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. . . 1776 The Great Plains Province, located to the east ofCailon City, ranges in elevation from 3,500 to 5,000 ft. The mean annual precipitation ranges from 10 to 14 in. Land use is primarily agricultural along the river corridor with the remaining areas comprised mostly of rangeland. Over 411,000 acres in the Arkansas River Basin are irrigated, including about 56,000 irrigated acres in the upper basin (Dash and Ortiz, 1996). The majority of the irrigated areas are located east of Pueblo and are concentrated along the river corridor. Primary crops are wheat, com, sorghum, and alfalfa. Fruit crops and commercial vegetables are also produced. Over 600,000 head of cattle per year have ranged in the Arkansas River Basin (Colorado Department of Agriculture, 1981) with the majority of the cattle and feed lots located in the lower basin from Rocky Ford to Lamar. Sedimentary rock and alluvial fill underlay the region. The two largest population centers are Pueblo and Colorado Springs (plate I). These two cities account for over 70 percent of the 641,720 people in the Arkansas River Basin (U. S. Bureau of the Census, 1990). Pueblo is located on the main stem of the Arkansas River downstream from Pueblo Reservoir and at the confluence with .-----.---. - ..--. - -. .----- ...- Fountain Creek. Colorado Springs is located on Fountain Creek 40 miles upstream from the mouth. . East ofCailon City, the gradient of the streambed decreases to less than 9 feetper"mile as the river flows out of the mountains and onto the eastern plains. Just upstream from Pueblo, the river flows into the 357,678 acre-ft Pueblo Reservoir. The reservoir is a multipwpose facility completed in 1975 as part of the Fryingpan-Arkansas project. Fountain Creek flows into the Arkansas River within the city limirs of Pueblo. Downstream from Fountain Creek, the stream channel changes from a rock and cobble bottom to a shifting sand channel that meanders along the alluvial flood plain. Numerous diversion strUctures between Avondale and La Junta remove water from the river for irrigation purposes. In terms of streamflow contribution to the Arkansas River, some of the larger tribu- taries include Fountain Creek, Timpas Creek. and Purgatorie River (plate 1). The Purgatorie River flows into the Arkansas River just upstream from the 615,500 acre-ft John Martin Reservoir which was completed in 1943 by the U. S. Army Corps of Engineers as a multipurpose project. 6 - ~. . ':'1 t:.~\. ~~~ ~ ~~~~..." . b;~~;:~~~~~~~:.f~~~.;~.;t.:;~ ~ ~:.:.5;.;.~ .... . _".'!;:o...;d ~ OJ..a:;rl-':"'O.......- ..,- C;:Jbi~~ 10 K3Visicn ... .--' -~ 00 R'=I = ..~~ r)t; ~"OT QUOit:: I;' 'I;;_",J.~_ .::.- ;," 9 Ap....r.vol by Dire-ctor, i:.,:::nr.nn.... :-" 'J ... . U.S. Gso!c.gica\ Sur"l'2;'