River Fisheries

Rivers drain all but the most arid areas of the earth through channels that are regu- lated by physical laws that impose on them certain forms. The ideal form is rarely encountered in practice and represents an end point to which geographic process tend. In general a river may be divided into two principal zones, the steep and fast flowing rhithron upstream and the sluggish and flat potamon downstream. While conditions in an individual system are highly variable along its length, similar reaches of different rivers differ much less even between continents and at different latitudes. All continents have a series of major river systems which consist not only of the river channels but also the swamps, lakes and seasonally flooded lands associated with them. Most rivers are highly conditioned by the patterns of precipitation in their basins. Differences in rainfall intensity throughout the year generate a flood wave that progresses downstream in the majority of rivers (flood rivers), although singular geographic circumstances may distribute discharge more evenly throughout the year in some systems (reservoir rivers). The number of reservoir rivers is increasing through flow regulation and dam building. Although the basic nature of the river is determined by the rocks over which it flows, the flood regime seasonally modifies the physical and chemical conditions within the river particularly in the tropics. In higher latitudes other features of climate, such as insolation or air temperature exert an increasing influence.

Carte de livre

• Nombre de pages: 330
• Auteur: R L Welcomme

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