FOREST COVER IX PROTECTING RESERVOIRS 23 
and temperatures of the Plains result in high evaporation, which 
reduces still further the run-off of the Plains. The rainfall on the 
Plains, 10 to 30 inches, consists chiefly of summer precipitation, 
which augments the flow from the mountain sources for a short time, 
but in autumn becomes insignificant. Most of their upper basins are 
naked or sparsely timbered with open stands devoid of humus, ex- 
cept at the headwaters where the forested areas are small compared 
with the extensive unforested areas below. (PL 16, figs. 1 and 2.) 
The mountain streams which feed these rivers are of low turbidity, 
being fed by springs and the melting of snow on the north slopes and 
the higher peaks. The snow waters are reinforced by the midsummer 
rains of the Plains which are heaviest in June. 
It is from the treeless Plains that these streams obtain the greatest 
portion of the silt burden. The silt burden is so excessive that many 
of the rivers have meandering channels from the deposit of sediment 
which may be in transit for only relatively short distances before 
being deposited. In addition to the violent fluctuations between the 
maximum flow and minimum dry-season flow, the constant washing 
of banks and formation of silt bars in the shifting channels are 
serious problems. The material of these silt bars is largely deposited 
during the spring freshets, being carried in part from the banks of 
the smaller tributaries of the Plains and from the unconsolidated 
sands and silts which constitute the prevailing soils of the Plains ; to 
a less extent it is material brought down in flood by the mountain 
streams. 
The silt burden in these streams is among the highest in the United 
States. The Missouri above Ruegg, St. Louis County, Mo., with a 
drainage area of 528,700 square miles, has a yearly silt burden of 
more than 176,000,000 tons. The Arkansas above Little Rock, with a 
basin of 148,000 square miles, discharges more than 40,000,000 tons 
of earth. The maintenance of forest cover on the mountain slopes at 
the headwaters of these streams is essential. It is the mountain 
springs which largely maintain the late autumn run-off after the 
melting of the snows. 
STREAMS OF THE GREAT PLAINS 
The streams of the Plains include the Washita, the Cimarron, the 
Red, the Smoky Hill, the Republican, and the Kansas Rivers. So 
erratic is the flow, that the Smoky Hill River, one of the chief trib- 
utaries of the Kansas, with a drainage area of 8,000 square miles, has 
a minimum discharge during the low-flow season of only 10 cubic feet 
per second. A comparison of this with a typical eastern stream — 
the Potomac River at Point of Rocks, Md., where the basin has a 
drainage area of 9,000 square miles — shows that the minimum flow 
of the Potomac, which for a 12-year period is recorded as 990 cubic 
feet per second, is ninety-nine times as great as that of the head- 
waters of the Smoky Hill River. The headwaters of the Arkansas, 
which rise in the mountains of Colorado, show a much more steady 
flow than those of the Kansas. At Canon City, Colo., the Arkansas 
drains a basis of 3,000 square miles and has a* minimum flow of 1C8 
cubic feet per second, or a flow thirty times as great as that of the 
Kansas River. 
