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NORMAN E. PEHRSON , At this point, I wish to give both Los Angeles County and Orange County an encouraging pat on the back for the fine job they are doing - not only in their development of ground water supplies, but also in the successful uphill battle they are waging to protect those supplies against contamination by ocean water. Years of drought and the demands of a burgeoning population have contributed to a lowering of the water table in these counties. With lowering of the water table, ocean water moving landward has posed a serious threat to the ground water supply. Recognizing the need for immediate remedial measures, both counties have embarked - both singly and jointly - on massive programs for the prevention of sea-water intrusion by constructing hydraulic barriers (wells) along the coast. Some of the Colorado River water imported by Los Angeles and Orange Counties is pumped into injection wells that are part of the hydraulic barrier. The program, which is still continuing, has been highly successful in protecting the ground water supply. Now a word about important studies currently under way for Arizona's Santa Cruz River ba- sin, which comprises an area of 8,000 square miles, mostly in southern Arizona. We believe that our findings in these studies will be of inestimable value to an area periodically menaced by damaging floods and constantly threatened by a diminishing water supply. Within the basin are the cities and towns of Tucson, Nogales, Eloy, and Casa Grande. This rapidly growing area depends on underground reservoirs for its water supply. However, continuous pumping has caused a steady decline in the water table, a decline that averages from 2 to 4 feet per year in the Tucson area and that will continue unless the underground reservoirs are replenished. In developing a plan of improvement for flood control and water conservation in the basin, our basic aim will be to insure that the underground reservoirs in the basin are used as storage reservoirs for floodwaters wherever feasible. Water demand in the Tucson area is expected to increase from 260,000 acre-feet in 1963 to 510,000 acre-feet in 2000; and in the Nogales area, from 1,570 acre-feet in 1965 to 9,670 acre-feet in 2000. Ground water recharge will help satisfy these demands considerably. Before we complete the comprehensive report for the Santa Cruz River basin, we will prepare two interim reports for the Tucson area - one for the RiIlito Creek drainage area and the other for the Santa Cruz River from near the San Xavier Mission to the convluence with Rillito Creek. The need for flood control improvements in the Tucson area is urgent - hence, the need for interim reports. A po- tential disaster area exists at the confluence of Pantano Wash and Tanque Verde Wash, where an ex- pensive residential development has been built along the flood plain. Downstream, on Rillito Creek, more subdividing is under way in the flood plain. To provide badly needed protection against floods in the area and at the same time protect and augment the existing water supply, we are considering channel improvements operating in conjunction with storage dams. The plan of improvement being considered includes about 18 miles of wide earth bottom channel on RiIlito Creek and dams on Pan- tano Wash and at the confluence of Tanque Verde Wash and Sabino Canyon. The dams would release the water into the earth bottom channel, where drop structures placed at intervals would flatten the channel slope and permit maximum percolation of water to the underground reservoirs. The broadbrush sketch that I have just completed points upthe Los Angeles District's ground water recharge activities over vast areas of the Southwest, where the water supply problems grow in complexity each day and where in turn the work of each agency involved in water resources develop- ment becomes increasingly more complex. We expect that our experience will be supplemented by the many research studies now under way by various organizations and agencies to develop new measures and new techniques. Weather modification, reclamation of waste water, desalting, reuse of water, nuclear excavation - these are a few of the areas in which valuable study and experimentation are be- ing made. I am especially intrigued by the studies made at the Lawrence Radiation Laboratory of the University of California at Berkeley on the economics of ground water recharge by nuclear means. The report, prepared by Professor David K. Todd, stressed the need for artificial recharge of ground water, especially in water-deficient areas where surface storage facilities are inadequate and where suitable reservoir sites are lacking. It indicates that nuclear explosives could prove useful in creating , -13-