Rivers and streams
An aerial view of two rivers
Rivers are dynamic systems, with their courses and flows constantly changing due to natural and human responses. The world's main channel river length amounts to about 269 000 km, with the highest density of rivers located in South America. Major river and floodplain systems are very important to Asian countries. Rivers play a substantial role for capture fisheries - their contribution to food security is often underestimated as statistics do not include the component consumed by fishers.
Improved data collection on the Mekong River system - draining an area approximately 800 000 km2, 9 percent of which are wetlands --revealed that it produces 1.3 million tonnes of fish. Previous estimates were about half this amount. In Central and Eastern Europe and the Commonwealth of Independent States and the Baltic States (former USSR), the considerable catch of recreational fishers to supplement their diet often does not appear in official statistics. Catch statistics from rivers are often of low quality because of the difficulty inherent in collecting data from fisheries which operate from many landings dispersed along a system that may traverse several countries. Extrapolating data from systems well-known to other river systems provides a comparison between the river channel length and catch for selected rivers of the world.
Rivers of the world
Courtesy of the Great Globe Gallery
Multiple use of rivers -- for transportation, power generation, agriculture and industry -- and their abuse as a recipient of wastes and through channelization (especially in Europe, North America and Asia), has led to the loss of their original form and to the progressive loss of biological diversity. In China, the multitude of flood control and other hydraulic structures, together with river pollution, has caused a drastic decline in river fish stocks and fisheries, on which tens of thousands of people fully depended in the past. Although some mitigation measures have been introduced, these have resulted in only minor benefits for fish stocks.
The deterioration of rivers on a global scale has led to calls for amelioration of negative impacts and, in some situations, to the reclamation of the natural habitat wherever possible.
Length and production of the world's major rivers