Page:Popular Science Monthly Volume 71.djvu/143

Rh these stages is commonly 50 feet. Any works in the river must be strong enough to withstand the enormous body of water of the flood stage. The locks in the chutes or side channels within the highwater levees will be inundated. The movement of the water must cause scour; and weirs, locks and abutments leading thereto would be threatened with imminent destruction.

The ratio of sediment carried by the river to the stream discharge is large. Much of this sediment is rolled along the bed of the river in waves transverse to the direction of stream flow. It is estimated that the total amount of sediment yielded to the Gulf yearly is a little over 400,000,000 tons. Enormous quantities of sediment find a temporary lodging place during the low-water season along the bed of the river. It is because of this sediment that the present dredging project is extant. It is admitted by the enthusiasts for canalization that the sediment would be injurious to a canal plant. They offer as a remedy the removal of the sediment by catchment basins before the navigable portions of the river are reached, or the removal of the sediment from the tributaries by some similar process, or the retention of the 400,000,000 tons of sediment "in the townships where it belongs." Any one of the remedies mentioned means a task as great if not greater than the original project. Catchment basins are easily constructed in small streams, but to suggest such a thing for large streams with a variable flow, the Cumberland and Tennessee Rivers for example, is to gamble with success. The retention of the sediment "in the townships where it belongs" is visionary. Some of the sediment comes from the caving banks of the river itself. This could not be eliminated. Furthermore, if the stream is deprived of much of its sediment, the power which ordinarily is expended in carrying its load can then be spent in further scour of its banks. Thus, to some degree, the deprivation of sediment will work to the harm of the protective levees.

A flood often falls rapidly in stage. During a sudden drop, the waters dump quantities of detritus along the bed. If we deny that during flood stages the sediment can be removed from the tributary streams, we must have fear for the effect of the sudden falls in river stage. Because of the resistance offered by any construction in the flooded stream, sand would tend to accumulate about such works. It is subsequent to these sudden falls that the dredging corps has to exert its utmost power in order to maintain the nine-foot depth of the dredging project. The removal of the sand from the wiers and locks would not necessarily last throughout the low-water season. Oftentimes secondary rises of the river occur which move the sand waves down-stream and obscure the trace of any previous work in dredging. Canalization, furthermore, would not make unnecessary the dredging plant. It is likely that as large a plant would be required. Dredging is carried on in the canalized rivers of Europe.

The river is long and tortuous. It has a width varying from