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bicarbonate is converted to the equivalent quantity of car-
<br />bonate that would remain in the evaporation residue. In this
<br />report, the term "carbonate equivalent" refers to the sum
<br />of carbonate and bicarbonate as carbonate. Where carbonate
<br />and bicarbonate data were missing, alkalinity was used to
<br />compute carbonate equivalent. Carbonate equivalent concen-
<br />trations and loads were used in all subsequent analyses and
<br />are listed in tables of data in this report. Bicarbonate was
<br />the primary form of dissolved carbon at all sampling loca-
<br />tions. The concentrations and loads of sodium and potassium
<br />also were combined in a single value in this report. Potassium
<br />generally accounted for only a small fraction of this sum.
<br />As a river flows downstream, it naturally accumulates
<br />dissolved solids from diffuse and point sources. Diffuse
<br />sources add dissolved solids gradually throughout long
<br />reaches of the river; these contributions may come from
<br />overland flow, from ground-water movement through
<br />underlying soils and rocks, and from dissolution of materials
<br />in the stream channel. Natural point sources are primarily
<br />ine springs that discharge into the stream. Human activities
<br />that can produce sources of dissolved solids include municipal
<br />and industrial development, mining and drilling operations,
<br />disturbance or inundation of surface material, and operation
<br />of irrigation systems. Diffuse sources contribute more
<br />dissolved solids to the Colorado River system than do point
<br />ources (Iorns and others, 1965) and are more difficult to
<br />measure and control. Any process that increases the quan-
<br />tity of dissolved solids transported downstream may be called
<br />dissolved-solids loading.
<br />Dissolved-solids concentration may increase because
<br />of the diversion of water that has a relatively small dissolved-
<br />solids concentration from the stream system. In the Upper
<br />Colorado River Basin, this concentrating effect primarily is
<br />caused by the export of water to other basins, evapotranspira-
<br />tion from irrigated agricultural lands, and evaporation from
<br />stream channels and reservoirs.
<br />To address the concern about dissolved solids, Con-
<br />gress passed the Colorado River Basin Salinity Control Act
<br />of 1974 (amended in 1985), which authorized planning and
<br />construction of salinity-control projects in the Colorado River
<br />basin. The U.S. Bureau of Reclamation was named the lead
<br />agency for coordinating salinity-control activities.
<br />As part of a cooperative agreement with the U.S.
<br />Bureau of Reclamation, the U. S. Geological Survey has
<br />undertaken studies to provide information that will help in
<br />evaluating and planning salinity-control needs. This report
<br />evaluates historical streamflow and dissolved solids in the
<br />Upper Colorado River Basin.
<br />Purpose and Scope
<br />The purposes of this report are to (1) determine an-
<br />nual and monthly concentrations and loads of dissolved solids
<br />and the major dissolved constituents for all streamflow-
<br />gaging stations in the Upper Colorado River Basin that had
<br />adequate periods of record; (2) determine sources of dis-
<br />solved solids; (3) determine trends in streamflow, dissolved
<br />solids, and the major dissolved constituents during various
<br />periods of record; and (4) determine possible causes of
<br />trends.
<br />This report describes the water resources of the drain-
<br />age basin of the Colorado River upstream from Lee Ferry,
<br />Ariz. Dissolved solids were estimated at 70 streamflow-
<br />gaging stations in the Upper Colorado River Basin by using
<br />surface-water and water-quality records from the U. S.
<br />Geological Survey data base. Records through the end of the
<br />1983 water year were evaluated.
<br />Previous Investigations
<br />The first description of the Colorado River basin was
<br />reported by John Wesley Powell (1875), documenting his
<br />expeditions during 1869 and 1871-72. Several U.S. Geo-
<br />logical Survey publications in the early twentieth century
<br />have described the region, its water resources, and its poten-
<br />tial for water use (La Rue, 1916, 1925; Follansbee, 1929;
<br />Woolley, 1930). Other comprehensive studies were compiled
<br />by Iorns and others (1965), the Upper Colorado Region State-
<br />Federal Inter-Agency Group (1971), and the U.S. Environ-
<br />mental Protection Agency (1971).
<br />Using tree-ring dating techniques, precipitation trends
<br />and natural streamflow in the basin have been evaluated for
<br />the last 450 years (Stockton and Jacoby, 1976; Dracup,
<br />1977). Using streamflow, reservoir storage, and depletion
<br />records, virgin streamflow at Lee Ferry, Ariz., has been
<br />estimated from 1896 to 1984 (Upper Colorado River Com-
<br />mission, 1984).
<br />Numerous studies concerning dissolved-solids concen-
<br />tration and water quality in the basin have been done since
<br />the 1960's in response to international agreements, national
<br />legislation, and increasing demands for potable water.
<br />Ground-water investigations include those of Price and
<br />Arnow (1974), Taylor and others (1983), and Warner and
<br />others (1985). The U.S. Bureau of Reclamation has published
<br />biennial progress reports since 1963 documenting salinity,
<br />water use, and salinity-control measures (U.S. Department
<br />of the Interior, 1985).
<br />Beginning in 1985, the U.S. Bureau of Reclamation
<br />and the U.S. Department of Agriculture jointly published
<br />an annual evaluation of salinity-control programs (U.S.
<br />Bureau of Reclamation, 1985b). Prior to 1985, their reports
<br />were published separately (for example, U.S. Bureau of
<br />Reclamation, 1983; U.S. Soil Conservation Service, 1983,
<br />1984). The U.S. Environmental Protection Agency (1971)
<br />also has investigated mineral quality in the Colorado River
<br />basin. Analyses of trends in dissolved solids in the basin were
<br />done by Kircher and others (1984) and Moody and Mueller
<br />(1984).
<br />2 Characteristics and Trends of Streamflow and Dissolved Solids in the Upper Colorado River Basin
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