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. .-.- CHAPTER 1 INTRODUCTION The Arkansas River Basin in southern Colorado originates on the eastern face of the Rocky Mountains near Leadville and follows a southeast and eastward course for a distance of about 360 river miles before it crosses the State boundary into Kansas. John Martin Dam spans the river about 58 miles upstream from the State boundary. The subbasin above the dam comprises an area of 18,915 square miles. This subbasin area has a width of about 25 miles in the headwaters region to a maximum of about 140 miles in the vicinity of Pueblo; then narrowing to a maximum of about 120 miles in the vicinity of the reservoir. Drainage area data for the subbasin area upstream of John Martin Dam are given in Table I-I. The headwaters area of the Arkansas River is one of the highest regions in North America. There are 25 peaks over 14,000 feet; including Mt. Elbert at elevation 14,431 feet, the highest point in Colorado. The river drops rapidly from its source and emerges from the mountainous area through the Royal Gorge. Below the canyon, the valley gradually grows wider through the foothills merging into the Great Plains in the vicinity of Pueblo. The decreasing stream gradient and progressive widening of the valley permits large scale irrigated agricultural development in the fertile Great Plains region of southeastern Colorado. . The principle tributaries above Pueblo in downstream order are the South Arkansas River, Grape Creek, Oil Creek, Hardscrabble Creek, and Beaver Creek. These tributaries are typical snow and spring-fed mountain streams which cascade through deep canyons and contribute much of the main stem flow. Large magnitude floods are relatively rare. Above Canon City the river descends over 6,000 feet in a distance ofless than 130 miles. Stream gradients often exceed 50 feet per mile. Between Canon City and Pueblo, the valley is generally narrow and the streambed is well defined. The channel capacity in this reach is approximately 3,000 to 5,000 c.f.s. (cubic feet per second) and a gradient ranging from 10 to 23 feet per mile. Below Pueblo the river valley widens and varies in width from one to three miles throughout eastern Colorado. Population centers along the Arkansas River, in downstream order, are Avondale, Boone, Fowler, Manzanola, Rocky Ford, Swink, La Junta, and Las Animas. The gradient below Pueblo fluctuates between five and nine feet per mile. The channel capacity downstream of Pueblo gradually increases reaching the maximum of 10,200 c.f.s. at La Junta and then diminishing to 6,500 c.f.s. at Las Animas just upstream of John Martin Dam. . The principal tributaries from Pueblo to John Martin Dam are Fountain Creek, Huerfano River, Apishapa River, and Purgatoire River. Fountain Creek originates on the slopes of Pikes Peak and flows generally southward to join the Arkansas River from the north at Pueblo. The Huerfano, Apishapa, and Purgatoire Rivers originate in the Sangre de Cristo Range along the southwest rim of the subbasin and flow generally northeast across the high plains to enter the Post Flood Assessment Report I Chapter i-introduction Draft Revised 09/09199