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" Ii 8 ,t . . diversions from t~e tolorado RIver without increasing consumptive use was still pending during 1970. The amendment was requested in 1968 by Secretary of Interior Udall, but California officially requested additional in- formation and analyses from the United States to support the necessity of increased diversions and to substantiate the claim that diversions could be increased without in- creasing consumptive use. In February 1970, the Department of Inte- rior indicated by letter that continuing water measurements and ecology studies were be- ing conducted at Topock Marsh by the Bu- reau of Sport Fisheries and Wildlife for its water management plan. The final inlet and outlet flow records at Topock Marsh for calendar years 1967, 1968, and 1969 were transmitted to the Board in July, 1970 by the Bureau of Reclamation. The records will be included in the annual compi- lation of records in accordance with Article V (B) of the decree. The U .S, Geological Sur- vey has rated the accuracy of the data as "poor" according to its standard for surface water flow measurements. This rating means that about 95 percent of the daily discharge measurements are not within 15 percent. Lower Colorado River Return Flow Study Task Force In the course of establishing Lower Colo- rado River diversions and return flows dur- ing the studies for development of the Ope- rating Criteria for Colorado River Reservoirs in 1969, the Bureau of Reclamation recog- nized the position of Arizona and California that there were substantial quantities of un- measured subsurface return flows and agreed to look into the problem. In 1970, the Bureau and the U.S. Geological Survey developed a program for measuring these return flows. A proposed study was presented on July 10, 1970, in Boulder City, Nevada, at a meet- ing of federal and state representatives. A Task Force on Ground-Water Return Flows to the Lower Colorado River was formed, consisting of state representatives from Ari- 42 zona, California, and Nevada, and of federal representatives from the Bureau of Reclama- tion, Geological Survey, Bureau of Indian Affairs, and Regional Solicitor's Office. The Task Force is to coordinate the work and advise the Geological Survey and Bureau of Reclamation on the study. The Principal Hy- draulic Engineer was appointed as Cali- fornia's representative on the Task Force. The study will include all sections of the river where subsurface inflow from prior di- versions is expected to occur. The first reach of the river to be studied will be between Laguna Dam and Morelos Dam. Details of this study area were discussed with the Task Force at meetings with the Bureau of Recla- mation and Geological Survey representa- tives. The gradients of the shallow, interme- diate, and deep ground water bodies will be measured and permeabilities will be comput- ed from tests using clusters of piezometers located at approximately 18 cross-sectional locations along the river. The return flow to the river will be calculated from these data. After physical measuring devices are in- stalled in the Yuma area, it is planned to be- gin work on the river reaches upstream. One matter not yet resolved in the conduct and analysis of the underflow study is whether the return flows should be meas- ured at the river's edge or at the edge of the irrigated areas. The latter method would give the states greater credit for return flows since phreatophyte areas generally consume additional quantities of water before the re- turn flows finally reach the river. It was fi- nally agreed that the main thrust of the pro- gram should be to measure return flows at the river's edge since the decree in Arizona v. California defines the returns as being to the river. An evaluation will also be made of the amount of return flows intercepted by phreatophyte areas. By the end of 1970, the installation of the piezometer clusters in the Yuma area was underway. \