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This paper gives an outline of the effects on the regimen of the river only in part foreseen, which were produced by the construction of Hoover Dam on the great sediment‐carrying Colorado. The effects fall in two categories: (1) The many phenomena that occur in an arid region when a great lake, more than a hundred miles long and in places more than 600 feet deep, storing almost two and one‐half average years' yield of the watershed, is created within a few years. The phenomena have to do with the distribution within the volume of the reservoir of the great incoming sediment load, with the travel of the incoming water through the stored volume, of the stratification of the lake due to temperature and salinity, and, finally, to the seismic effect of the great accumulated weight. (2) The phenomena occurring on the river above, and particularly below, the dam when the river bed adjusted to a sediment‐laden stream is suddenly forced to accommodate a flow of clear water, and when the yearly fluctuations of flow from flood to drouth are changed to an almost steady discharge. The sedimentation survey of Lake Mead, formed by Hoover Dam, undertaken in 1947–1950, yielded, for the first time, precise data on the travel and deposition of sediment, on the circulation of the water within the lake, and on the general subsidence of the Earth's crust caused by the weight of the water. The paper gives an outline of the main results of the survey pending the publication of a complete report. The paper also gives an outline of the 20 years' accumulation of data on the regimen of the Colorado River below Hoover Dam the changes which have taken place because of the construction of the dam, and the physical work which has been executed and is still in progress so as to help the river to adjust itself to the new conditions.