In connection with this investigation, and as a matter of value in showing the forces which are in operation to affect the river outflow, the fictitious or possible evaporation of the six great valleys referred to were calculated, in cubic miles of water, from July, 1887, to July, 1888, and also the average amounts of water in the air as vapor, and the amount required to saturate the air in the same valleys during the same period.
During the year 1882, the year of great flood in the lower Mississippi valley, the outflow at Red River Landing, La., was 202.7 cubic miles, of which the upper Mississippi river above St. Louis furnished 16 per cent., the Ohio 43, and the whole Missouri above Omaha, 4 per cent. The upper Missouri valley (that is, from the mouth of the Yellowstone up to the sources), and the middle Missouri valley (from the mouth of the Platte to the Yellowstone), each furnished only about 2 per cent. of the entire amount of the water which passed Red River Landing. The lower Mississippi valley, including the Arkansas, etc., furnished 32 per cent.
During March, April and May, 1882, the time of highest stage of the water of the lower Mississippi, the outflow at Red River Landing and through the Atchafalaya measured 82.7 cubic miles. 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 percent. 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 Missouri 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, supplemented 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' observations, 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.