Page:Popular Science Monthly Volume 69.djvu/253

Rh basin. It, too, has a portion of its drainage area protected from the prevailing winds and approximates to the rainfall condition of the Mississippi. The Ganges and the Hoang Ho are in regions of copious seasonal rainfalls.

The portion of the Mississippi Basin subject to inundation includes land on both sides of the river south of Cape Girardeau, Mo., and aggregates 29,700 square miles, or an area equivalent to the state of South Carolina. Through this alluvial basin (Fig. 2) the river winds in a rather tortuous path, the distance by river (1,700 miles) being nearly three times as long as a straight line drawn from Cape Girardeau to the Gulf of Mexico (GOO miles).

It is characteristic of many flood plains that the land immediately adjacent to the river is higher than the more distant parts of the plain (Fig. 3). These higher parts are called natural levees by some writers. If, therefore, the river rises beyond the limit of its banks, there is likely to be a general inundation of the alluvial basin. This feature of flood-plain form is further illustrated by the tributaries of the lower river. From an ordinary valley slope a river flows fairly direct into the trunk stream. If a tributary crosses a flood plain of some size, the stream must force an entrance against the rise of the back slope of this plain and finally must breach the banks of the river.