The Pacific Coast of the United States has a very narrow Continental Shelf, and few bays or capes. With the exception of Puget Sound, the Bay of San Francisco, and the harbor of San Diego, there is scarcely a noticeable break in the continuity of the coast line.
Islands.—There are many small rocky islands along the coast of Maine, and on the southern New England Coast is a group to which belongs Long Island, the largest of the islands of the United States. Farther south, off the Atlantic Coast, and also in portions of the Gulf of Mexico, are many low sand-spits lying parallel to the coast and having behind them shallow channels, lagoons and swamps. On the Pacific Coast there are no islands of importance except the Santa Barbara group off the southern coast of California.
Rivers.—The rivers of the Atlantic Plain rise in the Appalachian system, and are comparatively short. In many cases they are too rapid to be of much value for navigation, but are valuable for supplying water power. These rivers almost without exception have good harbors at their mouths. The chief are: the Hudson, the Delaware, the Susquehanna, the Potomac, the James, and the Savannah.
The Great Central Plain is drained by the Mississippi-Missouri river system, the basin of which covers half the area of the United States, and is equal in area to about one-third the area of Europe.
The Mississippi rises in Lake Itasca, in Minnesota, at about fifteen hundred feet above sea-level. After flowing for about one hundred miles in an easterly direction it turns south, and is joined by numerous tributaries. The chief are: St. Peter’s River, which joins the main stream nine miles above St. Anthony’s Falls; the Missouri, which enters the Mississippi just above St. Louis; the Ohio, which joins the main river at Cairo; the Arkansas, the Wisconsin, the Illinois, and the Red River.
The Mississippi-Missouri has made a broad flood plain, varying in width from thirty to sixty miles. This plain is subject to severe inundations, for it slopes very gently away from the river bed, which is in many parts of the river above the level of the surrounding plain. The river carries a vast amount of silt, which it deposits at its mouth, thus forming a delta which stretches a series of long, narrow, tentacle-like arms seaward.
Other rivers falling into the Gulf of Mexico are the Mobile and the Rio Grande. The Mobile, which enters the gulf at the town of Mobile, is the union of the Alabama and the Tombigbee. The Rio Grande forms the boundary between Texas and Mexico.
The rivers flowing into the Pacific are comparatively short, owing to the nearness of the coast ranges to the sea. The Colorado River flows into the Gulf of California, after crossing an arid plateau. (See [description] below.)
The San Joaquin and the Sacramento rivers unite and flow into the harbor of San Francisco; these and the Columbia are the only important rivers entering the Pacific.