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Show Chapter 2 The River and Its Tributaries The Missouri River, originating on the Conti- nental Divide in Yellowstone and Glacier National Parks and western Montana, flows for more than 2,400 miles through or along seven States and drains parts of three others. Its course is generally north- east to a point 30 miles downstream from Fort Ben- ton, Mont., then eastward into North Dakota, and southeasterly to Kansas City. From there it turns eastward to join the Mississippi about 15 miles above St. Louis. The two together form the longest waterway in the world, stretching from the Mon- tana Rockies to the Gulf of Mexico. The Missouri's drainage basin is an area of physical extremes, with the elevation ranging from well above 14,000 feet down to less than 400 feet. Stream characteristics vary greatly. Some trib- utaries are only a hundred miles long, others are as much as a thousand. In the upper reaches, the fall of the rivers and creeks is rapid, steadily dimin- ishing until in the lower reaches of some of the tributaries and in the main stem, the current is slow and sluggish. Of the many tributaries which flow into the Missouri, the principal ones are the Milk, Yellow- stone, Little Missouri, Cheyenne, James, Platte, Kansas, Grand, Osage, and Gasconade Rivers. Wide variations in seasonal flow characterize the Missouri. The winter season is a period of low flow. From December to February, ice may cover the stream as far south as Kansas City. The spring thaws greatly increase stream discharges. A typical rise on the Missouri during April is caused by the melting of the snow blanket over the interior plains area. This rise occasionally results in peak flows and some flood damage. A much greater rise is characteristic of June when snow melt from the higher plateaus and mountains is combined with sustained spring rainfall in the lower basin. This is the period of maximum flow of such tributaries as the Yellowstone, in which the increased spring flow makes a significant contribution to the main stem. Serious ice jam floods also occur. Following the June rise, low water prevails in the summer and early autumn, interrupted by minor rises caused by occasional heavy rains in the lower basin. Increased flow in late autumn results from widespread precipitation over the basin. Near the mouth of the river, the average flow is 64,000 cubic feet per second, with an estimated maximum of 900,000 and a minimum of 4,200 cubic feet per second. The annual natural flow of the Missouri at Yankton from 1898 to 1948 is estimated to have been 23,500,000 acre-feet,1 or 32,500 cubic feet per second. The flow at Yank- ton has varied from a maximum of 282,000 to a minimum of 2,700 cubic feet per second.1 (See figure 4.) In major floods many millions of acre-feet drain off the watershed into the main streams in a mat- ter of weeks. These waters, dealing destruction on the way and carrying millions of tons of valuable soil, are permanently lost to the basin. At other times, flows are so low as to cause concern for pub- lic health. Floods have always ravaged the basin. Great floods occurred centuries ago, when forests and grasses covered the land. For example, the greatest discharge of the Missouri may have occurred in 1844, when the flow may have reached 900,000 cubic feet per second. Great damage to flood plains has been caused by major floods in the past decade. The catas- 1 U. S. Corps of Engineers. 911610-51- -13 171 |
