The first step toward improvement of the Mississippi river was taken in 1820, when the U.S. Congress authorized a survey of the Mississippi and Ohio rivers. Two years later, Army engineers re porting upon this survey recommended that the removal of snags be undertaken first and, in 1824, $75,00o was appropriated for that purpose. As snagging improvements progressed on the river above the mouth of the Ohio (Cairo, dams and lateral nals with navigation locks were constructed at the more obstruc tive rapids and sand bars were removed by dredging. A project was adopted in 1878 for a 43-ft. channel at low water from the Ohio river to St. Paul. Succeeding projects provided for an 8ft. depth to the Missouri river and a 6f t. depth to Minneapolis. In the meantime, by an act adopted in 188o, the construction of five headwater reservoirs was authorized to impound water for release during the low water season. Subsequent legislation has author ized the construction of concrete dams to replace the five original timber dams and the construction of the sixth reservoir at Gull Lake, Minnesota. This work has been completed as follows : In 1927 the project depth from the Ohio river to St. Louis was increased to 9ft. and in 1930 this project depth was extended to Minneapolis, Minnesota. Between the Missouri river and Min neapolis this depth is secured by means of 26 locks and dams.
The canalization project for the Mississippi river between the mouth of the Missouri river and Minneapolis constitutes one of the world's greatest river navigation projects. The dams are de signed to maintain water levels during periods of low stream flow such that a minimum depth of 9ft. will be available for the entire length of the project. In operation, the water surface in the pools varies from a practically level condition at low flows to the normal open river slope at higher flows, which averages about o.45ft. per mile. The pools created by the dams are in general within the natural low banks of the river. At a few lo calities immediately upstream from the dams, overbank areas will be submerged to shallow depths. During periods of high dis charges the gates of the dams are raised well above the water surface, permitting floods to pass essentially unmodified as to stage or discharge. Pertinent data for this canalization project are tabulated as follows: Typical lock and dam structures on the upper Mississippi river consist of a lock section, a control gate section or spillway nor mally occupying the entire width of the low-water channel, and a section of fixed dam extending to the bluffs on either side. The latter consists of non-overflow earth dikes, but in a majority of cases also includes a spillway section of concrete or of stone paved overflow dike construction to aid in the passage of flood flows. The spillway is comprised of a roller gate section nor mally located near mid-channel, flanked on one or both sides by a section of Tainter gates. The supporting structure for the
gates consists of concrete piers usually resting on piles driven into the sand, and gravel bed of the river, since suitable rock foundations are not available at most sites. These piers are sur mounted by a service bridge along which a locomotive crane operates for the placing or removal of emergency bulkheads to enable unwatering and for removal of ice and logs which may lodge against the gates. A concrete apron, with a series of baffle piers, and a belt of heavy stone protection extend downstream to dissipate the energy of the water flowing through the gate open ings and to protect the river bed from scour. Seepage is checked by a steel sheet-pile diaphragm at the upstream edge of the sill. A similar diaphragm penetrating a lesser depth at the downstream end of the apron provides protection against undermining by scour.
Roller gates consist of large steel cylinders which roll on tracks, slightly inclined from the vertical, embedded in recesses in the concrete piers. The diameter of the cylinder is less than the nominal height of the gate opening, the remainder of the height being made up of one or two aprons attached to the cylindrical drum. The gate is raised and lowered by means of a specially de signed chain wrapped around one end of the cylindrical drum and actuated by a hoist housed in a building on top of the pier.
Tainter gates in their simplest form consist of a water-tight segment of a cylinder supported at each end by radial arms ro tating on pins anchored in the supporting piers. The first Tainter gates designed for the present Mississippi river project were of this simple type and were 35ft. in length. The more recent gates have been increased in length to 6oft., and gates Soft. in length have been designed for Dam No. 24, the last of the dams to be placed under construction.
contracting the lower 8.3mi. of the pass to a width of r,75oft. by means of jetties, spur dikes, and inner bulkheads. The jetties are composed of a substructure of brush mattresses surmounted with a superstructure of concrete or large rip rap. Spur dikes are corn posed of crib-work of round piling filled with willows and stone or with a curtain of round piles along the upstream face. The South Pass (13.5mi. long) has been improved by similar works and its navigable depth has been increased to 3o feet.
Between the mouth of the Missouri river and St. Louis plans for navigation improvement include a lateral canal about 8mi. long to by-pass the Chain of Rocks area where rock ledges cross the river and it is impracticable to maintain the 9ft. channel depth at extreme low water. A lock is proposed for construction in the downstream end of the lateral canal.