The Gallery Collecting System of the Des Moines Water Company, Des Moines, Iowa

Abstract

The Des Moines Water Company is one of the pioneers, if not the first, to develop its water supply thoroughly by means of infiltration galleries. A number of cities have water supplies developed from underlying sand and gravel strata by means of wells but few, if any, have adopted the method used here. In order to understand the practical application of this method, it will be necessaryfirst to fix clearly in mind the fundamental features of the waterbearing area, before undertaking to follow through the various stages of its development. The water is obtained primarily from the Raccoon River. This stream lies in a southwesterly direction from Des Moines and discharges into the Des Moines River in the heart of the city. It has a drainage area of 3677 square miles above the city and a minimum flow of sufficient quantity to more than meet the needs of the city for years to come. The most interesting feature of the river, from a water supply standpoint, is the geology of its valley in the vicinity of Des Moines. Here we find a broad valley, nearly 100 feet deep and a mile wide, which has been eroded in the clay, shale and rock of the natural topography and extending for many miles upstream with relatively steep sides, terminating in rolUng plains above. It is evident that this valley was eroded by a preglacial stream which eventually cut its way to a hard bottom and later covered this bottom with clean sand and fine gravel brought down from the glaciers, and then, at a still later period, covered this gravel bed with from 5 to 10 feet of alluvium. As might be expected, this stratum of sand and gravel terminates at the foothills on either side but extends upstream for many miles. Borings have shown this to be true and many made by the Water Company affirm the fact that it is from 10 to 30 feet in thickness, very clean and uniform in size. An analysis of the bor-