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. o - 3 - resulting from the entire integrated program is estimated to range from 57 percent for the le-year storm to 26 percent for t he 50-year storm and 3 percent for the so-called maximum storm. Apparently the reductions accomplished by the small detention dams are computed on the storage space behind these dams, which is estimated to average 20 acre-feet for each 640 acres of land in the plains portion of the watershed. A large number of such small detention dams were built by the United States Department of Agriculture in many areas throughout the United States during the latter part of the 1930' s. Surveys by the United States Geological SurveYl\Ild other i"gencies to determine the effectiveness of these structures at the present time has brought out the following information: 1. Small detention dams require careful and continuous maintenance if they are to retain their initial effectiveness. 2. If they are not b,ult to store or pass flood ,volumes resulting from the great storms of rather infrequent occurrence, a numbGr of the dams may be expected to fail each year in an area such as the Fountain River Watershed due to localized centers of high precipitation. 3. The detention dams are usually located on small tributaries and gullies which transport large amounts of sediment. Storage capacity must be provided behind the dams to accolTImodate this sediment; othe~vise, the structure soon becomes useless so far as further flood control effectiveness is concerned. The report does not indicate what provision has been made for sediment storage in the detention reservoirs. The over-all average annual reduction in sediment load of the Fountain River and its tributaries resulting from the proposed program is claimed to be about 58 percent. Somewhat over half of this amount is claimed to result from reduction in bank cutting and the remainder from reduction in sediment originating .from surface erosion. The reduction in bank cutting is based on a limited number of measurements in representative reaches of Fountain River and its tributaries. However, the report includes no substantiation for the reduction in sediment origi- nating from surface erosion. The estimates of annual sediment movement in the Fountain River were based upon data of sediment samples, included in the Corps of Engineers reports. It is not \mown whether these data adequately evaluate the bedload of the stream since we '\mow that the Fountain River carries a very large sand load. Sediment-rating curves are shown for the Fountain River but apparently flow-duration curves were not pre- pared 'to compute the sediment load from these rating curves. Recent studies by this Branch indicate that in computing the reduction in sediment load due to water- shed improvement, it is not always possible to reduce the discharge and then enter the sediment-rating curve with this reduced discharge to obtain the reduced sediment load. The flow of the stream may be competent to carry a larger sand load than is indicated by the sediment-rating curves" When this is the case, the amount of sand carried by the stream is controlled by the amount of sand brought into the main stream by the tributaries. Thus, the predicted reduction in sediment transportation and yield of Fountain River is dependent upon (1) the land treatment and complementary and structural measures proposed in the report attaining their full effectiveness, (2) the actual effectiverless of these measures in'producing the flood peak reduction which has already been questioned in this review, (3) the continued and effective maintenance of detention dams, bank stabilization measures, and other structural treatment. In vie'~ of the above, it is believed that the anticipated 58 percent reduction in sediment yield of Fountain River by the proposed program is highly optimistic.