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0755 " intensive gain-loss investigations for a 473-ft3/s release made August 21-24, 1972, for the reach between the Granite and Nepesta gaging sta- tions. The results of these two gain-loss investigations are summarized in figure 5, Because the flow of the Arkansas River has been progressively ad- justed for all inflows and outflows, figure 5 represents the cumulative total effects with distance of the ground-water contribution to river flow. For example, figure 5 shows that river flow in the reach between the "near Wellsville" (07093700) and "at Parkdale" (07094500) gaging stations is consistently supplemented by ground water, but at a rate less than the reach between the "at Buena Vista" (07087200) and "near Nathrop" (07091200) gaging stations. Similarly, the Arkansas River between the "at Salida" (07091500) and "near Wellsville" (07093700) gaging stations loses water to the ground-water system. Bank storage was obtained by subtracting the channel storage from the total effects as shown in table 3. The difficulty with this method is that large errors are possible due to the sensitivity of the stage-discharge relationships at the gaging stations, and any errors are accumulated through the reach. For these reasons, the bank storage can be determined only for particular points in time when actual flow measurements were made. Data derived by extrapolation are subject to considerable error. . Observation well studies During January 1972, 16 observation wells were installed at six sites along the Arkansas River in the vicinity of Salida, Colo, At each site two or three observation wells were placed at varying distances perpendicular to the river. The wells closest to the river were about 10 feet from the bank and consisted of 4-inch pipes with a 5-foot slotted section and instrumented with a float-driven digital recorder. The wells farthest from the river were about 120 feet from the river bank and, along with other intermediate wells, were of l\-inch galvanized pipe with sand point and not instrumented, Each site also had a staff gage in the river directly adjacent to the 4-inch well which established the datum to which the wells were leveled, . Response in all wells and in the river was repeatedly measured for several days before and after the 445-ft3/s release of Arpil 6, 1972, passed the sites, For each site, the observation well hydrographs were used to determine the response of each well at selected times since the first release impulse reached the site. Using the responses, the rate of bank storage for the selected times and, hence, the average bank storage rate for the interval between the selected times, could be deter- mined. The average response of all sites and an estimated storage coef- ficient, S, of 0.15 were used in the calculations. The average bank storage rates were express in cubic feet per second per mile for both sides of the river. \3