The hydrographic network of the USA is clearly organized according to the structural features of the vast territory: therefore the first fundamental division distinguishes the tributaries of the Atlantic from those of the Pacific. Most of the country, ca. 2/3, pours its waters to the Atlantic, in relation to the position, very displaced towards the West, of the Rocky Mountains, on which runs the watershed that separates the two sections (Continental Divide). A further subdivision can be made, within the Atlantic basins, between rivers directly tributary of the ocean and rivers flowing into the Gulf of Mexico. The latter includes the main river of the USA, the Mississippi, which drains the entire vast section between the Rocky Mountains and the Appalachians. The Mississippi basin, including that of Missouri, is approx. 3,328,000 km² (3rd of the world) and includes very varied regions from the point of view of rainfall. The richest food comes from the left, Appalachian tributaries, including the rich and placid Ohio; the right tributaries, which originate from the Rocky Mountains, have overall larger basins, particularly Missouri (which is more properly the main spring branch of Mississippi), whose catchment area is approximately 1,400,000 km², but where rainfall is lower. Indeed, they cross semi-arid areas subject to severe erosion and their waters reach the Mississippi laden with debris, as well as with a very irregular regime; however, Missouri is controlled by a gigantic dam system. Thin and full of the Mississippi are very sensitive: its flow at the mouth, which on average is 20,000 m3 / s, it rises up to 40,000 m 3 in the period of flood, at the beginning of summer/ s, due to the spring precipitation which affects almost the entire basin. Visit healthvv.com for North America geography.
The river, which including the Missouri-Red Rock spring branch extends for 5970 km, is navigable up to the city of Cairo, at the confluence with the Ohio, where the vast flood plain ends practically (on the sides of the river course, which in some points widen up to 2.5 km, large areas are periodically flooded as the river now runs on a hanging bed); compared to the past, its navigability goes much further upstream and a canal connects it to the Great Lakes. The function of the Mississippi as a communication route remains important, even if it is no longer what it had at the time of the conquest of the inland areas of the USA and which was remarkable despite the limits placed on penetration by the Alabama E Mississippi, the Brazos and especially the Rio Grande, on the border with Mexico, W. The section of US territory that is part of the real Atlantic basin is essentially drained from San Lorenzo (Saint Lawrence) and Appalachian rivers, those on the eastern, outer side of the chain (those inland mostly descend to Mississippi via Ohio, like Tennessee). Rich in water, given the good rainfall in the region, and with a not excessively irregular regime, the Appalachian rivers do not, however, generally have extensive basins: they are essentially escarpment rivers, with a short course, directed from the mountainous watershed to the sea. Their trend is basically normal to the coast and it is here that all their importance is measured, as the estuary mouths, the result of recent marine ingressions, constitute wonderful and well protected penetrations, suitable for the creation of ports.
From N to S the most important of these estuaries are that of the Hudson, a river that originates from the Adirondacks, partially navigable and today connected with canals to the Great Lakes, that of Delaware and that of Susquehanna (Chesapeake Bay), which draws the waters far within the Great Valley.Further to the S, the rivers have no estuary, given the existence of the coastal alluvial plain, reached by crossing the gap that separates this plain from the Piedmont: it is the aforementioned Falls Line, along which a whole series of hydroelectric power plants has been created. As for the San Lorenzo, this imposing river (3058 km of course, 1,550,000 km² of basin), although not part of the territory of the USA, represents a fundamental waterway directly inserted in the US geographical framework. This is because it connects the Atlantic with the Great Lakes, hydrographic elements of primary importance in the USA (as well as in Canada to which almost half of them belong), now fully navigable. The Big Five – Upper, Michigan, Huron, Erie, Ontario – which occupy ancient glacial basins a few hundred meters deep, form a single lake space, the maximum freshwater in the world (248,500 km²), thanks to their natural or artificial connections; the differences in height are minor, if we exclude the one that separates Lake Erie (175 m) from Lake Ontario (75 m), largely represented by the famous Niagara Falls. In the W-band Continental Divide, tendentially arid region, the hydrography is partly esoreica and partly area drains. The latter includes the entire Great Basin, into which there are several salt lakes, including the Great Salt Lake, the largest in the USA (4144 km²), excluding the Great Lakes. Two important rivers are attributed to the Pacific Ocean, the Colorado and the Columbia, the latter with its large Snake tributary.
The Colorado (2350 km of course, 676,000 km² of basin) draws its waters from the mountains that dominate the north-central section of the homonymous state, flowing, after a course mainly from NE to SW, in the Gulf of California; Columbia, shorter (1954 km) but with a larger basin (772,000 km²), originates in British Columbia, in Canadian territory, but mainly affects the USA, flowing into a deep estuary downstream from Portland (Oregon); their courses take place embedded in large canyons, such as the spectacular one, already mentioned, of Colorado. Not navigable, however, they are used, through a massive series of barriers, for the production of electricity and, especially in Colorado, for the supply of large quantities of water to the semi-arid areas of southern California. The largest US river after the Mississippi-Missouri, however, is the Alaskan Yukon (2897 km of course; 855,000 km² of basin), which crosses the entire state from NE to SW, flowing into the Bering Sea (Pacific Ocean); however, it is scarcely navigable because it is frozen for most of the year and generally of modest interest as it concerns a semi-inhabited area. Naturally, the watercourses that descend from the maritime side of the Coastal Chain have limited developments; a very characteristic hydrographic system is finally that of the California Valley, crossed by two rivers directed in the opposite direction: the Sacramento, which comes from the N, and the San Joaquin, which comes from the S and which pays tribute to the Sacramento at the mouth of this last in the San Francisco Bay.
