AZ vs. CA:24 California Exhibits, Vol. XXIV; Miscellaneous

page 147

PROBLEMS OF IMPERIAL VALLEY AND VICINITY- 147 topography is too rugged or the concentration of water supply insufficient for irrigation development except in small isolated tracts usually at high altitudes. Elevations range from 13,000 in the San Juan mountains to 4,000 at the mouth of the Dolores River, with the plateau region generally from 6,000 to 8,000. Precipitation ranges from 50 inches or more in the highest mountains to less than 10 inches at the lower end. In the mesa,region, where irrigation development is most feasible, the rainfall is 13 to 20 inches, of which roughly one-half comes in the growing season. The mean annual temperature is 40 to 48 degrees. The growing season is comparatively long, considering mean temperatures, being frost free from 110 to 140 days. Under these conditions, an average diversion of 2 acre-feet per acre will probably prove sufficient. Practically all of the region has a " chocolate" loam soil of 5 to 30 foot depth, underlain with shale and sandstone. Natural drainage will prevent water-logging over most of the areas. A consumptive use of 1.25 acre7feet per acre of diverted water would seem ample allowance under these conditions. The San Miguel River by reason of its high drainage area has a well-sustained summer flow, the Dolores falling off in June for lack of similar high drainage area. The combined run-off of these streams averages 730,000 acre-feet annually. The data here given are based on information obtained from United States Geological Survey Water Supply Papers, and particularly No. 395 (E. C. La Rue), from reports by the State engineer of Colorado, from unpublished reports to the Reclamation Service by C. B. Smith and H. F. Burkhart, from information furnished by private irrigation companies, and from general information. POWER DEVELOPMENT. On the headwaters of the San Miguel River near Placerville, three small high head plants have been constructed with an aggregate installation of approximately 5,000 kilowatts. While definite data are lacking, it is believed that further small developments of the same type as now installed may be feasible on the San Miguel and to a lesser degree on the Dolores watershed. The utmost development by this means would in any event be relatively small and insufficient for transmission to outside markets. On the main rivers, the relatively small flow available during most of the year combined with lack of concentrated fall preclude cheap development. Storage for equating the San Miguel run-off is not available. On the Dolores River this may be done at the Dolores and Bedrock reservoir sites, The former is the only means of utilizing Dolores water for irrigation in Colorado with diversion 173 feet above low water in a 230-foot reservoir. Under these conditions, a power development without loss of water for irrigation purposes would be impractical. At the Bedrock site there would be no interference with irrigation interests but the development of the Dolores, Montezuma, Disappointment Valley, and West Paradox projects would reduce the present limited water supply from 357,000 acre-feet annually to an estimated 115,000 acre-feet. This flow would be reduced to a very small amount in periods of low run-off and storage sufficient to equalize annual flows would entail heavy evaporation Tosses and produce a continuous flow of 100 second-feet at most. On the whole, then, it may be said that