CO-OP History

Interstate Commission on the Potomac River Basin

1960s Forecast of Shortage

Key Documents

The history of cooperative water supply management in the Washington, D.C., metropolitan area begins in the early 1960s, when projected growth in demand for Potomac water exceeded available supply.

Drought of 1966

In 1963, the U.S. Army Corps of Engineers conducted a comprehensive study of the Potomac River basin in order to identify solutions to the anticipated shortfall in meeting projected demand. The proposed construction of 16 large multi-purpose reservoirs in the Potomac basin met unexpected difficulty with public acceptance, and authorization and appropriation of construction funds. In the drought of 1966, flow in the Potomac was lower than the projected future demand; unrestricted water use would cause the river to go dry. Increasing population and droughts in the 1960s and 1970s added to the motivation to develop new resources. Only Jennings Randolph Lake was constructed of the 16 proposed projects. Jennings Randolph was originally called Bloomington Lake, and was completed in 1981.

Solutions Presented

Other structural solutions were examined in addition to the proposed multipurpose reservoirs. Inter-basin transfers were studied, a pilot estuarine treatment plant was constructed and tested, and an emergency estuarine pumping station was constructed.

Growing demand for potable water had the area scrambling to avert drinking water shortages.  When the Washington Suburban Sanitary Commission’s (WSSC Water) request for a weir that would allow withdrawal at low river flows was denied, and Fairfax County, Va., sought construction of a Potomac River intake, concerns over running the river dry during droughts came to a head.

Coordinated Operation

Negotiations in the mid 1970s among the utilities, Maryland, and Virginia, took several years as the Corps of Engineers sought a drought allocation scheme that would protect the District of Columbia’s access to water and provide for the growing suburbs in Maryland and Virginia.

The 1978 Potomac River Low Flow Allocation Agreement allocates water during a drought based on each utility’s usage during the previous five winter seasons while preserving a 100 mgd flow-by to maintain the river’s ecology.

Concurrently, a study of the situation was being conducted which treated the combined distribution areas of the three major Washington metropolitan area utilities as a single regional demand center, and investigated the coordinated operation of all the resources then available. The three utilities were the Fairfax County Water Authority (now Fairfax Water),  Washington Suburban Sanitary Commission (now WSSC Water), and the Washington Aqueduct Division (WAD) of the U.S. Army Corps of Engineers. The study showed that coordinated management of the water resources from a systems perspective led to gains in reliability of the water resource. The results of the latter analysis and its lower cost non-structural features led to the adoption of its results with the signing of the Water Supply Coordination Agreement in 1982.

The study found that the coordinated operation of the resources would allow the utilities to meet demands through the year 2025 even under a repeat of the drought of record. This is possible because of synergistic gains in total yield realized under the cooperative management strategies.

Cooperative Solutions

The signatories to the Water Supply Coordination Agreement were the District of Columbia, Fairfax Water, WSSC Water, WAD, and the Interstate Commission on the Potomac River Basin (ICPRB). The ICPRB’s Section for Cooperative Water Supply Operations on the Potomac (CO-OP) was designated by the Water Supply Coordination Agreement to be responsible for coordination of water resources during times of low flow. The management objectives embodied in the agreement and practiced by CO-OP are to keep the utilities’ resources balanced while meeting environmental requirements and municipal demands for water. The resources include Jennings Randolph, Savage, Little Seneca, Occoquan, and Patuxent reservoirs.

Each of the three utilities gives up a small measure of autonomy in order to gain the substantial benefits of reduced capital costs through coordinated cooperative operations of their individually and jointly owned resources. As an independent inter-jurisdictional organization, ICPRB is particularly well suited to engage in multi-state coordinated cooperative functions. 

Low Flow Allocation Agreement

As noted above, the Potomac River Low Flow Allocation Agreement divided available water among suppliers during drought conditions. Much work was needed to set up the agreement as well as to address issues that followed, as can be seen in this LFAA timeline.

To address concerns by the Department of the Interior (Fish and Wildlife Service and National Park Service) the U.S. Army Corps of Engineers issued a Memorandum of Intent noting that the LFAA signatories would agree to an environmental flow-by that would maintain sufficient flow to meet minimum aquatic habitat requirements between the Little Falls Dam and the head of tide at Chain Bridge, as well as supply water to the C&O Canal National Historic Park.

An acceptable environmental flow-by was determined with the Potomac Environmental Flow-by Report (1981), conducted by the Maryland Department of Natural Resources. The parties to the LFAA agreed to abide by the study’s recommended 100 mgd flow-by, “absent essential need.”

LFAA Review, 2018

A review of the LFAA was completed by a contractor in 2018. The review included research into historical background as well as current laws and regulations.  In depth interviews with the parties to the Agreement were conducted also.  The review concluded that the Agreement remains a cornerstone of water supply management for the region, it identified areas that might benefit from revision, and it provided options, though not recommendations, for how those updates might be done.

Resolution on Enhancing Water Supply Resilience for the Washington Metropolitan Area, 2021

In 2021, the Commissioners of the ICPRB passed a Resolution on Enhancing Water Supply Resilience for the Washington Metropolitan Area to facilitate the following action items:

  • A Task Force on the Water Supply Coordination Agreement;
  • Convene a Work Group to discuss the ten sets of options identified in the 2018 review of the LFAA;
  • Convene scientific workshops on state-of-the-art approaches to environmental flows for large river systems.
Potomac Environmental Flows Workshop 2022

To address the third action item of the Resolution, a virtual workshop was held in May 2022. Workshop attendees included government and regional agencies, non-profit organizations, and water suppliers.  They were tasked with the explicit purpose of answering the following questions:

  1. Are there other approaches now for determining environmental flows in large, relatively unregulated rivers like the Potomac?
  2. If there are, what data, analysis tools, and assessments are needed to make a scientifically defensible change?

Organized by ICPRB, the workshop consisted of one day of presentations on approaches for determining environmental flows and on the availability of data for Potomac River fish and macroinvertebrates. This was followed by a half-day of discussion among the stakeholders. The report, Potomac Environmental Flows Workshop 2022, shares a summary of the proceedings, including the thoughts and reactions of the participants.

Final products of the workshop:

The management of the cooperative water resource systems’ operations is overseen by a committee of water utility representatives. The water utilities provide the funding for these activities at the Interstate Commission on the Potomac River Basin.