Ch. 22] DEPOSITION ON LAND 395 



tected at such points or the minor watersheds are protected, storm 

 flow in these waterways may breach the canal and deposit sediment 

 for some distance along it. 



Deposition on Land, Improvements, and Habitats 



Sedimentation (with associated scouring and swamping) on flood 

 plains and other areas of overflow damages (1) agricultural land; 

 (2) property, facilities, transportation, and communication; and (3) 

 recreation and wildlife habitats. 



The damaging effects of infertile sediment deposits, flood scouring 

 of topsoil, and swamping due to drainage derangement on valley agri- 

 cultural lands are so widely distributed that nearly every part of the 

 country has such problems (Brown, 1945c; Happ, 1945; Happ et al., 

 1940; U. S. Congress, 1942, 1946; U. S. Bureau of Reclamation, 1948b). 

 These are, like soil erosion, cumulative damages to land. Whereas 

 flood-water damage can be correlated with stage, duration, frequency, 

 and season of flood flows, and its recurrence interval can be computed 

 by available hydrologic techniques, sediment damage is progressive and 

 can be evaluated only with respect to the average rate of development. 



Studies made by the Soil Conservation Service (Happ et al., 1940) 

 have shown that conditions in most valleys, especially in the head- 

 water reaches, were essentially stable prior to deforestation and culti- 

 vation. Changes due to stream meandering and overbank flow took 

 place slowly. In areas with more than 20 inches of rainfall, old, slowly 

 developed, dark topsoils now serve as key horizons from which subse- 

 quent changes during the period of accelerated erosion can be measured. 

 In hundreds of valleys these old topsoils have been buried by rapidly 

 accumulated and, generally, less fertile "modern" sediment. The valid- 

 ity of this dating has been frequently confirmed by fence posts, mine 

 wastes, bricks, and other artifacts buried in the "modern" sediment 

 but never below the dark topsoils; by releveling old profiles; and by 

 studies of the contrasting physical characteristics and watershed 

 sources of modern sediment and old soils. Valley agricultural damage 

 per acre of overflowed land is generally greater in headwater valleys 

 and becomes progressively less downstream. In some watersheds, how- 

 ever, there is evidence that the locus of greatest damage is moving 

 progressively downstream. 



The causes of damage to flood-plain agricultural land are as varied 

 as the problems and their locales. In many places local channel ob- 

 structions are forcing flood flows out of bank, causing scour and dep- 

 osition. More often, however, damages can be attributed to greatly 

 increased loads of coarse sediment and greater frequency of overflow. 



