A River basin is the portion of land drained by a river and its tributaries. It encompasses all area of the land surface dissected and drained by many streams and creeks that flow downhill into one another, and eventually into one river. The final destination is an estuary, a sea or an ocean. There are basins which are closed and loss the totality of their water resources to evaporation and see page. As a bathtub catches all the water that falls within its sides, a river basin sends all the water falling on the surrounding land into a central river and out to the sea. The following are some of main features:
- A River Basin is a natural depression in the surface of the land often with lake at the bottom of it.
- It is the entire geographical area drained by a river and its tributaries.
- A drainage basin (also often: watershed, river basin, or catchments): is an area that topographically appears to contribute all the water that passes through a given cross section of a stream.
- A River basin is the portion of land drained by a river and its tributaries. It encompasses the entire land surface dissected and drained by many streams and creeks that flow downhill into one another, and eventually into one river. The final destination is an estuary or an ocean. As a bathtub catches all the water that falls within its sides, a river basin sends all the water falling on the surrounding land into a central river and out to the sea.
Generally a river basin with in a country is a hydro-geographical boundary of an area of land that feeds the water it receives to a common river or lake or swamp before it crosses the international boundary or terminates in sea or ocean. How this occurs is the product of the interactions between land and water, particularly the watershed's underlying geology, rainfall patterns, slope, soils, vegetative cover and use. It is a natural boundary that is permanent as opposed to Political or Administrative Boundary that is liable to change with change in governance.
Ethiopia has 12 Basins, 8 of which are River Basins, 1 Lakes Basin and the remaining 3 Dry, with no or insignificant flow out of the drainage system. The dome shaped nature of the country, with high raising mountains and a high tableland at the center that descends in all direction and the surrounding lowlands that circumscribe the plateaus is a peculiar nature of Ethiopia . The Ethiopian Rift Valley that dissected the highland plateau from North-east to Southwest which tilts from the center in the North-easterly and South-westerly direction is also responsible for creation of the Lakes Basin and determining the direction of flow of some rivers that terminate in the Rift-Valley System.
Due to the legacy of its proximity to the equator, and high altitude that rises above the surrounding regions, the upper catchments of the Ethiopian River Basins is relatively endowed with a better rainfall that is in excess of evaporation and seepage resulting in surface run-off that cascades to the surrounding lowland in all directions. The historical settlement in the highlands associated with fast growing population and total dependence on natural resources of the basins, resulted in the sever degradation of these highland areas.
The complex interaction between the climate, biophysical and socio-economic characteristics of Ethiopia resulted in important features of the basins such as high level of spatial and temporal variability of flow, enormous turbidity, and tremendous potential for hydropower in the highlands and of irrigation in the lowlands and sceneries along the major gorges.