RIVER AND LAKE DEPOSITS 179 
portion. So the floods deposit a layer of sediment, 
and in time these accumulations build up a plain, whose 
surface is below the level of the flood water, but above 
that of the ordinary stages of the river. 
Among mountains, floodplains may be made of 
coarse gravel deposits; but in the gently sloping lower 
Fig. 91. 
Bars in Green River, Wyoming. Gunnison Butte in background. 
reaches of the river, the plains are almost invariably 
composed of very fine sediment. In the greater rivers 
of the world, the floodplain may be a broad and fertile 
tract of land, so valuable for agriculture that expen- 
sive earthworks are thrown up, as levees, to prevent 
