METHODOLOGY Use of RAINFLO, a hydrologic rainfall-runoff model developed by the author to emulate the U.S. Army Corps of Engineers HEC-1 standard. Determination of the topology of the drainage network. Conversion of the 1:50,000-scale maps of the Mexican cartography (INEGI) to the 1:24,000 scale of United States cartography (USGS). Conversion of the data to the metric system. Compilation of California precipitation data for return periods from 2 to 100 yr. Extension of the precipitation data to return periods of 200, 500 and 1000 yr. Selection of the SCS storm type I, applicable to Southern California. Calculation of the maximum mean monthly levels in Morena and Barrett reservoirs. Collection of cartographic data for soil type and land use in California. Evaluation, using stereoscopic pairs, photographic mosaics and field visits, of the soil type and land use conditions in the Mexican portion of the basin. Evaluation of the average land slope and average stream slope in the drainage network of Cottonwood Creek - Arroyo Alamar. Evaluation of typical cross-sections. Model runs for the 2-, 5-, 10-, 25-, 50-, 100-, 200-, 500-, and 1000-yr frequencies, using the maximum average monthly level in Morena and Barrett reservoirs. Model runs for the 2-, 5-, 10-, 25-, 50-, 100-, 200-, 500-, and 1000-yr frequencies, assuming zero infiltration in the project channelization reach. This is necessary to evaluate the impact of a postulated concrete channel on the aquifer recharge.