392 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



ceptions, these losses may also be attributed to widespread soil erosion 

 on tributary watersheds. 



On a few western streams and in the headwater tributaries of some 

 eastern streams, the quantity of sediment transported by flood flows is 

 sufficient to cause a higher stage and greater area of inundation than 

 if the flows were free of sediment. Maximum daily concentrations as 

 high as 13.8 percent by weight in the Colorado River at Grand Canyon, 

 Arizona (Howard, 1947), and 21 percent in the Rio Grande at San 

 Marcial, New Mexico, have been recorded. Samples taken on the Rio 

 Puerco, a major tributary of the Rio Grande, have contained more than 

 60 percent of sediment by weight, and samples from small streams in 

 Iowa have ranged as high as 27.6 percent of sediment. In the small, 

 steep mountain watersheds above Los Angeles, California, flood flows 

 caught in debris basins (Eaton, 1936) contained 85 percent of solids 

 by volume (after settlement) and only 15 percent of free water (above 

 the amount required to saturate the deposit). These examples are 

 extreme conditions for both large and small streams. Most of the large 

 streams of the country seldom transport more than 1 to 3 percent by 

 weight of sediment at any time. Many eastern streams never approach 

 this value. As a concentration of 10 percent by weight causes a volume 

 increase of only about 4 percent in the flow, it is obvious that only high 

 concentrations will significantly increase the flood stage and resulting 

 area of inundation. 



Coarse sediment passing through the penstocks of run-of-the-river 

 power plants has been known to abrade the turbine blades and neces- 

 sitate frequent replacement. This material is almost entirely bed- 

 load material, the immediate source of which is stream-bed scour. 



Sedimentation in Channels, Hakbors, Ditches, and Canals 



Sedimentation (and scour) of channels affect (1) flood damages and 

 flood control by increasing or decreasing the size of stream channels 

 and floodways; (2) navigation of (a) inland rivers and (6) harbors; 

 (3) drainage through (a) open canals and (b) the level of the water 

 table under agricultural land; and (4) irrigation water-distribution 

 systems. 



If a stream channel is progressively becoming smaller because of 

 aggradation, the area of inundation for a given flood discharge will 

 progressively increase. Conversely, if the channel is enlarging by ero- 

 sion, the area of inundation will decrease. Almost without exception, 

 flood-damage-frequency studies have been based on an assumed con- 

 stant channel capacity. Actually, in many watersheds, particularly on 



