330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



SEASONS OF GREAT FLOODS 



It is no accident that great floods occur only in winter or spring, 

 even though the melting of accumulated snow is usually a minor or 

 negligible factor. Summer and autumn rains in the eastern United 

 States may be as great as those of winter and spring and are usually 

 more intense, that is, the rate of fall is more rapid. There is so much 

 more vapor in the air that strong general storms are not required for 

 the production of excessive rains, and when a hurricane puts the vapor 

 through the cyclonic wringer at high speed the rains exceed any that 

 can be produced in winter. Fortunately, however, there is in the 

 active vegetation of the warm season an effective intercepter of rain, 

 a mechanical hindrance to run-off, and a rapid user of ground water. 

 Hardwoods in Wisconsin when in leaf intercepted 25 percent of the 

 rainfall but when dormant only 16 percent; 10 but this difference of 

 9 percent of the rainfall is but a small part of the total change in run-off 

 from summer to winter for the same rainfalls. Besides the several 

 effects of vegetation, evaporation from the ground is greater in summer. 

 A comparison of run-off with rainfall at Knoxville, Tenn., 11 shows an 

 average run-off for 1900-23 of only 28 percent in summer and 30 

 percent in autumn, whereas in the 3 months January to March it 

 was 64 percent. Even in summer months with 7 to 12.5 inches of 

 rainfall the run-off was only 25 to 37 percent. The run-off of 4.06 

 inches from the 10.89 inches of rainfall in the hurricane flood of July 

 1916, and that of 4.38 inches from the 12.52 inches of rainfall in August 

 1901, did not equal the 4.22 and 4.54-inch run-offs from the 7.00 and 

 7.10-inch rainfalls of the March floods of 1922 and 1913. To produce 

 important floods in summer, monthly rains well in excess of 10 inches 

 are required, whereas half as much will suffice in winter. Summer 

 rains, in general, are spottier than winter ones, and excessive down- 

 pours are more restricted in area than ordinary heavy rains; so sum- 

 mer floods are usually more local and of shorter duration than winter 

 ones. In the cold season the low evaporation permits the ground to 

 stay wet for a long time after a rain, and a freezing of the ground may 

 stop percolation. Before the rivers have carried away the water from 

 one general rainstorm another may occur and add to the burden. 



THE OHIO-MISSISSIPPI FLOOD OF JANUARY-FEBRUARY 1937 



These general observations may now be examined in the specific 

 terms of the Ohio-Mississippi flood of January-February 1937. In 

 the eastern United States northwesterly or northerly winds normally 

 prevail in winter, pushed from the high pressure formed by air accumu- 



'» Ibid., p. 4. Cf. also Baldwin, H. I., and Brooks, C. F., Forests and floods in New Hampshire, New 

 England Regional Planning Commission, Publ. No. 47, pp. 5-9, Boston, 1936. 



» Voorhees, J. F., A preliminary study of effective rainfall, Monthly Weather Rev., vol. 53, pp. 63-65, 

 1925. 



