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<br />the naturally-occurring salt load originates on these federally-owned and administered lands, Human <br />activities. can influence the rate of natural salt movement from rock formations and soils to the river <br />system aild include: livestock grazing, wildlife management, logging, mining, oil exploration, road <br />o building,' recreation and urbanization, . <br />{\.) <br />W <br />W Approximately 53 percent of the salinity concentration in the water arriving at Hoover Dam, <br />as identified by EPA, results from various human activities. EPA estimated that out-of-Basin <br />exports account for about 3 percent of the salt concentration at Hoover Dam, with irrigation <br />accounting for 37 percent, reservoir evaporation and phreatophyte use accounting for about <br />12 percent, and about 1 percent attributed to municipal and industrial uses. Much of the salt load <br />contribution from irrigated agriculture is from federally-developed irrigation projects. <br /> <br />Salinity control activities necessarily include a water quality monitoring and analysis <br />component that provides basinwide information for program evaluation. The monitoring and <br />analysis component provides an essential database for future studies, supports state and regional <br />planning activities, and provides an objective basis for evaluating the effectiveness of salinity control <br />measures. <br /> <br />Continuing evaluations of the salinity of the Colorado River are made by Reclamation, the <br />U.S. Geological Survey (USGS) and BLM. The results of several studies have been published by <br />the agencies since the last Review (1999-2001). To evaluate changes in salinity, water quality and <br />streamflow data are obtained on a daily, weekly, monthly, and/or quarterly basis at various points <br />on streams throughout the Colorado River Basin by the USGS in cooperation (through fmancial <br />and/or direct services) with private entities, the states and other federal agencies. Figure 2-1 shows <br />the gaging stations in the Colorado River Basin which are of significance to the program and for <br />which streamflow and water quality records are available. Data and salinity reports are available <br />from Reclamation at www.uc.usbr.gov/orogact/salinity/index.htrnl. <br /> <br />Salinity data are based on total dissolved solids (TDS) as the sum of constituents, whenever <br />possible. The sum of constituents values are defined to include calcium, magnesium, sodium, <br />chloride, sulfate, a measure of the carbonate equivalent of alkalinity and, if measured, silica and <br />potassium. If a sum of constituents value could not be computed, TDS as residue on evaporation <br />(at 180 degrees Celsius) is substituted. Further, some reported salinity values are based on <br />correlatio;lU with specific conductance measurements. In this Review the terms "salinity," "TDS" and <br />"concentration" in mg/L are used interchangeably. <br /> <br />Average annual salinity concentrations and salt loads are determined on a flow-weighted <br />average annual salinity concentration. The flow-weighted. average annual salinity is the <br />concentration determined from dividing the annual total salt load passing a measuring station by the <br />total annual volume of water passing the same point during a calendar year. The flow-weighted <br />average annual salinity is calculated by first multiplying the daily concentration values by the daily <br />flow rates. These values are then summed over a calendar year and then divided by the sum of the <br />daily flow rate. <br /> <br />2-2 <br />