ABSTRACT

The Upper Mississippi Valley canalization project was authorized by Congress in 1930. The construction of 24 navigation darns in addition to the power dams already in operation at Fort Snelling, Minnesota (Dam No. 1) and at Koekuk, Iowa (Dam No. 19) enables a 9-foot channel to be maintained from St. Paul, Minnesota to St. Louis, Missouri. The new structures are low-head, movable, roller-gate dams, a type especially designed to reduce deposition in the pools. During excessive high-water stages the dam is out of operation and open-river conditions allow the current to make a clean sweep.

A re-survey of the pool bottom upstream from Dam 15 at Rock Island, Illinois, in October, 1943, on the same ranges as were surveyed in August, 1938, and in June, 1930. before the dam started operation, indicates that in the 10-year period: (1) deposition was active during the early stages of the pool's existence, mainly due to a shift of the channel, and (2) during the later stages scour exceeded deposition. The discharge of the river was abnormally high in the later years.

Pool 15 is located in a gorge where the slope is steep and there is no flood plain and therefore this study does not yield facts or conclusions which can be applied equally to all the dams. It may be stated that there will be no excessive silting like there was in the early years of the pool. The periodic high stages of water will sweep out the deposits and the abnormally steep gradient causes a fast flow which will not allow for much deposition. Long periods of low flow necessitating an almost closed dam are favorable for deposition but it must be remembered that low velocities transport much less sediment.