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from March 1999 and an algorithm based on iterative finite-difference methods (Hutchinson, 1989) that is implemented in the Environmental Sciences Resource Institute geographic information system software known as ArcInfo. Land-surface elevations were avail- able in digital elevation models prepared by USGS (REF). The depth-to-water map shown in figure 3 was prepared by subtracting the ground-water elevation model from the land-surface elevation model. Water-Table Map The generalized water-table map (fig. 2) indi- cates the direction of ground-water movement, which is perpendicular to the water-table contours. The map indicates that the general direction of ground-water movement in the study area was from west to east. The gradient of the water table, indicated by the spacing of the contours was comparable to that of the river- about 6 feet per mile. The map also indicates that in much of the study area, ground water was moving from the area of the Fort Lyon Canal and the Arkansas River toward the center of the study area; a condition indicating that the aquifer was receiving recharge from both the canal and the river. The gradient from the area of the Fort Lyon Canal to the center of the study area is sometimes steeper than the gradient from the river to the center of the study area, especially in the eastern part of the study area. Estimate of Ground-Water Storage The generalized water-table map (fig. 2) was used with information describing the elevation for the base of the local alluvial aquifer (Nelson and others, 1989), and with an estimate of the specific.yield of the aquifer (Watts and Lunsford, 1992), to estimate the amount of water stored in the aquifer. The difference between the water-table surface and the base of the aquifer represents the volume of the aquifer. The specific yield, estimated as 0.20 in the study area, is the ratio of the content of water that will drain, due to gravitational forces, to the water content of the aquifer from which it is drained. To obtain the estimate of water contained in the aquifer, the aquifer volume was multiplied by the estimated specific yield. These methods indicated an estimated water content of about 40,000 acre-feet; however, the entire estimated water content of the aquifer would be available to wells only under ideal conditions. Further, under similar ideal conditions, only the water contained in the aquifer that is above the elevation of the Arkansas River would be able to provide ground-water contributions to the river. Depth to Water The generalized depth-to-water map (fig. 3) indicates that water levels ranged from less than 5 feet below the land surface to about 50 feet below the land surface. In general, the range of depths to water was greatest in the eastern one-half of the study area. The digital elevation model for depth to water indicates that water levels were within 5 feet of the land surface in about 31 percent of the study area (table 2), mostly in a zone that parallels the rive:~ (fig. 3). Water levels were relatively shallow, from 5 to 10 feet below the land surface, in about 20 percmt of the study area. The area of relatively shallow W2.ter levels is prominent in the western one-half of the study area where the area is relatively wide compared to the eastern one-half of the study area where it generally is a thin strip; however, there is an area in the eastern part of the study area, mostly in section 21 of township 23 south, range 54 west, where relatively shallow depths to water are more extensive. Maps made at different times have different locations for the Arkansas River. These differing loca- tions indicate the shifting nature of the river and also indicate that the river channel location, on a decadal scale, is variable. HISTORICAL WATER LEVELS The USGS has monitored ground-water levels in the study area for several decades. Water-level records are maintained in a system known as the Ground-Water Site Inventory (GWSI) data base. As part of the study, the GWSI data base was accessed to obtain water-level records from wells in and aroLlnd the study area; information from 224 wells was obtained. A subset of these wells consisting of 14 I wells that wells that have been measured more than one time, is listed in table 3. A brief summary of the information available for each well also is incl uded in table 3. HISTORICAL WATER LEVELS 7