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Water Science for Maryland, Delaware and the District of Columbia

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Streamflow

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.

January 2018 Streamflow


Monthly mean streamflows were in the normal range at 61 percent, or 20 of 33 selected USGS streamgages, although the values were estimated in 29 of the streams due to ice. (See the explanation of the effects of ice on streamflow at the end of the streamflow section). Streamflow was below normal at 13 streamgages in Maryland, Delaware, and the District of Columbia, including 11 streamgages in the 10th -24th percentiles, and 2 streamgages in less than the 10th percentile. Streamflow decreased at the Nanticoke River and increased at the remaining 32 streamgages between December 2017 and January 2018.


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.

At the Nanticoke River near Bridgeville in Sussex County, Delaware, the monthly mean streamflow decreased slightly between December and January to 46.8 cubic feet per second (ft3/s), which is below normal. The normal streamflow range for January at this streamgage is between 65.3 ft3/s and 165 ft3/s. Record-keeping at this streamgage began in April 1943.

At Piscataway Creek at Piscataway in Prince George’s County, Maryland, the monthly mean streamflow increased between December and January to 10.4 ft3/s. The normal streamflow range for January is between 32.6 ft3/s and 71.8 ft3/s. Record-keeping at this streamgage began in October 1965.

Effects of Ice on Streamflow

In the Mid-Atlantic region, ice in streams can typically occur between December and March. Below freezing air temperatures can lead to the formation of ice in many channels and may result in erratic water-level readings. Ice in streams may result in biased gage height records, invalidating the known stage-discharge relation.

When erroneous gage height values appear, they are flagged or removed from the web display until they can be reviewed. However, if the gage heights are considered to be accurate but the ice in the channel is causing a bias on the stage-discharge relation, then a heavy blue line will mask discharge values, as shown in this example at the Savage River in Maryland. Hydrographers will later analyze the data available and estimate unit value discharges to derive a daily discharge when the values are affected by ice. Weather records, discharge data obtained through direct measurements (made during the ice-affected period) and (or) hydrographic comparison with non-ice affected streamgages in the surrounding area all play an important role in making these estimates.

 

Below-freezing temperatures may also affect the amount of water in a stream channel. In winter, a natural freeze/thaw cycle can occur, as shown in the hydrograph below from Georges Creek, Maryland. When the water in smaller tributary streams upstream of a gaging location freezes, typically after dark, then less water is able to pass by the gage, causing the sudden drop off in flow as seen in the graph below. When the sun comes out the next day and if the air temperature rises, upstream channels will thaw and release the water, resulting in increased streamflow at the gage.


Frozen ground leads to less infiltration and groundwater movement (baseflow) to streams, which could result in lower streamflow, but as temperatures get warmer, the water is released. Fluctuations related to the freeze/thaw cycle may represent actual flow conditions, which would not require corrections to the data.

 



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