Ch. 22] SEDIMENTATION IN CHANNELS 393 



the headwater tributaries, channels and first-bottom floodways are 

 being progressively reduced in cross-sectional area by aggradation; 

 consequently future flood flows of the same volume and duration will 

 cause a higher stage and greater area of inundation. 



The Middle Rio Grande Valley is a classic example in the United 

 States of channel and floodway aggradation (Happ, 1948; Rittenhouse, 

 1944) . Throughout the 150-mile length of this valley the river is gen- 

 erally confined by levees except for a stretch about 17 miles long above 

 the original head of Elephant Butte Reservoir. Cross-section surveys 

 showed that the average annual sediment accumulation in the floodway 

 during the 5 years from 1936 to 1941 was 3,312 acre-feet. This is 

 equivalent to an average aggradation of about 1 foot in 12 years. 



In the Kickapoo River watershed, Wisconsin, the channel and first 

 bottom have aggraded at an average rate of approximately 1 foot in 

 20 years since the beginning of cultivation, about 1850 (Happ, 1944) . 

 From Ontario to the mouth of the river, flood heights increased an 

 average of 2.5 feet for the same discharge up to 1940. Continuation of 

 this trend until 1960 would cause an estimated increase of approxi- 

 mately 40 percent in damage to seven urban areas built on low ter- 

 races adjacent to the present flood plain. Similar conditions have 

 been found in some streams in almost every section of the United 

 States (Adams, 1944; Brune, 1942; Gilbert, 1917; Happ, 1945; Happ 

 et al, 1940; U. S. Congress, 1942; U. S. Bureau of Reclamation, 1948b) . 



In some places channel aggradation is a local phenomenon resulting 

 from backfilling of sediment behind obstructions in the channel, such as 

 fallen trees. The stream would be capable of passing through the sedi- 

 ment load delivered to it if these obstructions were periodically re- 

 moved. In many watersheds, however, the present rate of coarse- 

 sediment production exceeds the stream's capacity to deliver this part 

 of its load to its mouth, and general aggradation is resulting (Happ et 

 al., 1940). The only permanent solution in such cases is to reduce 

 the supply of coarse sediment or to increase the stream's transportation 

 capacity. Reservoirs for holding back bed sediment and sills or bar- 

 riers for stabilizing channel scour and stream-bank and gully erosion 

 are generally the most immediately applicable control measures. 



In some watersheds where the load is largely fine and carried in sus- 

 pension, the flood-plain surface is aggrading while the channel bed 

 remains constant or is being degraded. This condition is, of course, 

 decreasing the stage and area of inundation for a given flood discharge. 



Reservoirs upset the regimen of alluvial-bed rivers, causing aggrada- 

 tion of channels upstream and degradation downstream (Hathaway, 

 1948). On the Colorado River at Needles, California (U. S. Bureau of 



