Leaving aside the tributaries of Lake Winnipeg – such as the Red River, the border between the states of North Dakota and Minnesota, the Souris River, which winds through North Dakota, and other minor ones – and still small stretches of some rivers water flowing into Oldham (South Saskatschewan), the United States does not participate in the Arctic Sea basin and its dependencies; their waters go to the Atlantic and the Gulf of Mexico on one side, to the Pacific on the other, and the main divide line) is made up of the Cordillera. These move away from the Pacific much more than in Canada, thus allowing the formation of significant river basins, such as those of Columbia and Colorado, which collect part of the water from the inland plateaus and from the intermontane basins. However, it should not be forgotten that in this western region large areas are devoid of drainage (about 600,000 sq. Km. Overall), and that there is no shortage of areic regions either (see America, II, p. 851). Only the waters of the state of Michigan and some parts of the other states bordering the Great Lakes go to the San Lorenzo basin; so that essentially there are three main hydrographic networks: that of the Pacific, that which flows into the Mississippi and that of the Appalachian rivers.
The Pacific tributary regime is characterized by large seasonal fluctuations and very strong summer meats. These are attenuated only in the central part of the Rocky Mountains and in the Sierra Nevada, due to the melting of snow and ice; but nevertheless the Columbia has average level variations of not less than 18 m. in Dalles, at 11 in Portland, and Colorado has a completely torrential character. The rivers of New Mexico and Texas – including the upper reaches of the Rio Grande and its tributary Pecos – are typical steppe streams with violent floods of very short duration, almost without water in the long dry season. The irregularity of the regime and the very wide seasonal fluctuations also characterize the rivers that form the Mississippi basin, except for the minor left tributaries of the lower section from the Appalachians. Ohio normally has high waters in April, low waters in late autumn, sometimes even in late summer; the amplitude in Cincinnati reaches 16 m .; terrible floods occur when melting snows are associated with heavy rains and exceptional duration. The tributaries coming from the Appalachians have a similar character, whose irregularity has become rather accentuated following extensive deforestation. Upper Mississippi has less significant fluctuations (high waters in June; low waters in December); but on the other hand, all the tributaries coming from the Prairies are extremely irregular in terms of regime, due to the distribution of rains on a continental basis. This is especially true for Missouri, the Arkansas and the Red River, which have violent floods in late spring or early summer and then rapidly subside until they shrink to thin water strands in late summer. In the Red River, the floods are made particularly violent by the clogging phenomena produced by the masses of floating wood (rafts). In the lower Mississippi the composition of the various regimes results in a condition of relative equalization, with high waters in May-June, low in October-November; the average amplitude in New Orleans is 5 m. approximately. But sometimes the contemporaneity of the floods of Ohio and Missouri, or the occlusion of the right tributaries determined by the main current already in a state of flood, determine catastrophic floods throughout the low plain, despite the numerous and complex embankments; this happened, for example, in 1897, 1912 and 1927 (see, for more information, mississippi). Only the Appalachian rivers have a more regular regime and a more defined succession of flood and low water periods; as a consequence of the more regular distribution of precipitation; the melting of the snows has no influence. Normally there is high water in spring or early summer, low water in autumn. The seasonal fluctuations tend to increase towards S., and the rapidity with which the floods follow the periods of more intense rainfall is also accentuated, as occurs, for example, in Florida, where the rivers are already in full flood in late summer. Disastrous floods are not uncommon, not least as a result of barriers caused by floating timber.
The Appalachian rivers were also the first to be used for both irrigation and waterways. Then was added the use of the driving force developed by the Piedmont jumps. But in all these respects the rivers of the Mississippi Basin are of far greater importance: excellent navigable arteries that are easy to link together through the low watersheds; generous power supplies for irrigation branches; rich reserves of hydraulic energy, even if not everywhere so far exploited for the unequal industrial development of the regions included in the basin. The tributaries of the Pacific are less suited to navigation, which on the other hand, also due to morphological conditions, present great possibilities for the creation of basins for industrial and irrigation purposes.
