USGS Water-Year Summary 2024

01638500 POTOMAC RIVER AT POINT OF ROCKS, MD

LOCATION - Lat 39°16'24.9", long 77°32'35.2" referenced to North American Datum of 1983, Frederick County, MD, Hydrologic Unit 02070008, on left bank at downstream side of bridge on U.S. Highway 15 at Point of Rocks, 0.3 mi downstream from Catoctin Creek (Virginia), 6 mi upstream from Monocacy River, and at mile 159.5.

DRAINAGE AREA - 9,651 mi².

REVISIONS HISTORY - WSP 192: 1895-1905. WSP 1432: 1899, 1901-2, 1904-5, 1912, 1914(M), 1915, 1917(M), 1918, 1919(M), 1920, 1921-23(M), 1924, 1925-28(M), 1930(M).

SURFACE-WATER RECORDS

PERIOD OF RECORD - February 1895 to current year.

GAGE - Water-stage recorder and crest-stage gage. Datum of gage is 199.91 ft above North American Vertical Datum of 1988. Prior to Oct. 28, 1929, nonrecording gage at same site. Prior to Sept. 2, 1902, at gage datum about 0.45 ft higher.

REMARKS -

Water Years 2017-2024: Records good, except those for estimated discharges, which are poor.

Water Years 2014-2016: Records good, except those for estimated daily discharges, which are poor.

Water Year 2013 and prior years' accuracy statements can be found in Annual Water Data Reports.

Low flow affected slightly from 1913 to July 1981 by Stony River Reservoir; since December 1950 by Savage River Reservoir (see station 01597500); and since July 1981 by Jennings Randolph Lake. Low flow affected extensively at times by run-of-the-river hydroelectric plants. U.S. Geological Survey satellite data-collection platform at station. 

EXTREMES OUTSIDE PERIOD OF RECORD - Flood of June 2, 1889, reached a stage of 40.2 ft, from floodmarks, discharge, about 460,000 ft³/s from rating curve extended above 300,000 ft³/s, on basis of adjustment of figure of peak flow at station near Washington for inflow and storage, and slope-area measurement of peak flow.

WATER-QUALITY RECORDS

PERIOD OF RECORD - May 2021 to current year.

INSTRUMENTATION - YSI EXO2 multiparameter sonde. 

REMARKS -

Missing record due to periodic instrument malfunction or times when the sensor is being serviced.  Occasional localized spikes in water quality data can be caused from boating, fishing, swimming, and other recreational activities, as well as high winds.  These localized spikes are believed to be real, though they may not be representative of the entire river.  Due to the width of the river, braided channel upstream of the gage, and inflows coming into the river upstream of the gage, the continuous water quality data may not be representative of the entire river at all times.

Turbidity values greater than 1,000 formazin nepthelometric units (FNU) exceed the instrument calibration range.  Values above 1,000 FNU are included in the record but are presumed to have lower accuracy.  

The accuracy limit of the turbidity sensor in low turbidity water is +/- 0.3 FNU.

October 1, 2024:

Values are censored and qualified as Less Than (<) 0.3 FNU if they are recorded below 0.3 FNU

EXTREMES FOR PERIOD OF RECORD -

WATER TEMPERATURE: Maximum, 32.9°C, June 22, 2024; minimum, -0.1°C, on multiple days throughout the period of record.

SPECIFIC CONDUCTANCE: Maximum, 481 microsiemens/cm, Nov. 19, 2023; minimum, 126 microsiemens/cm, September 23, 2021. 

DISSOLVED OXYGEN: Maximum, 17.3 mg/L, January 21, 2025; Minimum, 3.7 mg/L, July 29, 2021. 

pH: Maximum, 9.0 standard units, on multiple days throughout the period of record; Minimum, 7.3 standard units, on multiple days throughout the period of record. 

TURBIDITY: Maximum, 628 FNU, December 23, 2022; Minimum, 0.3 FNU, November 4, 2022.