Topography.— The main part of the United States presents four physical divisions: two ele vated and two lowland regions. The elevated are the Appalachian Mountains (q.v.) in the east and the Rocky Mountains (q.v.) or Cor dilleran system in the west. The eastern low land mass is along the Atlantic Coast, broad at the Gulf of Mexico and narrowing toward the north where the mountains are but a short dis tance from the ocean. The southern part of the Atlantic lowlands joins the central lowland region south of the Appalachian Mountains and about 70 miles from the Gulf of Mexico. It is much less in extent than the central lowland division but it contains a large population, is the oldest portion settled by Europeans and was the chief battle ground in the war for Ameri can independence. The northern and southern parts of this lowland section differ materially in formation and soil. The mountains in the northern part approach the ocean so that the foot-hills are almost at the sea line; the low lands are in some places really series of low hills, masses of rocks, sandy soil, large areas covered deep with glacial deposits and with masses of rock formation which show the marks of mighty ice-forces. Beginning with and including the southern portions of New Jersey, and continuing to the Gulf and Florida Strait, is a plain of low, almost level, land, ex tending in a gradual slope from the mountains to tide water. The soil and climate contribute to the extensive growth of fruits, tobacco, corn and cotton. The northern section of the At lantic lowland is a worn-down mountain region and the southern section at no ancient period was sea-bottom and even now the line of de marcation between the coastal plain and the con tinental shelf is very slight in many places. This section has received the name of ((Tide Water Country,D on account of its being a gift to the continent from the sea, and also be cause many of its rivers are tidal streams for some distance from the ocean. The central low land lies between and on the lower slopes of the two great uplifts. It is called the Mississippi Valley on account of the greater portion being in the basin of the Mississippi River. The higher slopes, merging into the foot-hill region of the Appalachian on the east and the Pacific on the west, become the plateau lands. The so-called Piedmont Plateau lies west and northwest of the central part of the great Atlantic lowlands or plains. The whole plateau seems not to have been brought down to the level of the sea, but to have inclined slightly to seaward and toward the valleys of the streams; so that, in reference to its plain condition, the region is often called a pene plain. When the land portions of the Atlantic lowland was raised above sea-level the same system of upheaval extended throughout the Piedmont region, and has continued with inter mission to the present time, until the old plains have now reached an altitude which constitutes them a plateau as now understood. The plateau is bounded on the northwest by the Ap palachian Mountains. It was not upheaved evenly and simultaneously in all its parts. Where first lifted, the gorges and valleys are convex; where last lifted they have concave profiles. The large grassy, almost treeless areas in this section are called prairies. This great lowland region of the United States is a part of the central lowland section of North America, which is called, in Canada, the Hudson Bay and Mackenzie regions. In the southern part of this section are vast areas of flood plains, and also land which at no very remote period was wholly under water. Beginning with the At lantic plain or lowland at Florida, extending west and including the southern part of the central lowland section, there are broad areas only a few feet above sea-level, in many places less than 100 feet. The northernpart of this central section is bounded by the Great Lakes. The divide between the streams that flow into the Great Lakes and those which flow into the Gulf of Mexico, by way of the Mississippi, is very slight. The three long slopes in this division are the one from the northern part to the Gulf ; the one from the Appalachian divide on the east to the Mississippi; and the third from the Rocky Mountain divide on the west to the Mississippi. There are numerous sand bars and swamp lands along the southern coast. The eastern slope is shorter and less steep as a whole than the western slope. There are high bluffs along many of the rivers of the western part, even in sections where there are large areas of rolling prairie lands. The huge boulders and mountain peaks of the north west section seem like outposts on the begin ning of the plateau region. In the southwest the lowlands extend around the southern part of many of the mountain ohains, so that the greater part of the United States south of the low rocky hills which form the extremity of the Appalachian Mountains, and extending west to the Guadalupe Mountains, is one continuous lowland mass. The mountains in Missouri and Arkansas, south of the Missouri River and just west of the Mississippi, are the most im portant highlands in this whole section. In Missouri these highlands are called Ozark Mountains or Ozark Plateau, in Arkansas Ouachita Mountains.
Along the Pacific Coast is a narrow strip of low land of not sufficient extent to be classed among the great physical divisions, but of great value from an economic point of view. The southern part of this lowland border is consid erably wider than that of the northern part.
The numerous parallel valleys, in some places valley arms, which are on the eastern border of the Pacific lowland strip, furnish a considerable area of productive farm lands.
The eastern uplift, the Appalachian Moun tains, are the older and less extensive of the two great highland sections of the United States. (See APPALACHIAN MOUNTAINS). They con sist chiefly of mountain ranges which are nearly parallel with the Atlantic Coast and extend from near the Gulf of Mexico north into Can ada. Nearly all of the western part of the United States, beginning about the 104th merid ian, belongs to the Rocky Mountain region. (See ROCKY MOUNTAINS). This portion of the United States has a greater altitude and extent than the mountain lands of the Atlantic region. The Rocky Mountains extend from Mexico to Canada. The ranges which constitute this group are by no means as regular in arrange ment as are the Appalachian chains; some ex tend nearly parallel with the coast; many lofty ranges are at almost right angles with the north and south ranges and others run northeast and southwest. Enclosed by ranges of these moun tains are the Great Basin (q.v.) and the Yellow stone Park (q.v.). The Great Basin region is a series of basins isolated to all appearances from each other so far as drainage lines, and differing in soil and geological formation. The chief basins are the Carson, Humboldt and Great Salt Lake. The mountain ranges within the Great Basin trend mostly north and south. The vast area of the volcanic region of the Rocky Mountains contains many extinct vol canoes. On both the eastern and western border are numerous high peaks connected by high plateaus. In the southern cart or the por tion drained by the Colorado River is a region of high plateaus crossed by streams which flow through deep canons, some of which are over 2,000 feet deep. The Grand Canyon (q.v.) of the Colorado is in places 6,000 feet deep.
Hydrography.— The United States may be divided into four great drainage slopes,— the Atlantic, Great Lake, Gulf and Pacific— al though the second and third of these belong broadly speaking to the Atlantic. All the streams of the Atlantic slope drain into that ocean by river mouths within the territory of the United States. All the streams of the Great Lake slope ultimately discharge into the Saint Lawrence River, also within the territorial lim its of the United States. The Gulf slope in cludes all of that part of the United States which drains into the Gulf of Mexico. In pop ular usage, most of this is called the valley of the Mississippi, while small areas are drained into the Gulf by streams not tributary to the great river. East of the Mississippi this slope is drained by the Apalachicola, Mobile and Pearl, and to the west by the Sabine, Brazos, Colorado, San Antonio, Nueces and Rio Grande. Along the western border of the Gulf slope the country is arid, having a rainfall of less than 20 inches annually. In this arid re gion there are many small streams whose waters are not carried away by ocean-feeding rivers, as the small streams are lost in the sands. On the Pacific slope all streams that ultimately reach the sea do so within the ter ritorial limits of the United States, but the district drained by the Colorado River of the West reaches the Gulf of California by passing a short distance through Mexican territory. In a large part of the Pacific slope there are many small streams that discharge their waters into sands, where they are evaporated and lost from the ocean-reaching drainage; but the val leys in which they are evaporated incline to ward streams draining into the Pacific Ocean. We thus have an Atlantic slope, a Great Lake slope, a Gulf slope and a Pacific slope. The lines of demarcation between these slopes are very Between the Atlantic and the Great Lake slopes the divides are sometimes in mountains and sometimes in hills; between the Atlantic and 'Gulf the divides are in part in mountains and in part along the low ridge of Florida; between the Gulf and Great Lake slopes the divide is an inconspicuous elevation, so low in many places that the waters may easily be diverted from one slope to another. The divide between the Mississippi region and the Pacific region is sometimes mountains, with peaks from 8,000 to 14,000 feet above sea level. Of the large river systems the chief are the Mississippi, the Saint Lawrence, the Co lumbia and the Colorado. The Mississippi (q.v.) is the largest, including within its basin nearly all the region in the central lowland sec tion, and a large area of the Rocky and Appa lachian Mountains. The chief tributary is the Missouri (q.v.) which has a drainage area of about 530,000 square miles. Next in drainage area is the Ohio River (q.v.), the basin of which is over 200,000 square miles; the Ar kansas, 185,671 square miles; the Red River, nearly 90,000 square miles. The total area drained by the Mississippi, its tributaries, and other streams which enter the Gulf, is 1,726,000 square miles. The Rio Grande, which also en ters the Gulf of Mexico, has one large tribu tary, the Pecos. Further streams which flow into the Gulf of Mexico are the small Colo rado, the Brazos and others in Texas, and sev eral rivers in Mississipni, Alabama and Geor gia. The Mississippi and its tributaries are navigable for thousands of miles. To the great central waterways is due the early development of the interior of the United States, first as a section with trading posts and forts at conven ient places on the navigable streams, and later as a farming and manufacturing region. West of the divide in the Rocky Mountains the drain age is to the Pacific Ocean. Nearly all the rivers, except those in the Great Basin, flow directly, or through a main stream to the open sea. The Colorado River enters the Pacific Ocean through the Gulf of California. The largest river of the Pacific Basin is the Colum bia (q.v.). Some of the other important rivers are the Sacramento, San Joaquin, Klamath and a number of short streams. The rivers in Cali fornia and some of the other valleys follow the course of the valleys, but the Columbia, Colo rado and branches of the Columbia break through the mountains in several places and thus form high waterfalls and series of cas cades. The Columbia has several large tribu taries, chief of which is the Snake River. The rivers of the Atlantic Basin east of the Appalachians have mostly rapid currents, and have had great influence in the development of the manufacturing industries of the country. Many of the streams which flow into the At lantic, or into wide bays which are arms of the ocean, are tidal streams for some distance inland. (See DELAWARE ; HUDSON, etc.). The principal rivers of this basin are the Kennebec, Merrimac, Penobscot and Manchester in the northeast; the Connecticut, which flows into Long Island Sound; the Housatonic, the Hud son, a magnificent stream, alike remarkable for its scenery and its navigable importance, which flows south for 300 miles and contributes to form the harbor of New York; the Delaware, which after a course of 300 miles enters the Delaware Bay and is navigable for large steam ers to Philadelphia, a distance of 40 miles; the Potomac, which flows into Chesapeake Bay, and is navigable for the largest vessels to Washing ton, a distance, including the bay, of 200 miles; and the Savannah, which enters Savannah Bay and is navigable for large vessels for 17 miles, to the city of Savannah, where it forms an im portant harbor. Besides the rivers, one of the most remarkable features of the United States, as also of Canada, is the chain of large fresh water lakes: Superior, Michigan, Huron, Erie and Ontario. The lakes drain an area of about 90,000 square miles, and send their waters into the Saint Lawrence, after precipitating the greater part of them in an accumulated mass over the renowned Falls of Niagara (q.v.), which are between Erie and Ontario. The rivers of the United States which flow into the Great Lakes (q.v.) are of no great length. The chief streams are the rivers of northern New York and Vermont, some of which enter the Saint Lawrence River through Lake Cham plain; the Genesee River in the west central part of New York, and a number of small streams which enter Lakes Erie, Huron, Mich igan and Superior. (See SAINT LAWRENCE RIVER). The Red River of the North enters the Atlantic Ocean through Lake Winnipeg and the Hudson Bay. In the interior of the United States are many groups of small lakes which have been mentioned in articles on the different States. The inland seas or salt-water lakes within the Great Basin are of special interest as being the remnants of large inland seas. The fresh-water lakes in the Appalachian sec tion, and even the Great Lakes, were once much larger than at present. The chief characteris tics of the whole drainage system of the United States are that by far the greatest portion of the waters are carried south or in a southern direction, and reach the Atlantic Ocean through the Gulf of Mexico. The Great Lakes with their large outlet, the Saint Lawrence River, receive but a small portion of the drainage; their chief supply comes from the melting snows of the Rocky Mountains. The rivers which enter the Pacific are small streams, ex cept the Columbia and Colorado. The Red River of the North is the largest stream which flows north.