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and Canadian Rivers in the Albuquerque District. Maximum floods ex- perienced in the lower Pecos River are the result of decadent hurricane activity which seldom extends more than a few hundred miles inland. Furthermore, elevations in Colorado average much hi~her and thus there is less precipitable water in the airmasses over the upper Arkansas River subbasin. 6-04. Las Animas is located near the confluence of the Arkansas and Purgatoire Rivers. Therefore the degree of protection from flood- inR induced by the Purgatoire River should be comparable to that af- forded against Arkansas River flaws. The recurrence interval of a discharRe of 140,000 c.f.s. on the Arkansas River at Las Animas with Pueblo Dam operating would be 143 years. For a comparable recurrence interval the flow on the Purgatoire River at Las Animas with Trinidad Dam operating would be 102,000 c.f.s. 6-05. During project document studies, use was made of the envelope curve of maximum experienced peak discharges for the Albu- querque District in use at that time. It was found that the project desi~ flood of 140,000 c.f.s. for the Arkansas River at Las Animas was 52 percent of the envelope curve. The same percentage value of the curve for the uncontrolled 2,813 square mile area of the Purgatoire River near Las Animas resulted in a flood of 97,000 c.f,s. - 6-06. Although the areal extent of the 1921 storm transposition used to determine the standard project flood for the Arkansas River at Las Animas was not large enough to result in significant runoff from the Purgatoire River, it is reasonable that with the discharge of the project design flood there would be some coincident flow in the Purgatoire River. Conversely, it is likely that during the project design flow on the Purgatoire River there would be coincident flow on the Arkansas River. Therefore, for purposes of determining water sur- face profiles on the respective streams during project design flows, it was assumed that a flow of 20,000 c.f.s. in the Purgatoire River would coincide with the project design flow of 140,000 c.f.s. in the Arkansas River, and that a flow of 8,000 c.f.s. in the Arkansas River would concide with a flow of 102,000 c.f.s. in the Purgatoire River. VII - WATER SURFACE PROFILES 7-01. Arkansas River f1.oodLJay.- The water surface profile for the design discharge was determined by backwater computations using "n" values of 0.020 for channel flows and "n" values ranging from 0.035 to 0.060 for overbank flows. Channel velocities for the design flow of 140,000 c.f.s. would range from B,to 26 feet per second. Veloci- ties in the overbank would vary from 2 to 14 feet per second. Levee heiRhts were determined by routing a flow of 246,000 c.f.s. through the project reach. This discharge was determined to be the capacity of the critical section with flow three feet above the design flood 25