Ch. 22] SOURCES OF. SEDIMENT 383 



density, soil and rock types, and particularly land use (Brown, 1946) . 

 The larger the watershed in any given hydrologic region, the more 

 these differences are ironed out. In certain regions a tendency toward 

 decrease in the rate of sediment production per unit area with increas- 

 ing size of area has been noted. Apparently this trend is caused by the 

 progressive downstream deposition of sediment on flood plains and else- 

 where throughout the watershed. However, in several major drainage 

 basins, incoming tributaries cause a progressive downstream increase. 

 For example, the sediment load per unit area of the Missouri increases 

 downstream to Sioux City, and of the Mississippi downstream to the 

 mouth of the Missouri (Brune, 1948). Selected rates of sediment pro- 

 duction are given in Table 1. 



SOURCES OF SEDIMENT 



Practically all stream-borne sediment comes from one or more of the 

 following sources: 



Sheet erosion of the land surface. Sheet erosion is the more or less 

 uniform removal of soil or soil material from the land surface by the 

 forces of raindrop impact and surface runoff, or wind action. Most 

 sheet erosion by water involves the formation of rills or minor gullies. 

 If these are not too deep (more than about 6 inches) to be obliterated 

 by cultivation or to be crossed by wheeled vehicles, they are considered 

 part of the phenomena of sheet erosion (Fig. 2) . 



Gullying. Gullies are channels, generally more than 6 inches deep, 

 eroded by concentrated runoff in soil, alluvium, or decomposed and un- 

 consolidated rock. It seems desirable to confine the term gully to chan- 

 nels cut where no well-defined or continuous channels formerly existed. 

 Thus defined, the arroyos or valley trenches in alluvium-filled western 

 tributary valleys could be considered gullies, but enlargement of for- 

 merly existing channels of a watershed drainage system would be 

 classed as stream-channel erosion (Fig. 3). 



Stream-channel erosion. This is defined as erosion of the banks and 

 scouring of the beds of geologically developed perennial or intermittent 

 streams. In so far as this form of erosion is a source of sediment 

 production, a net enlargement of the channel is implied. Many streams 

 are eroding their banks, often causing severe losses of agricultural 

 bottom land, but the losses are balanced by deposition within the chan- 

 nel, so that there is no net increase in the sediment load reaching a 

 point downstream (Fig. 4). 



Mass movements. These include landslides, slumps, avalanches, soil 

 creep, etc. (Fig. 5). 



