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the questions of how and where the silt would be deposited were largely forgotten, although some short-lived exc~tement was created in 1935 when mudqy water was suddenly discharged through the outlet pipes. At about the same time an investigator at California Institute of Technology was experimenting with a stream of colored salt water, slightly heavier than ordinary water, introduced ihto clear water of ordinary weight. He found that under certain circumstances, the stream would travel long dis- tances through clear water without greatly losing identity. To study these phenomena further, the National Research Council in 1937 established a Committee on Density Currents. The committee realized that the muddy discharge at Hoover Dam, in 1935 might have indicated that a current of muddy water had traveled the entire length of Lake Mead. Similar phenomena had been observed at Elephant Butte Dam. Thus, there was evidence of density currents not only in the laboratory but ihreservoirs. A subcommittee of the Density Current Committee, formed to trace possible density currents in Lake Mead, reported in 1941 that at certain times of year inflow followed the old river bed along the bottom of the reservoir, without greatly intermixing with the standing water, and finally became di~sipated at the dam. It also dis- closed that substantial volumes of silt ha,d accumulated immediately upstream from the dam. Since the lake is perfectly Clear, this silt could have been transported the long distance from the head of the reservoir only by means of density currents. ' The volume of silt deposited near, the dam was only a small portion of the estimated volume of the total silt inflow. Where had the rest 'of the silt been deposited? Only a thorough sedi~entation survey could furnish the answer.- Thanks to the wholehearted cooperation of the Navy, Geological Survey, Coast and Geodetic Survey, and Bureau of Reclamation, this unprece- dented survey was completed in 1950. From the time the reservoir began; to fill until the survey was done, approximately 2,004,000,000 tons of silt entered the reservoir. It occupies a space of 1,420,000 acre-feet, corresponding roughly to an average unit weight of 64.8 pounds per cubic foot., The greatest depth of silt was at the upper reaches of the reservoir, far'above the silt pocket provided when the dam was designed, and thus largely within the useful operating volume of the lake. The maximum depth was;270 feet. Proceeding toward the dam, the depth of silt decreased, but deposits 45 to 160 feet deep were found along the original river channel almost the entire distance'to the dam. At the dam the deposit had a thickness of 106 feet. Smaller accumulations were in the Virgin Basin and at the mouth of Virgin River. Only 42 percent of the total silt volume was within the desigqated silt pocket below the lowest outlet gates. On the basis of data 'gathered in the survey, and taking into account the increasing compaction as additional layers are added, it is estimated that with no additional storage upstream, it would be almost 500 years before all storage capacity of Lake Mead were lost. However, it may be assumed that other storage will be constructed upstream long before the operating volume of Lake Mead is reduced, to any important extent. Related phenomena investigated during the sedimentation survey included the salinity of the lake, evaporation from the surface, and the general subsidence of the area covered by the enormous weight of water and -36-