River Basins

Rivers may be conveniently divided into fast-flowing, upland reaches and slow-flowing reaches on the lowlands. Rapids reaches tend to divide into pools and riffles, which alternate along the river channel. The lowland reaches are normally flanked by floodplains, which often extend over large areas. To a great extent the ecology of a river is conditioned by its flood regime and the living organisms of the system including the fish are highly adapted to these seasonally rep etitive events. Because of the extent of the flooded zones and the increase in production which occurs during flooding, the lowland reaches of rivers usually support fisheries of exceptional value. The fishing communities which exploit rivers are equally adapted to the changes in flood regime and migrate from one area to another within the system or have alternative occupation for the flood periods when fish are less available. The fishing gear also tends to be adapted to the seasonal changes occurring within the fish communities, particularly where such fisheries have existed for a long period of time.