394 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



Reclamation, 1948a) , for example, an aggraded channel condition can 

 be traced to bed scour upstream induced by construction of Hoover 

 Dam, which desilted the river flow. This illustrates the fact that the 

 cause of a channel problem must be understood in order to plan proper 

 corrective measures. In this case, erosion control on the watershed 

 area below Hoover Dam would have no significant effect on the prob- 

 lem. 



The basic causes of problems incident to the maintenance of navig- 

 able channels on inland rivers are not always clear. Usually they 

 appear to be due to natural river meandering and shifting of the bed 

 with rising and falling stage (Fisk, 1947; Friedkin, 1945). Engineers 

 familiar with the navigation problems of the Mississippi, Missouri, 

 Ohio, and other large rivers are predominantly of the opinion that in- 

 creased sediment production from the watershed is not a primary factor 

 in channel maintenance. They believe generally that minimization of 

 maintenance costs depends on securing the most favorable alignment, 

 grade, bank protection, and control of discharge. 



On the other hand, the maintenance of most harbor channels is 

 caused by deposition that takes place when the inflowing river can no 

 longer transport its load after reaching tidewater (Gottschalk, 1945). 

 In some places tidal currents and wave action bring littoral sediment 

 into harbors, but available data indicate that this is a minor cause of 

 harbor-maintenance costs compared with the deposition of stream- 

 borne sediment generally derived from disseminated sources on tribu- 

 tary watersheds. 



Silting of open-drainage canals is an aggravated problem in many 

 parts of the country. Runoff coming directly from watershed lands 

 often carries more coarse sediment than the canals, with their lower 

 grades, can transport. Canals draining swamps and marshes, on the 

 other hand, ordinarily carry desilted water and have no sediment 

 problem. Vegetative growth in canals increases their roughness, thus 

 decreasing their sediment-carrying capacity. It has been found that, 

 sometimes, the sediment load is "excessive" only because of this growth. 

 Its removal enables the flow to erode the channel to its original dimen- 

 sions. More generally, however, control of soil erosion on the drainage 

 area is the only applicable remedy. 



Irrigation canals are subject to much the same problems as drainage 

 channels. Since water is diverted into them, however, their sediment 

 content can often be controlled by properly designed diversion works 

 or desilting devices (Parshall, 1947). Because irrigation canals usu- 

 ally do not follow the natural drainage lines of a watershed, they 

 must cross many minor waterways. Unless they are adequately pro- 



