Geography of the Avr. 57 
During this time there flowed through the upper Mississippi 
river above St. Louis, 14 per cent. of the amount ; through the 
Ohio, 38 per cent., and through the Missouri 6 per cent.; while 
the rivers of the lower Mississippi valley contributed 41 per 
cent. The water that passed Omaha was 1.92 cubic miles, or 2 
per cent. of the flow of the whole Mississippi during the same 
time. The water which flowed from the upper and middle Mis- 
souri valleys during March, April and May, 1882, was for each 
valley, probably only 1 per cent. of the water that flowed through 
the lower Mississippi river. The flood of the lower Mississippi 
was undoubtedly due to the great discharge of the Ohio, supple- 
mented by heavy river inflow below the mouth-of the Ohio, and 
the unusually heavy rainfall in the lower Mississippi valley. 
The ratios of river outflow to rainfall over the catchment 
basins, as derived by Professor Russell from the two years’ ob- 
servations, 1881 and 1882, were as follows : 
Upper and Middle Missouri valleys, about 335,000 square 
miles, 13 per cent. 
Lower Missouri valley, about 210,000 square miles, 12 per cent. 
Entire Missouri valley, about 545,000 square miles, nearly 13 
per cent. 
The upper Mississippi valley, about 172,000 square miles, 33 
per cent. 
Ohio valley, about 212,000 square miles, 40 per cent. 
Lower Mississippi valley, about 343,000 square miles, about 27 
per cent. ; 
The above percentages, while showing the averages for two 
entire years, and so of decided value, are not to be depended 
upon for special years or months. For instance: in the Ohio 
valley in 1881, the outflow was 33 per cent., while in 1882 it was 
50 per cent., and as the rainfall in 1882 was 180 cubic miles 
against 151 cubic miles in 1881, it appears evident that a much 
greater proportional quantity of water reaches the rivers during 
seasons of heavy rainfalls than when the precipitation is moder- 
ate.or scanty. 
Evaporation is also a very potent cause in diminishing river 
outflow, and as this depends largely on the temperature of the 
air and the velocity of the wind, any marked deviation of these 
meteorological elements from the normal, must exercise an im- 
portant influence on the ratio of outflow to rainfall. 
