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<br />1920 <br /> <br />ielations of Discharge and Specific-Conductance <br /> <br />Trends to Reservoir Operations in the Lower <br /> <br />Arkansas River Basin, Southeastern Colorado <br /> <br />By Michael E. Lewis and Daniel L. Brendle <br /> <br />\ . R'-( RcFOi.;' <br />?\;-SUM\NA E.V\S\C~ <br />SUBJE.CT TO R <br /> <br />Abstract <br /> <br />To provide for the berter management of streamflow in the lower Arkansas River. two mainstem reservoirs <br /> <br />were constructed between 1948 and 1975. John Martin Reservoir. constructed near Las Animas in 1948, and Pueblo <br /> <br />Reservoir, constructed near Pueblo in 1975, provide for flood control, irrigation and municipal water-supply man- <br /> <br />.ement, and recreation. Both reservoirs bave the potential to alter specific conductance in the Arkansas River by <br /> <br />streamflow manipulation. <br /> <br />Step-trend analysis of discharge and specific-conductance data for the Arkansas River was used for determin- <br /> <br />ing if the operation of either Pueblo Reservoir or John Martin Reservoir bad affected discharge or specific conduc- <br /> <br />tance in the lower Arkansas River. The analysis of discharge trends was conducted because discharge and specific <br /> <br />conductance are typically strongly correlated, therefore, cbanges or trends in specific conductance can often be <br /> <br />explained in terms of the associated change or trend in discharge. Oat" collected at five gaging stations on tbe lower <br /> <br />Arkansas River and at one station on tbe upper Arkansas River were analyzed for trends. The upper basin station <br /> <br />was included in the analysis to belp differentiate between trends in the lower basin that were caused by differences <br /> <br />in the quantity or quality of inflow from the upper basin or by reservoir operations witbin tbe lower basin. The period <br /> <br />of record varied between stations, therefore. trend-test results for different stations were not directly compared. Data <br /> <br />from the upper basin station and ftom the three stations located between Pueblo Reservoir and John Marrin Reser- <br /> <br />.r were analyzed for trends that may have occurred after 1974, which corresponds to the construction of Pueblo <br /> <br />, <br />