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I I I I I I I I I I I I I I I I I I I ARKANSAS RIVER FROM OTEROIPUEBLO COUNTY LINE TO JOHN MARTIN RESERVOIR CHANNEL CAPACITY AND RIPARIAN HABITAT PLANNING STUDY 3. HYDROLOGIC AND ECOLOGIC INVESTIGATIONS 3.1 GEOMORPHOLOGICAL ASSESSMENT Watershed Characteristics. The Arkansas River originates in the central Rocky Mountains near Leadville, Colorado, and drains about 18,000 square miles to the John Martin Dam and Reservoir in Colorado. There are at least 25 mountain peaks in the upper watershed that exceed 14,000 feet in elevation. The Arkansas River is a torrential mountain stream until it emerges from central Colorado's Royal Gorge Canyon. Downstream from the canyon, at about elevation 5,300 feet, the valley gradually widens and descends through the foothills to reach elevation 4,700 feet at Pueblo, Colorado. From Pueblo to John Martin Dam and Reservoir the river flows across the prairie of eastern Colorado through a broad well-developed irrigated valley. In addition to discharge regulation at Pueblo Dam, natural stream flows are affected by trans-mountain diversions, storage reservoirs, irrigation diversion and return flows, groundwater extraction, and power developments along the river. Historic Changes in the Streambed. Geomorphological changes on the Arkansas River between 1870 and 1977 were reported by Nader (1978). He identified changes in three distinctive reaches using historical mapping through 1252 and measurements in 1977. ..The first reach was upstream from John Martin Reservoir between La Junta and Las Animas. He compared bankfull widths at specific cross sections over time and calculated averages for the reach. The average bankfull width increased from about 575 feet to 700 feet between 1870 and 1892. There were considerable deviations from the average at individual cross sections, and widening did not occur at every section. The average width remained constant between 1892 and 1926 although changes occurred at individual cross sections. Between 1926 and 1952 the average width decreased significantly to about 150 feet. No clear trend was observed between 1952 and 1977. In addition to channel width changes, Nadler documented planform changes. The river changed from braided to meandering. Nader attributed changes in the river to the following: a. Irrigation Improvements. This includes smoothing of the annual hydrograph (reduction in peaks) due to storage and release of irrigation water, and reduction in sediment load due to abstraction into irrigation canals. Irrigation improvements caused the river to become a perennial steam instead of an intermittent stream. Annual flow was also increased due to diversions from other watersheds. 23