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0755 <br />intensive gain -loss investigations for a 473 -ft /s release made August <br />21 -24, 1972, for the reach between the Granite and Nepesta gaging sta- <br />tions. The results of these two gain -loss investigations are summarized <br />in figure 5. <br />Because the flow of the Arkansas River has been progressively ad- <br />justed for all inflows and outflows, figure 5 represents the cumulative <br />total effects with distance of the ground -water contribution to river <br />flow. For example, figure 5 shows that river flow in the reach between <br />the "near Wellsville" (07093700) and "at Parkdale" (07094500) gaging <br />stations is consistently supplemented by ground water, but at a rate <br />less than the reach between the "at Buena Vista" (07087200) and "near <br />Nathrop" (07091200) gaging stations. Similarly, the Arkansas River <br />between the "at Salida" (07091500) and "near Wellsville" (07093700) <br />gaging stations loses water to the ground -water system. Bank storage <br />was obtained by subtracting the channel storage from the total effects <br />as shown in table 3. <br />The difficulty with this method is that large errors are possible <br />due to the sensitivity of the stage- discharge relationships at the <br />gaging stations, and any errors are accumulated through the reach. For <br />these reasons, the bank storage can be determined only for particular <br />points in time when actual flow measurements were made. Data derived <br />by extrapolation are subject to considerable error. <br />a Observation well studies <br />During January 1972, 16 observation wells were installed at six <br />sites along the Arkansas River in the vicinity of Salida, Colo. At each <br />site two or three observation wells were placed at varying distances <br />perpendicular to the river. The wells closest to the river were about <br />10 feet from the bank and consisted of 4 -inch pipes with a 5 -foot slotted <br />section and instrumented with a float- driven digital recorder. The wells <br />farthest from the river were about 120 feet from the river bank and, <br />along with other intermediate wells, were of lk -inch galvanized pipe <br />with sand point and not instrumented. Each site also had a staff gage <br />in the river directly adjacent to the 4 -inch well which established the <br />datum to which the wells were leveled. <br />Response in all wells and in the river was repeatedly measured for <br /> t several days before and after the 445 -ft /s release of Arpil 6, 1972, <br />passed the sites. For each site, the observation well hydrographs were <br />used to determine the response of each well at selected times since the <br />first release impulse reached the site. Using the responses, the rate <br />of bank storage for the selected times and, hence, the average bank <br />storage rate for the interval between the selected times, could be deter- <br />mined. The average response of all sites and an estimated storage coef- <br />ficient, S, of 0.15 were used in the calculations. The average bank <br />storage rates were express in cubic feet per second per mile for both <br />sides of the river. <br />13 <br />