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NORMAN E. PEHRSON , steep slopes. The total fall for each of the Los Angeles and San Gabriel Rivers in its short length is greater than the total fall of the Mississippi River as it traverses the nation. (The total fall of the Los Angeles River is 770 feet in 50 miles; of the San Gabriel River, 775 feet in 33 miles; and of the Miss- issippi River, 725 feet in about 2,400 miles.) The simple solution of carrying damaging floodwaters to the sea with the utmost dispatch would have meant disaster from the standpoint of water needs. Therefore, it was essential that the area be protected against damaging floods and, at the same time, the ground water supply be protected and improved, if possible. Hence, each step in the planning and development of the LACDA project was a carefully thought-out procedure closely coordinated with local water agencies. Our work on LACDA began with improvement of the Los Angeles River, an important source of water for replenishing the underground basin, upon which settlers depended more than 200 years ago when California was still part of the Spanish empire. When we started planning for the Los Ang- eles River channel, the City of Los Angeles expressed its concern on the effect that the channel would have on the underground basin near Griffith Park. To replenish this sour_ce of water supply with some of the floodwaters to be collected within the flood control system, we constructed a 6-mile reach of the channel near Griffith Park with concrete side slopes, an invert paved with cobblestones and a div- ersion structure at the upstream end for off-channel spreading. (You will note that the recharge areas are shown in blue on this and subsequent slides). To regulate floods on the river and its principal tri- butaries, we built three reservoirs (Hansen, Sepulveda and Lopez) in addition to a reservoir (Pacoima) constructed by local interests. We built debris basins on tributary streams to control flood-borne de- bris issuing from the canyon mouths. As our work progressed on LACDA, protection of the recharging capabilities of the under- ground basin became increasingly important. The San .Gabriel River channel is an interesting example of diversity in improvements to protect the underground water supply and at the same time provide protection against floods. From the mouth of San Gabriel Canyon downstream to Santa Fe Reservoir, we constructed an earth bottom channel with revetted levees on each side of the channel. Because Santa Fe Reservoir is on an alluvial cone and no cutoff was constructed under the dam, the entire res- ervoir is actually a recharge area. From Santa Fe Reservoir downstream to Whittier Narrows Reser- voir, we again provided an earth bottom channel with revetted levees along the San Gabriel River. In this reach, we constructed 15 concrete drop structures at appropriate intervals to maintain nonscour- ing velocities. Each drop structure effects a 10-foot lowering of the invert grade and a commensurate loss of energy. Downstream from Whittier Narrows Reservoir, an earth bottom channel is being con- structed along that reach of the San Gabriel River that supplies the Central Basin aquifers. The rest of the channel downstream from the aquifers will be concrete lined except near the ocean. During the water year September 1966-67, we operated the Federal reservoirs in the LACDA project in conjunction with the Los Angeles County Flood Control District reservoirs in such manner as to conserve about 397,000 acre-feet of water, comprising 262,000 acre-feet of local runoff and 135,000 acre-feet of water imported from the Colorado River. On the basis of 150 gallons per day per capita, the water salvaged served about 2,400,000 people. Additional water was salvaged by waste- water reclamation, which I will discuss later. Another important California project in the Los Angeles District is the Santa Ana River Basin and Orange County project, which comprises five reservoirs and an elaborate system of levee and chan- nel improvements. During the water year 1966-67, we operated the reservoirs in the project so as to conserve about 200,000 acre-feet of water, comprising about 96,000 acre-feet of local runoff and 104,000 acre-feet of imported Colorado River water. Again, on the basis of 150 gallons per day per capita, the water salvaged served about 1,200,000 people. Further, the local flood control and water conservation districts in the area salvaged additional water by diverting flows in the upstream part of the basin into off-channel spreading grounds that recharge the underground reservoirs. Also, addition- al water was salvaged by waste-water reclamation. , -11-