The Upper Mississippi River Embayment is a region of about 45,000 square miles in the Mississippi River Valley extending from the vicinity of the 34th parallel northward to the mouth of the Ohio River. It includes parts of Arkansas, Mississippi, Tennessee, Kentucky, and Missouri. In 1940 about 2,700,000 people inhabited the region.Ground water is one of the most valuable natural resources of the region. It is estimated that 95 per cent of all the water used in the Upper Embayment comes from the ground.Structurally the Embayment is a downwarped, downfaulted trough in Paleozoic rocks, in which have been deposited sediments ranging in age from Cretaceous to Recent. Very productive water-bearing formations extend throughout most of the region but the total quantity of water perennially available is unknown, and little information is available on the quality of the water.The formations comprise a natural hydraulic system underlying parts of several States, and a study of the geology and hydrology of the entire region is needed. Such a study is necessary because additional ground-water supplies will be required with the future development of the region. In order to develop the region wisely the quantity and quality of the water perennially available must be known.