LEVEES. 



95 



serious import to the stability and permanence of the levee itself, may be carried into 

 the nearest branch of the drainage system without difficulty, because its quantity at 

 any one point is not great; but that which enters through the gap formed by the 

 outfall may be considerable, and if it were not for the slope of the flood-plane already 

 mentioned, the entire basin would be inundated. To more fully illustrate this we will 

 quote Mr. Starling:* "If the front of a basin be sealed by levees, joined to the hills at 

 its head and extending as far as the mouth of its drainage stream at its foot, the water 

 can get access to it only through the gap caused by the entrance of the tributary. The 

 plane of the Yazoo basin has a mean slope of about 8 inches to the mile. At a distance 

 of 15 miles above the lower end of the levee system, therefore, the level of the back- 

 water from the Mississippi will be about 10 feet below that of the river-water. Now, 

 the hills which bound the basins usually approach the river gradually, so that the lower 

 end of the basin has much less than the average width. In the instance cited the 

 levees extended to the mouth of the Yazoo River. The area of the alluvial tract from 

 its mouth to a line 15 miles above it would be about 250 square miles. In the case of 

 the St. Francis it would hardly be more than half of this amount. Of these areas 

 large tracts have so high a situation that they are several feet above back-water. Part 

 of the remainder is irreclaimable swamp." 



The question of drainage is, by reason of physical features, not so readily solved 

 in many cases as in that of the Mississippi, and recourse must be had to pumping and 

 other devices. The operation is a difficult and expensive one, and involves the creation 

 of ditches and drains in the lowest levels, with works for the regulation of flow at their 

 outer or lower extremities. These works must not only provide for outward flow but 

 must prevent, as far as practicable, that which would come inward. During periods 

 of low water these regulating gates remain open, but upon the approach of a flood 

 they have to be closed, and the waters within the levee must remain and gradually 

 rise over the lands unless removed by pumping. Whether this can be done economically 

 depends upon a number of conditions, among which is whether the value of property, 

 crops, etc., which may be injured by reason of this elevation of the water-surface, will 

 justify the expense necessary to preserve it. 



Influence on Flood Heights. As levee systems are developed and narrow the field 

 of inundation, it becomes necessary to raise their heights, because the elevation of high 

 water is increased by reason of this restriction. The Mississippi systems are not yet 

 complete, but sufficient progress has been made to indicate decidedly that the flood 

 level has been raised materially, and that the completion of the various systems will 

 be the signal for greater flood levels. It will then be a struggle to hold the river within 

 certain prescribed limits. 



Experience in other countries has shown that in this struggle to keep rivers within 

 restricted bounds they often regain their domains, and it becomes necessary to rebuild, 

 strengthen, and increase heights of levees. 



* Levees of the Mississippi, p. i. 



