The Potomac River Flow

At the last meeting of the Potomac Watershed Roundtable, Curtis Dalpra, Communications Manager for the Interstate Commission on the Potomac River Basin (ICPRB) presented the results of a study to examine what the impacts of various senarios on the flow of the Potomac River which supplies much of the drinking water in the Washington Metropolitan Area. At one point in his presentation he stated that the flow of the Potomac has fallen, but did not have the data on hand to specify over what period. The region had only recently emerged from an extended drought and so I went back to the study that the ICPRB performed to examine adequacy of the Potomac River flow with projected land use and population growth without climate change to see if I could find the data. What I found did not indicate a reduction so much as a change in flow to the river.

Both Images from ICPRB

As shown in the two charts above taken from the ICPRB, there appears to be an increase in both precipitation and runoff (surface flow to the rivers) over the past century rather than a decrease. Mr. Dalpra might have been referring to the recent drought and changes in flow patterns. Low flows have been lower, and high flows higher, in the past than they are now. These flow changes are the result of some combination of changes in land use, consumptive water use, augmentation of low flows using reservoir water and possibly changes in the climate or to the extensive drought that hit this region several years ago and expanded to a multi-year drought.

The region seems to have recovered from that drought according to the Drought Monitor. However, in reviewing the data I did discover that about 70% of the Potomac is used in power generation, though it is not clear what proportion of that use is consumptive. In the United States over 90% of all power is generated by using the power source (coal, gas, nuclear and even solar) to heat water to create steam that drives the turbines and generates electricity. Nationally, less than half of the fresh water is used in power generation, but that is because irrigation water usage is much lower in the Potomac River basin where agriculture is much less dependent on irrigation.

The Potomac is the fourth largest river along the Atlantic seaboard. 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 more than 500 million gallons of freshwater daily to those living in its watershed, as well as irrigation water , and the more than 2 billion gallons of water a day for power plants.

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 500 million gallons per day the river supplies for drinking water to the region and the essential environmental services. The ICPRB was born out of the severe and extended drought in the 1960's when water withdrawls to supply drinking water to the region from the River reduced flows to such an extent that the River practically ran dry, leaving only mud between Great Falls and the tidal river.

Ultimately (after more than a decade) the ICPRB was created to manage the water withdrawls from the Potomac to ensure that essential services like wastewater assimilation and habitat maintenance. The ICPRB monitors river flows and withdrawls to ensure the 100 million gallons per day minimum flow- at Little Falls and the 300 million gallons per day from Great Falls to Little Falls. These minimum flow levels have been observed since the early 1980's, but during that time flows have rarely been that low.

It is possible that River median flows at the measuring points of Little Falls and Great Falls is changing and may be decreasing due to population growth and associated land use changes that have taken place in the past 15 years. 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.

The original old growth forests of the region were largely destroyed by slash-and-burn agriculture and by logging to build the region. After destroying 60%-70% of the original forest cover by the 1890’s the trend reversed. Marginal agricultural lands were abandoned and young forest began to reestablish. The new forests that grew up in the 20th century helped restore ecological and hydrological functions. Increases in forest acreage continued until the late 1900s, when expanding urbanization began to reverse that trend and the wooded lands at the suburban edge began to be developed in increasing numbers.

Increased development reduces groundwater infiltration, increases the demand for drinking water and the demand for power. Even without climate change impact, human uses of land and water can interrupt or break many of the natural connections between river flow and precipitation. Analysis by the ICPRB found that land use change is a greater source of hydrologic alteration than dams, impoundments or water withdrawals.