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In any plan for the development of water and land resources, hydrology must be a basic consideration. Because of the Missouri River Basin's large size and the many associated variations in hydrologic characteristics, hydrology is of special significance.
The Missouri River drains an area of approximately 530,000 square miles, and it is the longest river in the United States (2,315 miles). The source of the Missouri River is the headwaters of the Red Rock River in Montana, and the mouth is at the Mississippi River near St. Louis, Missouri. Part or all of 10 states and 9,700 square miles in Canada are contained in the Missouri River Basin.
The basin's physiographic features and the resulting effects on the basin's hydrology are extremely varied, since the basin extends into three major physiographic divisions, the Rocky Mountain System, the Interior Plains, and the Interior Highlands. The Interior Plains Division contains by far the greatest part of the Missouri River Basin.
The Missouri River is characterized by wide variations in seasonal flow; lowest flows occur during the winter months. A complicating factor during the winter is ice, which may cover the river as far south as Kansas City, Missouri. When the ice cover breaks up, significant ice jams may form and cause resultant flooding. Discharges are significantly increased in the spring due to snowmelt, and low flows generally prevail during the late summer and early fall.