THE OHIO AND MISSISSIPPI FLOODS OF 1912. By H. C. FRANKENFIELD, Professor of Meteorology. THE DRAINAGE BASIN OF THE MISSISSIPPI RIVER. A full description of the character and extent of the basin of the Mississippi River will be found in Bulletin E (Weather Bureau, 1897) and in the Annual Report of the Chief of the Weather Bureau, 1896-7, and those who desire more detailed information are referred to these publications. From them it is learned that the great drainage basin of the Mississippi River comprises an area of about 1,240,050 square miles, or about 41 per cent of the total area of the United States, exclusive of Alaska, and extending between the Allegheny and the Rocky Mountains through 56° of longitude and 21° of latitude. There are six grand subdivisions, five of them comprising the watersheds of the largest tributaries, and the names and areas of the six subdivisions are as follows (see chart 1, appendix): In the Ohio River and in the Mississippi River below Cairo, Ill., years without floods are exceptional, while in the Mississippi River above Cairo stages above the flood line, as measured by the gage heights at St. Louis, Mo., are the exception rather than the rule, occurring on an average about one year in every four or five. Prior to the nineteenth century little is known of the occurrence of floods except through more or less authentic tradition. Of the lower Mississippi floods, the most notable occurred in 1815, 1828, 1844, 1849, 1850, 1851, 1858, 1859, 1862, 1865, 1867, 1874, 1882, 1884, 1890, 1893, 1897, 1903, and 1912, being 19 years in all, or an average of one flood to about each six years. In the upper Mississippi River the years of marked high water, as measured on the St. Louis gage, were 1785, 1811, 1823, 1826, 1844, 1851, 1855, 1858, 1862, 1881, 1883, 1892, 1903, and 1909, being 14 in all, or an average of one flood to about each nine years. The greatest lower Mississippi flood, measured by the flood height, occurred in 1912, while in the upper Mississippi the greatest flood was probably that of 1785, "L'annee des grandes eaux," when the river at St. Louis is said to have reached a stage corresponding to a height of 42 feet 11 on the present gage, or 0.6 foot higher than the stage of June 28, 1844. However, the stage of 42 feet in 1785 is not one of actual record, but of tradition only, and the flood of 1844 is usually considered to have been the greatest of upper Mississippi and lower Missouri floods. CAUSES OF MISSISSIPPI RIVER FLOODS. The normal rains of late winter and early spring over the lower Mississippi Valley are usually sufficient to bring the rivers almost to the flood stage from Cairo to the Gulf of Mexico. Then, if the spring rains over the Ohio Basin are heavier than usual, an enormous volume of water from the main river and the swift-running mountain tributaries is brought down upon the lower Mississippi, already at bank-full stage, and a disastrous flood results. If the winter happened to be a moderately cold one, with plenty of snow laying upon the ground, there would be a further increment from the melted snows, as the spring rains are almost invariably accompanied by high temperatures. Thus the Ohio and the lower Mississippi Rivers alone can produce a great flood, independently of the upper Mississippi and the great western tributaries of the lower river. The upper Mississippi, while in itself incapable of causing a flood in the lower river, yet, rising later than the Ohio, as a rule, serves to prolong the high water and at times to increase somewhat the stages from Cairo southward. Fortunately the lower western tributaries thus far do not appear to have played an important part in flood causation, yet the possibility of simultaneous floods in both the eastern and the western tributaries and in the main stream is ever present; and should this condition arise, the resulting stages between Cairo and the Gulf would probably be higher than have yet been recorded. It appears that the precipitation that directly causes the Ohio and Mississippi floods is due to a single type of storm known as "The Southwest" type, for the reason that usually it is first observed in definite formation over the southwestern portion of the country. These ' storms move in from the Pacific Ocean, the majority of them by way of the north coast, whence they move southeastward through the great plateau until they reach western Texas, when they turn northeastward over the Ohio Valley and the Lake region with increased development and velocity of movement, accompanied by heavy rains and high temperatures over the Gulf States and the Ohio Valley, and frequently by snow to the northward. A small portion moves inland by way of California, and a still smaller portion by way of Mexico, but, with rare exceptions, all reach Texas and move northeastward as indicated above. None appears to develop true storm conditions until Texas is reached, and about 95 per cent of them are preceded and accompanied by heavy rains and high temperatures to the eastward and southeastward, and by rains and snows to the northward. If the temperatures to the northward range between 25 and 32° F. the snowfall will be heavy, both as to quantity and character. Flood probability depends upon the number and time of occurrence of these southwest storms, and, of course, somewhat upon the antecedent conditions. The storms usually begin in February and continue during March and at times a portion of April. If the early winter has been a cold one, resulting in a frozen soil and the accumulation of a considerable amount of snow over the Ohio watershed, the flood probability will be increased. If there should be but a single storm, or even if there should be several storms separated by considerable intervals of time, the flood wave will be short and the river stages only moderately high, but if there should be a series of storms, separated by intervals of only a few days, as in the present year, a severe flood is certain, and its intensity will be limited only by the amount of precipitation, which in any event is almost certain to be heavy. The accumulated snowfall will usually go out with the first storm, and, if the early winter has been cold, the run-off from the first one or two storms will obviously be greater than the normal run-off. The normal winter and spring rainfall over the Gulf States is comparatively heavy and any considerable increase in the amount, if distributed over an extended period of time, will bring the lower Mississippi and the Yazoo Rivers to stages above the flood line and while in this condition the extensive flood wave from the Ohio pours in, provided the direction of storm movement has been about normal. If, by any chance, |