Most rain falls into the oceans but the lesser amount that falls on the land is absorbed by roots of plants and is transpired back into the atmosphere, is percolated into the soil, or is collected into depressions forming lakes, and some evaporates. In addition, a great deal of the rain that falls on land runs off into the streams, creeks, runs, licks, and rivers.

A river can be of any size and its depth and flow may vary from time to time. The Mississippi-Missouri River is the longest on the North American continent, while the Virgin River that runs through Zion Canyon in Utah is little more than a creek. During periods of heavy rainfall, the Freemont River in Central Utah becomes a roaring, rampaging flood, but in times of drought it can be easily forded on foot or on horseback. The rivers drain the continent, carrying silt, salts and nutrients, rocks, stones, pebbles, and gravel to the sea. The flow of the water responds to the pull of gravity and rivers follow the least resistant course.

Some of the most interesting rivers in the United States have formed as a result of glacial activity. There seems to have been a time, prior to the Wisconsin glacial period, when a great river system originated somewhere in the mountains of Tennessee or North Carolina and flowed north and westward through what is now the Kanawa River Valley of West Virginia. It crossed the Ohio River Valley, continued to the Scioto River Valley, probably connected at some point with the Wabash or the Illinois, made a loop to the west connecting the Mississippi River drainage, heading south in mid-continent.

The system of rivers that included the Kanawa River which flows through Charleston, W. Va., to the Ohio River which, in turn, flows from Pittsburgh through Portsmouth, and the Olentangy which joins the Scioto River at Columbus, Ohio, and flows through Circleville and Chillicothe to the Ohio, all resulted from the Wisconsin glaciation. The Kanawa River flows north to the Ohio in a river valley that widens as it proceeds northward, while the Scioto narrows as it approaches the Ohio River. It is an interesting geological phenomenon, for apparently the continental glacier blocked the flow of the rivers to the north, impounded the water, and formed lakes—the beaches of which can still be found—and as the glacier retreated, a sufficient amount of debris was deposited in the river valley to reverse the flow of the stream. The Ohio River Valley formed at the edge of the melting glacier and represents a new river that was cut from part of several river valleys. The drainage flowed to the southwest because in that direction lay the lowest point in the ridge of mountains where the impounded water in front of the glacier could flow. A water gap was cut at Portsmouth, Ohio, and the river continued to flow in that direction.

The Mississippi, Missouri, and Ohio Rivers drained the great continental glaciers and the entire river system of the North American continent was influenced in some way by the glaciation, to the extent that it is now difficult to determine what the land surface and the river systems were like originally.

The more common word for drainage is watershed, which can be defined as an area of land that is drained by a single stream or creek. But streams and creeks flow into rivers, so in the larger sense of watershed we really mean the river drainage; hence, the Ohio watershed, the Monongahela watershed, the Allegheny watershed, the Conomaugh watershed, and so forth.

it was an unfortunate accident of history that when the Upper Mississippi was discovered, the significance of the tributary at the confluence of the Missouri was poorly understood; for if we look at the river system of the central continent, it is obvious that the continuous river system is the Mississippi-Missouri, and that the Upper Mississippi is merely a tributary of this great river system. The specifications for the Louisiana Purchase were that it should comprise all of the land drained by the Mississippi-Missouri River system. It is easy to understand why Thomas Jefferson was so anxious to acquire the port of New Orleans since it was the gateway to the continent as it was known at that time.