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<br />~. <br />O' <br />00 <br />CD, <br /> <br />natural causes including salt contributions, from saline springs, ground water discharge into the <br />river system (excluding irrigation return flows), erosion and dissolution of sediments, and the <br />concentrating effects of evaporation and transpiration. The natural causes category also included <br />salt contributions from non-point (excluding irrigated agriculture) or unidentified sources or from <br />the vast, sparsely-populated regions of the drainage, much of which is administered by the BLM <br />or other government agencies. Of the land within the Colorado River Basin, about 75 percent is <br />owned and administered by the Federal Government or held in trust for Indian tribes. The <br />greatest portion of the naturally-occurring salt load originates on these federally-owned and <br />administered lands. Human activities, such as the following, can influence the rate of natural salt <br />movement from rock formations and soils to the river system: livestock grazing, wildlife <br />management, logging, mining, oil exploration, road building, recreation and urbanization. <br /> <br /> <br />Approximately 53 percent of the salinity concentration in the water arriving at Hoover <br />Dam, as identified by EPA, results from a number of human activities. BPA estimated that out- <br />of-basin exports account for about 3 percent of the salt concentration at Hoover Dam, with <br />irrigation accounting for 37 percent, reservoir evaporation and phreatophyte use accounting for <br />about 12 percent, and about 1 percent attributed to municipal and industrial uses. Much of the <br />salt load 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 basin-wide 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 <br />control measures. <br /> <br />Continuing evaluations of the salinity of the Colorado River are made by Reclamation, the <br />U.S. Geological Survey (USGS) and the Bureau of Land Management (BLM). Several were <br />published by the agencies during the period of this Review (1993-1996). To evaluate changes in <br />salinity, water quality and streamflow data are obtained on a daily, weekly, monthly, andlor <br />quarterly basis at various points on streams throughout the basin by the USGS in cooperation <br />(through financial andlor direct services) with private entities, the states and other federal <br />agencies. Gaging stations in the basin which are of significance to the programs, and for which <br />streamflow and water quality records are available, are shown on Figure 2-1. <br /> <br />Average annual salinity concentrations and salt loads are determined on a flow-weighted <br />basis using the most accurate data available. To compute the flow-weighted average annual <br />salinity concentration, the average flow of the River in acre-feet per day at a measuring point and <br />the average concentration of salts in the water in mg/L are determined on a daily basis. <br />Concentration of salt may be measured directly by chemical analysis of dissolved constituents <br />(TDS) or indirectly as specific conductance and correlated to TDS. Daily flows are multiplied <br />by daily salinity concentrations and then summed to produce an annual mass figure. The annual <br />mass figure is then divided by the total flow for the year at the measuring point (sum of the daily <br />average flows) to yield the flow-weighted average annual salinity for the station. <br /> <br />2-2 <br />