Scientific Investigations Report 2005-5286

Scientific Investigations Report 2005-5286

Discharge and Physical-Property Measurements from Virgin River Narrows, AZ, to Lake Mead, NV, February 12, 2003

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Synoptic-discharge measurements were made on February 12, 2003, along the lower Virgin River from the Narrows gage to Lake Mead, Nevada. Discharge, water-temperature, and specific-conductance data were collected at 19 sites to evaluate their spatial variability within the river during a period of low agricultural diversion and minimal ET rates. Five discharge measurements, on average, were made at each site within a time span of about 5 hours. A mean discharge was used to represent the discharge rates for each measurement site. The lowest mean discharge, 90.5 ft3/s, was measured at the Narrows gage. The highest mean discharge, 158 ft3/s, was measured at the Littlefield gage. Mean discharges between the Littlefield gage and Lake Mead ranged from 116 to 148 ft3/s. An increase in discharge between the two lower sites (09415240 and 09415250) was attributed primarily to an increase in streamflow and not from ground-water inflow.

The lowest average water temperature was 7.5°C at the Narrows gage. The highest average water temperature was 13°C at the Littlefield gage and at sites 09415020 and 09415105. Specific-conductance measurements ranged from 2,850 µS/cm at the Narrows gage to 3,430 µS/cm at site 09415250.

Between the Narrows and Littlefield gages, mean discharge, mean water temperature, and specific conductance indicated net increases of 67.5 ft3/s, 5.5°C, and 130 µS/cm, respectively. Between the Littlefield gage and site 09415250, mean discharge and water temperature indicated a net decrease of 42 ft3/s and 4.0°C, respectively, whereas specific conductance indicated a net increase of 450 µS/cm.

The general trends in discharge, water temperature, and specific conductance on February 12, 2003, in the lower Virgin River from the Virgin River Narrows, Arizona, to Lake Mead, Nevada, seem consistent with a river basin where surface water is diverted for agricultural and municipal purposes, and some diverted water is recycled back into the river through the shallow ground-water system downstream of the diversions.

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