Physical Setting And Hydrology

The Upper Potomac River, which has a drainage area of 11,560 square miles, is the major freshwater inflow to the estuary. Based on long-term (1931-1981) USGS data at Little Falls near the fall line, the mean annual daily flow is 11,406 cfs, with extreme

Figure 8-2 Location map of Middle and Lower Potomac River. (River miles shown are distances from the confluence of the Lower Potomac River with the Chesapeake Bay.)

discharge conditions of 374 cfs recorded during the drought of 1966 and 483,802 cfs recorded during the flood of 1936 (MWCOG, 1989). The long-term (1931-1988) mean 7-day, 10-year low flow (7Q10) at Little Falls is 628 cfs. Low-flow conditions typically occur from July through September, with the minimum monthly flow of 4,126 cfs recorded during September (Figure 8-3). The long-term (1951-1980) mean summer (July-September) flow for the Potomac River at Little Falls was 4,428 cfs (Figure 8-4).

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