FLOODS — BROOKS AND THIESSEN 329 



The run-off that produces great floods of the Mississippi is between 

 25 and 30 percent of the rainfall over the entire drainage area, which, 

 however, includes semiarid lands. 5 In Tennessee the average run-off 

 is estimated as 45 percent. 6 



The current weather conditions tending to make floods are con- 

 tinuously heavy rains and temperatures above freezing. Freezing 

 during the night is a flood deterrent. Warm rains and high tempera- 

 tures with strong winds melt the layer of snow if there is any; and 

 rain and snow water run off together. An old snow cover is so dense, 

 however, that much more time is required to remove it than freshly 

 fallen snow, the more so if it lies in the shelter of a forest. Ten inches 

 of rainfall in a day or two will produce a flood anywhere and at any 

 time in the eastern United States. Five inches will suffice if the ground 

 is bare or lightly snow-covered and already saturated or frozen. 

 Under such conditions 60 to 100 percent of the water may run off into 

 the streams. The Muskingum River (Ohio) flood-control plan is 

 based on a hypothetical 10 inches of rain in 5 days on frozen ground 

 with a 90 percent run-off. 



On the other hand, dense vegetation and a dry soil or a deep snow 

 cover on unfrozen ground may wholly absorb 5 or more inches of 

 rainfall. Thus in March 1936, in experimental plots in the upper 

 Susquehanna drainage, while 60 percent of a 5-inch rainfall over 

 several days ran off of open cultivated fields that were frozen, none 

 came from the unfrozen forest near by ; and at the Arnot Soil Conser- 

 vation Experiment Station in Schuyler County, N. Y., while a run-off 

 of 7.9 inches, including melted snow, came from frozen, open fields 

 after 6.4 inches of rainfall, March 10-19, 1936, only 0.1 inch came 

 from a beech-maple forest plot, where 12 inches of snow remained 

 unmelted after the snow had disappeared in the open. 7 Of the 27 

 inches of rainfall causing the great flood of the Yazoo River, Miss., 

 in 1931-32, 62 percent ran off from cultivated fields, 54 percent from 

 abandoned fields, 2 percent from scrub oak, and 0.5 percent from 

 undisturbed oak forest. 8 The Soil Conservation Service reported 

 that 8 inches or 95 percent ran off of plowed land in Ohio in the heavy 

 rains of January 1937, but only 2 inches from comparable areas under 

 grass and trees. 9 



« Frankenflcld, H. C, and others: The spring floods of 1922, Monthly Weather Rev., Suppl. No. 22, pp. 

 7-8, 1922; idem; The floods of 1927 in the Mississippi Basin, ibid., Suppl. No. 29, p. 31, 1927. 



« Forests in flood control, Supplemental Rep. to Committee on Flood Control, House of Repr., 74th 

 Congr., 2nd Sess., on H. R. 12517, p. 4, Washington, 1936. 



» Ibid., p. 6. 



« Ibid., pp. 3 and 51-59. 



• For other data on storm-rain run-oS, see Hoyt, W. G., and others, Studies of relations of rainfall and 

 run-off in the United States, U. S. Geol. Surv. Water-Supply Paper, 722, csp. tables 30, 33, 36, 39, 42, 45, 48, 

 51; Washington, 1936. 



