8i THE IMPROVEMENT OF RIVERS. 



when it should have been built in that situation to level grade. The consequence was 

 that the water ran over at the lower end, while it was still 1.5 feet below the crown at 

 the upper end. 



In order to successfully establish proper grades it is evident that considerable 

 skill and experience are required, as well as a thorough study of the immediate locality. 

 Safety will dictate a too great elevation, while economy will call for one which is scarcely 

 high enough. There is a mean which should be intelligently thought out and applied. 

 A Government report has the following on this subject: "The grade of levees for the 

 improvement of navigation should be at least the normal height of the.bank of the river, 

 or the height to which the bank would be built by such floods as recur with sufficient 

 frequency to exert an appreciable influence in bank-building or enlargement of water- 

 way. This grade is approximately indicated by points on or near the margin of the 

 river, well above general overflow, where deposit has been carried to a natural limit 

 unchecked by swift currents and undisturbed by caving. This grade should be supple- 

 mented by such additional height as will protect it against frequent injury or destruction. 



" Preparations must also be made in the grade for such increase of flood elevation 

 as will at first and during the period of readjustment ensue from the introduction of 

 greater volume between levees, when made more continuous. It is not possible to 

 predict at present to what this will amount, and it is probable that an exact conclusion 

 will only be reached by experience. 



"We therefore conclude that levees, such as have been herein described, are, in. 

 connection with an equalization of width and the prevention of caving, an important 

 part of any general and systematic plan for the improvement of the navigation and 

 the prevention of destructive floods; and we recommend the construction of new and 

 raising of existing levees along all parts of the river where the highlands are too remote 

 to check the passage of large volumes of flood water outside the bed of the river; or, 

 in other words, on the entire right and also on the left bank below Baton Rouge, and 

 from the Yazoo River to Horn Lake, below Memphis." 



Materials. I^evees are generally built of the material nearest at hand, whether 

 it be loam, sand, clay, gravel, or a mixture of two or more of these. The clays along 

 the Mississippi are known generally under the name of "Buckshot," the name being 

 given because of its peculiarity of breaking into fragments about the size of bullets. 

 Buckshot is not wholly clay, there being a varying percentage of sand in its make-up. 

 The sand used in levee construction is mixed with earth and is of fine particles. The 

 clay is usually plastic and of the blue variety, although other kinds are found. The 

 loams are composed largely of sand, and are light and fine, and very poorly suited to 

 embankment building. By reason of its resistance to the action of the waves, as well 

 as to percolation, clay is the best material found for this work. 



Sand makes a fairly good embankment when not subjected to wave action, but 

 it is not safe to build one of small thickness, because a cavity once formed will increase 

 rapidly and soon endanger the whole structure. In fact, sandy material cf any kind 



