94 THE IMPROVEMENT OF RIVERS. 



could not be suspected, or some circumstance that could not be seen or controlled. 

 Levees very seldom break from sloughing for that is generally remedied. They seldom, 

 of late years, break from insufficient cross.section ; they never break from sliding or 

 overturning, as they ought to do by rule. They do not often bieak from storms, though 

 sometimes they have run very narrow escapes. Crevasses are due mostly to three 

 causes: to being overtopped by the water, to holes through the levees, and to weak- 

 ness of the foundation. In former years the most common of all causes of breaks was 

 insufficiency of height, because even a thin stream of water running over a bank of 

 light or ordinary material will effect its destruction. Experience having taught that 

 this was the chief danger to be apprehended, construction has been generally turned 

 in the direction of increased height. 



" The closing of a levee break, or crevasse, as it is generally called, is an operation 

 which ranges, in point of difficulty, from the easy to the impossible, depending upon 

 the height of the levee, its material, the foundation, and a number of things. The 

 only plan which has been successful is one of the oldest employed. First, a number 

 of bents or trestles are set up, and connected by stringers; then hand-piles are driven 

 in front of the stringers, supported by the latter. The work is completed by filling 

 in with sacks and earth. 



"Another method consists of throwing out spu^-dikes at r.ght angles to the break 

 and thus destroying the effect of the current, while yet another surrounds the break 

 with a pile-dike, which is then rapidly filled with sacks of earth. As may be imagined, 

 earth is scarce at times of floods, and this fact will often defeat the closing of a crevasse 

 which might otherwise be successfully stopped." * 



Drainage of Inner Basin. The drainage problem of the areas inclosed along the 

 Mississippi is a much easier one than in many other sections where levees are 

 employed. The continuous slope of the flood-plane and its magnitude render great 

 service in this regard. Each system of levees incloses an area which is drained by 

 rivers, creeks, branches, etc., reaching every portion of the basin and finally emptying 

 into the main river itself. The highest land to be found is on the bank of the river, 

 from which point it gradually slopes back to the drainage tributary, which is generally 

 near the hill ground. Thus it will be seen that there is a complete system of drainage 

 for each of the various levee basins through the natural stream traversing the inclosed 

 areas, the final outlets of which are not closed by levees because, if this were done, it 

 would of course be necessary to exhaust the water from the basin by pumping. On 

 account of its great quantity this would be impracticable, to say nothing of the expense, 

 which would not be justified by the value of the lands inclosed. 



By starting at the hills at the head of a basin and carrying the levee along its front 

 as far as the outlet of the drainage tributary at the foot of the basin, it will be seen that 

 water in entering must either pass through or under the levee, or come in through the 

 tributary stream. That which passes through and under the embankment, while of 



* Levees of the Mississippi, p. 14. 



