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7 . 13. The principal tributaries of the Arkansas River above Las Animas are Fountain Creek, which rises on the slopes of Pikes Peak and flows generally southward to join the Arkansas River at Pueblo, Horse Creek which enters from the north a fewmi les upstream from l.as Animas, and the Huerfano and Apishapa Rivers which enter from the south. The largest tributary above John Martin Dam is the Purgatoire River which enters the main stem at the head of the reservoir, about three mi les downstream from Las Animas. 14. The Purgatoire River rises in the Sangre de Cristo Range west of Trinidad, where several peaks exceed 13,000 feet and one exceeds 14,000 feet, and flows eastward for a distance of about 30 mi les before it reaches Trinidad. The mountainous area above Trinidad comprises about 22 percent of the watershed. From Trinidad the river flows In a generally northeast direction through an alluvial valley about 35 mi les long, and then becomes antrenched in a rugged canyon for a distance of 105 miles. Below the canyon the river flows through a second al iuvlal valley for 17 mi les to its confluence with the Arkansas River. 15. Las Animas is the county seat of Bent County. The Arkansas River passes through Bent County from west to east with about ohe- third of the county on the north side and two-thirds on the south. The area north of the Arkansas River Val ley is featured by rol ling plains and benches with outcrops of shale and I imestone along most of the streams. Rol I ing hi lis and benches characterize about one-half of the area south of the valley; however, the southern third of the county Is broken with bluffs, canyons, and sandstone mesas. Most of the soi I is derived from the shale and limestone of the Benton and Niobrara Groups. The valley section consists of valley fi II and alluvial fans composed of si Ity sand and clays. There is a light clay loam topsol I and a heavy clay loam subsoi I to a depth of 5 feet or more. Outside of the irrigated areas, vegetation Is sparse consisting generally of cedar, juniper, cacti, and grass. Vegetal cover in the project area is most I y tamar i sk, cottonwoods, willows, and grass. 16. STREAM CHARACTERISTICS.- The Arkansas River above Canon City is a typical mountain torrent which descends over 6,000 feet in eleva- tion in a distance of less than 130 mi les, at gradients often exceeding 50 feet per mi Ie. From Canon City, elevation 5;300 feet, the river descends on a diminishing gradient to about 10 feet per lIJile at Pueblo, elevation 4,668 feet. From Pueblo to Las Animas the stream slope decreases to 8 feet per mi Ie and the channel capacity varies from 5,000 to 10,000 c.f.s.(I) The present channel capacity in the vicinity of Las Animas is 6,500 c.f.s. '17. Continuousrecords of the Arkansas River f lows at Las Animas began in May 1939 when the stream gage was instal led on the left bank . (I) Cubic feet per second.