OCR Text |
Show AVERAGE ANNUAL RUNOFF Water Years 1938-43 Missouri River Basin Under 1 inch 1-3 inches 3-5 5-10 10 and over Source: Department of Agriculture FIGURE 4. trophic flood of 1943 alone inundated over 2 mil- lion acres of land; flood losses in 1944 and 1945 were almost as severe. Again, in 1947, floods in the middle and lower basin inundated 3 million acres of la.nd. More than 6 million acres through- out the basin are subject to flood inundation. These are usually the most productive lands. A wide variety of conditions can cause floods along the lower reaches of the Missouri. The flood of April 1943 was caused by melting snow, ag- gravated fc>y ice jams. Snow cover over the greater portion of North Dakota averaged 1 foot, and in parts of South Dakota, Montana, and Wyoming, was about half a foot. Although precipitation for the area Ln which the floodwaters originated was very lights abnormally high temperatures in late March resulted in an unusually rapid thaw. This flood was the greatest on the Missouri between Bismarck and St. Joseph since 1881. The lower river flooded in May 1943, principally because of two protracted and heavy rainfalls over the Grand, Osage, ancd Gasconade Basins. The April 1944 flood was due principally to snow melt in the upper tributaries, followed by 1 to 3 inches of rain over the entire Missouri Basin. The principal damage occurred in the State of Missouri. The flood of 1945, which was not a major one, resulted from snow melt alone, although there was some scattered rainfall. The March-April 1947 floods were similar to the catastrophic flood of 1943 in that snow melt flowing into streams covered with thick ice was responsible. Severe damage occurred particularly in the Dakotas. Subsequent heavy rainfall from Sioux City to the mouth of the river resulted in the worst floods in over 100 years on the lower Missouri. Many tributary floods causing great damage have occurred. Thus in 1935, excessive rains in the Re- publican River watershed caused a loss of over 105 lives and many millions of dollars in damage. Flash floods in the Grand have caused severe floods and terrific soil loss. Flash floods from summer storms on the Front Range of Colorado have caused loss of life, as at Morrison, Boulder, and Denver. 172 |