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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Discharge Measurements

Discharge measurements were made at 19 sites in the study area. Of these sites, 14 were along the Virgin River, 1 site was at the confluence of Beaver Dam Wash with the Virgin River, and 4 sites were at agricultural diversions (table 1, fig. 1). At the 15 sites excluding the diversion, measurements were made hourly from 9:00 a.m. (PST) until 2:00 p.m. (PST) on February 12, 2003. Measurements at the four diversion sites were made at differing intervals between the same time periods. All discharge measurements were made with a current meter using standard methods of the USGS (Buchanan and Somers, 1969). Staff gages were installed at each site to monitor changes in stage.

Although river stage at most sites remained stable (within a few hundredths of a foot), discharge measurements at each selected site differed, on average, by about 8 percent with a maximum of 15 percent at one site. These differences reflect the inherent instability of sand-dominated channels as described by Rantz and others (1982). In spite of channel instability, each of the synoptic-discharge measurements made at the 14 sites along the Virgin River were rated good (within 5 percent of the actual discharge) based on estimates of standard error for each current-meter discharge measurement (Sauer and Meyer, 1992). Final discharge rates along the lower Virgin River were determined by computing a mean discharge from the synoptic measurements made at each site. The computed mean discharge and the maximum and minimum measured discharges are shown in figure 2 for each site.

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