The Severn Basin is irregular in shape, its topography and geology show great variety. Several geological systems are represented in the catchment area of the river.
Rainfall distribution in the area follows the topography, ranging from an annual average of 90 inches in the mountains of mid Wales to between 20 and 25 inches in the valley of the Warwickshire Avon.
Owing to the effects of topography, geology and rainfall, river characteristics show marked variation. The Upper Severn, Teme and their tributaries are fast-flowing, attracting migratory fish. Rivers in the eastern part of the area are most sluggish.