320 happ. STREAM-CHANNEL CONTROL [Ch. 18 



and check dams to retard erosion and runoff, constructed principally 

 as part of the federal soil conservation and forestry programs. 

 Straightening of channels by dredging or dragline excavation has been 

 extensively undertaken to improve drainage and reduce flooding of 

 valley farm lands in the central and southern United States. In Eu- 

 rope more effort has been devoted to improvement of smaller rivers 

 for navigation, and to stabilization of steep mountain torrents, as 

 described in treatises by Franzius (1936) and by Schoklitsch (1937), 

 translated by Straub and Shulits, respectively. Young (1933) has 

 summarized contrasts between European and American practices. 



STREAM-CHANNEL PROCESSES 



All natural streams carry sediment, derived from erosion of the soils 

 and rocks in their drainage basins, and form sedimentary deposits, or 

 alluvium, in places where the transporting capacity is locally de- 

 creased. Some alluvial deposits, such as channel bars, are temporary 

 and may exist only for periods of minutes or days ; others may become 

 incorporated into flood plains and persist for centuries. Most streams 

 therefore have their beds and banks formed mainly in their own sedi- 

 mentary deposits, which they continually rework by eroding the banks 

 in some places and redepositing the sediment farther downstream, un- 

 less artificially restrained. 



The heavier materials, commonly gravel or coarse sand, are moved 

 chiefly at flood stages and deposited chiefly in the deeper parts of the 

 channel ; finer sands usually accumulate chiefly as bars along the sides 

 of the channel; and silts and clays are usually deposited chiefly in 

 areas of shallow overflow on the flood-plain surface where velocities 

 are least. Thus, as a stream channel shifts about in its valley, it 

 sorts the alluvium into a sequence grading from finer material at the 

 top to coarser sands or gravel below. Most channel-control works are 

 concerned primarily with the trading processes by which sediment is 

 ordinarily being removed by stream erosion at some places and re- 

 deposited elsewhere farther downstream. 



It is natural for stream channels to be crooked, because channel pat- 

 terns are governed mainly by resistance of the banks to erosion, and 

 bank materials are normally variable in resistance. The formation 

 of a sedimentary bar in a channel produces asymmetry in the path of 

 flow and complementary asymmetry downstream, and induces forma- 

 tion of additional bars and sinuosities in the current. If the banks are 

 sufficiently resistant, the channel as a whole may retain a straight or 

 slightly curving path, but the bed will develop a series of bars alter- 



