Sunday, July 17, 2022

Drought and the Potomac River

For the first time in years,  the Interstate Commission for the Potomac River Basin, ICPRB, says there is an above normal probability of needing to releases water from the Washington metropolitan area’s back-up water supply reservoirs over the summer and fall. The need for water from the Jennings Randolph and Little Seneca reservoirs is triggered by low flows in the river resulting from a combination of low summer precipitation and low groundwater levels. The average precipitation in the Potomac Basin is below normal for the past 12 months. 

Between 2014-2018, total daily withdrawals between Seneca Pool and the Little Falls Dam averaged 354 million gallons/day and ranged between 257 and 503 million gallons/day (Ahmed et al. 2020, Ahmed, pers. comm.). While the average daily withdrawals are roughly 5% of the long-term river flow at little falls, they are a substantially larger percentage of the flow during dry and drought periods.

According to the ICPRB the Washington metropolitan area water utilities are planning to meet the  potable water demands in the face of increasing population and use. “Current demands would suck the river dry during a drought of record without the creation of reservoirs used to bolster river flows during extreme low flow. “ The stored water is used and managed by a process that provides water for each of the major independent water suppliers serving the area. A crucial aspect to meeting the drinking water demands of the region is preserving and protecting the ecological health of the Potomac River, leaving enough water in the river segment downstream of intakes for the river to reach the Chesapeake Bay. These so called minimum flow levels have been established for that segment of the river.

The 1978 Potomac River Low Flow Allocation Agreement continues to serve to allocate water during  drought periods based on each utility’s usage during the previous five winter seasons while preserving a 100 million gallons/day flow-by to maintain the river’s ecology. Loudoun Water has recently built its own river intake upstream and is being incorporated into the agreement which turns out not to be entirely straight forward. 

As part of an ongoing process of updating the agreement and water supply management, the ICPRB recently held a two-day Environmental Flows Workshop to assess possible new approaches for determining sustainable environmental flows and the data, tools, and types of assessment needed to create new flow rules that protect the Potomac as the climate and ecology continue to change. This is all intended get stakeholder buy-in and to ensure the Potomac can meet ongoing needs while retaining its living resources and other values.

It is possible that River median flows at the measuring points of Little Falls is changing and may be decreasing due to population growth and associated land use changes that have taken place this century. Population growth accelerates loss of forest and farmland, hardens surfaces, increases demand for water. Urbanization can significantly alter a river’s flow. Impervious surfaces of roadways, sidewalks, parking lots and building foundations increase stormflow peaks, frequency, and duration, impart greater erosive power to the water by increasing velocity, and reshaping stream contours. Rivers are sustained by groundwater between in drier periods, but urban and suburban development reduces recharge of the groundwater. Deforestation increases the proportions of rainfall running off the landscape instead of seeping into the ground where it can be taken up by plants or enter the groundwater. All this needs to be accounted for in allocations and ecological flows.

The Potomac River starts life as a spring at the Fairfax Stone in West Virginia. The river flows approximately 385 miles to the Chesapeake Bay increasing in size and flow from its tributary streams and rivers in West Virginia, Maryland, Pennsylvania, Virginia, and the District of Columbia. The Potomac River grows to become the Chesapeake Bay's second largest Tributary. The River provides drinking water to those living within its watershed, irrigation water , and the water for power plants and data centers.

The Potomac River is one of the least dammed large river systems in the Eastern United States. The combined storage capacity of all major reservoirs upstream of Washington, DC makes up less than 7% of median flow. Nonetheless, the Potomac River’s flow needs to be managed to assure the river supplies for drinking water to the region and the essential environmental services. The ICPRB was created to manage the water withdrawals from the Potomac to ensure that essential services like wastewater assimilation and habitat maintenance. The ICPRB monitors river flows and withdrawals to ensure the Potomac River will never again run dry.

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