on natural resources due to low flows. A subset of stakeholders, including local, state, and federal agencies, water supply utilities, and environmental groups, was set up as a Habitat Assessment Subcommittee (HAS) to focus on flow-related habitat issues.

A number of different approaches were reviewed for use in establishing the basis on which low-flow requirement decisions could be made but none were found suitable. Fortuitously, at the time a re-evaluation was being considered, record drought conditions in 2002 presented an opportunity to conduct a physical habitat assessment, including developing a habitat map, conducting a field survey of habitat types, and measuring hydraulic and water quality conditions. PPRP produced a report that provides background information describing the history of current low-flow requirements, a review of the studies conducted to support those requirements, and the 2002 habitat assessment conducted during low flow conditions. In April 2003, a workshop was convened with a special panel of nationally recognized experts on habitat assessment to investigate and develop methods to evaluate the environmental flow-by requirements. At this workshop, members of the special panel collectively considered and debated the various methodologies applicable to the Potomac River to address the flow-by issue. The final product of the workshop is a set of recommendations for 1) the best method or approach, given current financial resource limitations, to address the Potomac Flow-by Study objectives, and the level of confidence associated with their recommendations, and 2) an alternative long-term method or approach which could better accomplish those objectives, yet might exceed current resources or available data, and recommended guidelines for achieving the objectives in a longer time-frame.

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This page was updated on Dec. 8, 2005