22 BULLETIN 1430, U. S. DEPARTMENT OF AGRICULTURE 
South, ceased between 1880 and 1900. The rural population moved 
to the factory towns; the negro labor migrated to cities. Many 
counties in the southern Piedmont region showed a decrease in rural 
population during these decades, and in many there was a decrease in 
the total population. The surface of the farm land abandoned in 
this manner was quickly hardened and lost its porousness. Only a 
small amount of the heavy rains is absorbed by it, and much water 
that should be absorbed runs off, augmenting the floods; while the 
springs formerly fed from the water stored in the soil have failed. 
The land itself has eroded in deep gullies. (PI. 15, figs, 1 and 2.) 
More recently, in the decade 1910 to 1920, there has been a general 
decline in rural population throughout the entire United States. 
During this decade 38 per cent of all the counties in 16 Southern 
States, all of them agricultural counties, lost population. In Xorth 
Carolina, this loss in rural population was accompanied by a decrease 
of 1.500,000 acres in the area of cultivated land. But the loss in 
agricultural population and the accompanying shrinkage in the area 
of land in cultivation is by no means confined to the Southern 
States. It is a serious problem in every section. Much of the land, 
thus abandoned has been subject to destructive erosion of soil. 
The irregularity and concentration of the precipitation, associated 
with other unfavorable conditions, frequently produce high floods, 
particularly on such streams as the Cape Fear and the Xeuse Rivers 
which have narrow channels, not only in winter and early spring, 
when warm rains release the stored waters of snow and ice, but 
during the summer and occasionally in the fall as well. In these 
violent floods the rise of water at the fall line of the rivers frequently 
amounts to from 30 to 60 feet, the narrow channel having, however, 
an important influence upon the height of flood crest. On the other 
hand, during periods of drought, which occasionally extend to 60 
and 90 days, that of the summer of 1925 being of more than usual 
severity, and the deficiency in ground water having been carried for- 
ward into 1926, the stream flow decreases until the water in the chan- 
nels is very shallow. Rises of 60 or more feet above low water are 
known to take place in the Cape Fear, the Alabama, the Cumberland, 
and the Ohio, as well as in other streams. 
RIVERS OF THE EASTERN SLOPE OF THE ROCKY MOUNTAINS 
Rivers of the group draining the eastern slope of the Rockies 
embrace many streams of great length flowing east and south through 
the Plains. The most important are the Missouri, the Yellowstone, 
the Canadian, the Platte, and the Arkansas Rivers and the Rio 
Grande. 
These streams have an erratic flow. There is a period of high 
water in the late spring or early summer resulting from the melting 
of the accumulated snows of winter, folowed by a long period of 
extremely low water, with very high floods at irregular intervals. 
Many of them, however, are extremely valuable as sources of power 
and for irrigation, since it is probable that a large portion of the re- 
gion in which dry farming has failed may eventually be made highly 
productive through irrigation. After the early summer floods the 
autumn stages of these rivers are usually low, while the high winds 
