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'!!fORS" . .. ~, "'I~,:_,,'~J "'::, r!i:;;~ Mining is a'SQ~q~ofi ...... ... ; urbanizatio~~'is a'soJlrc~\'o '''':'. ," "~ 1~I,br:t,1Jii" ",~I~: ,!~}~;, ,;'i'", , !j~, ~ JiutJ;ients and,,6rganicch~mi~als;'and agpcultu&'i,s a'souiee of' s",llj,qi!Yi,anc;l nU1[ients't ui'fers and streams,~c:lsus~n'd'ea' seil.iIl\e. Ii'ts't.,O strea~$ j" "'v, ~; ffl. _ ~.,.I .! ,"~ Interbasin water transfers, mining, urbanization, and agriculture are the principal human activities that affect water quality in the UeOL study unit. In the study unit, these activities occur approxi- mately in a downstream order. Interbasin water transfers are in the headwaters, mining occurs in the mountainous areas, urbanization is in the valleys of the Southern Rocky Mountains and Colorado Plateau, and agriculture predominantly is in the valleys of the Colorado Plateau. Water Management and Use In 1993, interbasin water transfers (fig. 4) conveyed about 585,000 acre-feet of water (12 percent of the average annual streamflow of the basin) from the Upper Colorado River Basin to the South Platte, Rio Grande, and Arkansas River Basins. Inter- basin water transfers generally occur near the stream headwaters, and the amount of streamflow diverted can be a substantial part of the streamflow near these sources. By removing water from the system, interbasin water transfers decrease the dilution capability of the streams. The numerous reservoirs, water diversions, and municipal discharges in the basin (fig. 4) alter the natural stream- flow, which can affect the aquatic habitat and water quality of the streams. For example, reservoirs can trap sediment and decrease trace-metal loads downstream. ~..~~ l\... !.J~ "'," ,.~. /"-.... . ~~ ! ~o:..,J Va". 1 "~ . . --.J Glenwood '- I:;ll~(t' ~ ( . . - CI,!oroJdo Spri~g9 Avon ~e..."'. ...... :'p ,~ ~~,,"",f~ A.pen ~ :; De.e · ) ~ \ . --\ +''i;:~ .~ :.;, Gunnison , :::o.~ \.,. . ~.,.) '\ .~" J 0 .~.,~.r-J"" -f>..i-....... ;;- Grand ) Junction "-.,... .. \ . l, ~<.. ~ <';"o~ ~ . 'i~1't- ,. 20 40 MILES o 20 40 KILOMETERS EXPLANATION . Reservoir greater than 50.000 acre-feet Water diversion greater than 100.000 acre-feet per year . ~ Municipal discharge greater than 850 acre-feet per year Interbasin water transfer greater than 4,500 acre-feet per year Figure 4. Major water storages, uses, and exports in the Upper Colorado River Basin study unit. Water used in the study unit predominantly is from surface- water sources; however, ground water, which accounts for about I percent of the total water used in the basin, is important for domestic use in rural communities. Irrigation is the principal off- stream water use in the basin, which accounted for about 97 percent of all water use in 1990. Land Use Rangeland and forest (fig. 5) are the predominant land uses in the study unit (about 85 percent). Livestock grazing, logging, and year-round recreational activities on rangeland and forest can affect the water quality by adding sediments, nutrients, and other constituents to the water. Metal mining is an important activity that occurs in the headwater areas of the basin. Past and present mining activities have included the extraction of metals such as copper, gold, lead, molybdenum, nickel, silver, vanadium, and zinc. Several tributaries to the Colorado River-the Blue, Eagle, Roaring Fork, and Gunnison Rivers-drain one of the primary metal-mining regions in Colorado. Streams in the basin have been affected by point-source mine discharge and nonpoint-source runoff from mined areas (Wentz, 1974). Some streams in the basin, such as Red Mountain Creek (a tributary to the Uncompahgre River), are affected by acid mine drainage. Urbanization is one of the smaller land uses in the mostly rural Upper Colorado River Basin. However, in some mountain communities in the headwaters of the study unit, the population is increasing at a rate of about 10 percent annually (Bureau of Census, 1992). Point sources from urbanization include discharge from wastewater-treatment plants, leachate from septic systems, solid- waste disposal, leaking underground storage tanks, industrial discharges, and storm runoff, which also is a nonpoint source. All these sources potentially can affect the surface and ground water by adding nutrients, pesticides, various chemicals, hydrocarbons, trace elements, and salts depending upon the specific point source. Agriculture, predominantly in the Colorado Plateau physio- graphic province, is an important land use (about 7 percent of this physiographic province). There are several potential agricultural nonpoint-source issues in the lower region of the basin. Agricul- tural activities in the basin can cause increased levels of salinity, sediments, nutrients, pesticides, and selenium and other trace elements in receiving waters, which can affect the surface- and ground-water quality and aquatic biota. For example, irrigation return flows can increase salinity in the surface and ground waters of the study unit. In addition, partly because of reuse of irrigation water and leaching of naturally occurring trace elements from the soils and underlying shale bedrock, trace elements, such as selenium, are present in the water used for domestic and irrigation purposes. Selenium concentrations are high in the lower valleys of the basin, which also can be a concern for protection of fish and waterfowl. In the semiarid to arid climates of the Colorado Plateau, suspended sediment in the streams, as a result of sediment erosion by wind and water, can be increased by agricultural activities.