SOIL FORMATIONS 23 



the concave bank and deposits it on the convex. Finally 

 the curve becomes so great that the stream breaks through 

 the banks and straightens its course. In this way a broad 

 valley may gradually be covered by sediment deposited by 

 the stream. 



Changes in velocity of a stream, as when in flood after 

 heavy rains or melting snows, cause a change in its canying 

 power. Much material will be picked up by a stream in 

 flood that must be deposited as the flood subsides. A 

 stream may build up its bed so that the surface of the 

 water is higher than is the land at some distance on 

 either side. Such is actually the case in the lower Mis- 

 sissippi valley. 



23. Character and distribution of alluvial soils. — Allu- 

 vial soils may be sands, loams or clay, depending on the veloc- 

 ity of the stream and the nature of the eroded material. 

 It is likely to be the case that the alluvial deposits along the 

 upper stretches of a stream will be sandy, and that the 

 material deposited will become finer as the stream proceeds. 

 Soils of this formation have no very distinctive composition. 

 Naturally this character depends on the nature of the ma- 

 terial farther up the stream, and this, of course, varies in 

 different parts of the country. Even along any one stream 

 there may be a wide diversity of material picked up and 

 hence an alluvial soil is likely to be a heterogeneous one. 

 The content of organic matter is usually high, as this 

 is carried and deposited with the other matter. Alluvial 

 soil is generally regarded as rich soil, but there are many 

 exceptions. When situated along slowly flowing streams, 

 the land is likely to need drainage. 



Alluvial soils are naturally confined to the margins of 

 streams, but they are found along small as well as large 

 ones, and consequently the aggregate area of alluvial land 

 is large. The Mississippi valley and its branches contain 



