Page:Popular Science Monthly Volume 69.djvu/255

Rh The sequence of floods from the tributaries is first, the Ohio, then the upper Mississippi, followed by the Missouri and the western streams. Great floods are not annual occurrences. Recently the years marked by excessive floods have been the years of 1893, 1897 and 1903. The floods of the years 1898 and 1901 did not fall much short of the records of the seasons previous.

The hydrographs of floods (Fig. 4) and the profiles of rivers at different seasons (Fig. 5) show that the floods proceed down stream somewhat as a wave. The highest point or crest marks the extreme danger limit due to height of flood. The hydrograph also indicates that the crest flattens somewhat in its down-river progress. The reason for this may be seen in the river profiles which are drawn from three stages of the river's annual fluctuations. The down-stream slope (AC) of the flood wave has increased over the normal slope of the river (GE). This results in an increase of speed of the waters on this slope. On the other hand, the up-stream slope (BA) is a less one than the normal slope (DG), and a decrease of speed over normal flow results. At certain times the slope (BA) may be against gravity, and a further retardation will be experienced. This would occur when a tributary added a large volume of water to the Mississippi, as in the case of the Ohio during the spring freshets. The tendency of this increase of speed on the down-river side of a flood wave and the decrease of velocity on the up-river side is to reduce the size of the flood wave by draining off the excess of water faster than it can accumulate. If the river is long enough to allow this