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.. . .... INTRODUCTION From Pueblo Dam to John Martin Dam, a distance of 142 river miles, the sandy channel of the Arkansas River meanders across a broad flood plain. Due to diversions for irrigation, the mean annual flow in this reach ranges from about 536,000 acre-ft at Pueblo Dam to about 182,000 acre-ft at John Martin Reservoir. More than 80 percent of the annual flow occurs during the prin- cipal irrigation season, April through September. Throughout the study reach, the Arkansas River traverses an alluvial aquifer. This aquifer ranges frpm 0.3 mi to about 6 mi wide, is as much as .300 ft thick, and consists of gravel, sand, silt, and clay. Water pumped from more than 1,000 wells tapping the aquifer is used primarily for irriga- tion to supplement surface-water supplies and a seasonal precipitation of about 8 in. Irrigation water is also obtained from 12 major canals that from the Arkansas River between Pueblo Dam and John Martin Dam. divert an average of 650,000 acre-ft annually. They are according to the prior-appropriation doctrine by the Office of State Engineer. - divert water These canals administered the Colorado Purpose and Scope Colorado water law allows water users to transport their water in natural river channels from upstream storage res~rvoirs to a downstream delivery point provided allowances are made for transit losses (Radosevich and Hamburg, 1971). A transit loss of 0.07 percent of the reservoir release per mile of river length was established by court action for transmountain water del ivered along the Arkansas River in Colorado (Sunnyside Park Ditch vs. M. S. Hinderlider, State Engineer; Court Case No. 3345; 1944-~ The transit-loss for all water deliveries along the Arkansas River in Colorado is determined by the Division Engineer, Office of the Colorado State Engineer, who generally uses 0.07 percent per mile except during unusually low river conditions. The construction of Pueblo Reservoir near Pueblo (total storage capacity, 357,000 acre-ft) as part of the U.S. Bureau of Reclamations's Fryingpan-Arkansas Project together with the associated increase in trans- mountain diversions of water into the Arkansas River basin and proposed pro- gram of winter-water storage have increased the need for more definitive in- formation with regard to transit losses and traveltimes of reservoir releases along the lower Arkansas River in Colorado. In July 1974, the U.S. Geological Survey, in cooperation with the South- eastern Colorado Water Conservancy District, began a study to determine the transit losses and traveltimes associated with deliveries of reservoir water along the Arkansas River from Pueblo Reservoir near Pueblo to John Martin Reservoir near Las Animas, Colo. (fig. 1). The purpose of this report is to describe the results of that study. 2 ." "'-- ,~. ,". \~-:'~ ,~'"__v_ _'." ,-.-. "__"__"'~;_"'_;;":"-;~::~~::;:':::.':":::'.'~~:-:;-;'.~~::':"V