USGS - science for a changing world

Water Science for Maryland, Delaware and the District of Columbia


Streamflow data are most commonly used for assessing water supply and to determine the risk of droughts and floods. Streamflow data are also used to calculate loads of chemical constituents, and to assess how biological communities are affected by hydrologic conditions.

The USGS streamgages chosen for the monthly water summary were selected based on the following criteria:

  • Minimum period of record is 10 years of continuous data
  • Watershed areas greater than 5 square miles
  • Streamflow is not regulated, such as by a dam or diversion, and it has relatively natural flow
  • Streamflow data reflect to weather condition
  • Most of the surrounding area and watershed are not urban

Of the 33 streamgages used in this summary, 22 have more than 60 years of data, allowing for comparison to data from the historical droughts of 2002 and the 1960s. All 33 streamgages have at least 30 years of monthly mean streamflow data.

September 2018 Streamflow

Monthly mean streamflows were normal to above normal at all 33 streamgages. Streamflow at 21 streamgages was in the normal range. Streamflow at four streamgages was above the 90th percentile, and was between the 76th and 90th percentiles at the remaining eight streamgages. Monthly mean streamflow decreased at 28 streamgages (85 percent) and increased at 5 streamgages (15 percent) between August and September.

Click here to access the clickable streamflow map

In the hydrograph for the selected streamgages, the dark line in the 5-year hydrograph represents the monthly mean streamflow for this period, and the white band shows the normal range (25th-75th percentiles) based on the period of record. The maximum monthly mean streamflow is at the top of the blue shaded section, and the lowest monthly mean streamflow is at the bottom of the tan area. Each monthly mean streamflow is colored according to the percentile rank compared to the historical data for the month.

Monthly mean streamflow at Potomac River at Point of Rocks, in Frederick County, Maryland, was 6,930 cubic feet per second (ft3/s), which is above the 90th percentile. Streamflow had been above the 90th percentile at this streamgage, including two record highs, over the previous 5 months. The normal monthly mean streamflow range for September is between 1,720 ft3/s and 4,420 ft3/s. Record-keeping at this streamgage began in February 1895, and it is the oldest streamgage in Maryland.

At St Jones River at Dover, in Kent County, Delaware, the monthly mean streamflow was 24.2 ft3/s, which is in the normal range (between 5.68 and 28.9 ft3/s). Record-keeping at this streamgage began in January 1958.



Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo logo U.S. Department of the Interior | U.S. Geological Survey
Page Contact Information: