The story of the Imperial Valley begins with the formation, in remote geologic times, of the great shallow depression, or basin, which modern explorers have called the Salton Sink. Tens of thousands of years ago, before the appearance of man on earth, the long arm of the Pacific Ocean which is now known as the Gulf of California extended in a northwesterly direction to a point more than a hundred miles distant from its present head. Its terminus was then near the San Gorgonio pass, about ninety miles east of the place where Los Angeles now stands, and it extended across the Colorado Desert to the site of the present town of Yuma. If it had not been affected by external forces, it would probably have retained to the present day its ancient boundary line; but into it, on its eastern side, happened to empty one of the mightiest rivers of the Great West--the Colorado--and by this agency the upper part of the Gulf was gradually separated from the lower, and was finally turned into a salt-water lake, equal in extent to the Great Salt Lake in Utah. This detached body of ocean water, which had formerly been the upper part of the Gulf of California, completely filled the basin of the Salton Sink, and had an area of approximately 2100 square miles. “But how,” it may be asked, “could a river, however mighty, cut the Gulf of California in two, so as to separate the upper part from the lower and leave the former isolated?” Easily enough in the long ages of geologic time. A great river like the Colorado does not consist of water only. It holds in suspension and carries down to the sea a great load of sediment, which, when deposited at its mouth, gradually builds up a delta-plain of mud, and often changes topographical conditions over a wide area. It was this deposited sediment that cut the Gulf of California in two. The drainage basin of the Colorado and its tributaries extends from the Gulf of California to the southern edge of the Yellowstone National Park, and has an area of more than 260,000 square miles. Most of this area is mountainous, and the innumerable streams that tear down through its gorges and ravines erode and gather up vast quantities of sediment, which the river carries to the Gulf and finally deposits in its waters. How great a load of silt the Colorado brought down in prehistoric times we have no means of knowing; but it transports past Yuma now about 160,000,000 tons of solid matter every year, or enough to fill a reservoir one mile square to a depth of one hundred and twenty five feet.[1] Century after century, the river poured this vast quantity of silt into the Gulf opposite its mouth, and gradually built up a delta-bar which extended westward, year by year, until it finally reached the opposite coast. The upper part of the Gulf was then separated from the lower by a natural levee, in the shape of a delta-plain, which was perhaps ten miles in width by thirty in length, and which extended from a point near the present site of