The river Severn is Britain's longest river at 350km, with a catchment covering 11,000km2. Its source is in the Welsh Mountains on Plynlimon at 741m. The river falls quickly falls through steep-sided incised valleys to 198m by the time it reaches Llanidloes, a distance of just 19km. It then flows East and South to its mouth at Avonmouth near Bristol. The geology and climate of the River Severn's catchment help explain the pattern of flooding.
Bewdley Case Study: Catchment Overview
The geology of the upper catchment is made up of Ordovician and Silurian shales; these are hard rocks, resistant to erosion with many steep slopes. These rocks are also impermeable, allowing rapid runoff into the river, although Lake Vyrnwy and Llyn Clywedog help to control the flow of the headwaters in the upper catchment.
Further downstream, the geology is more varied with glacial and post-glacial sands, alluvium and river terrace gravels. These are mobile sediments that are transported and continuously re-worked by the river. The underlying bedrock here varies with some sandstones and limestones, but in the lower course it is mainly mudstones and Lias deposits. These softer rocks are more permeable, and result in fewer steep slopes. Much of the middle and lower course has deep loamy soils.
Geology map ©Cheryl Jones 2003 - www.geopark.org.uk
Annual precipitation in the upper catchment in the Welsh mountains is over 2,500mm but most of the rest of the catchment area is close to the average for Britain at under 700mm per annum. Rainfall in summer is mainly caused by convective storms. In contrast, precipitation during autumn and winter is generally due to weather fronts and low-pressure systems (depressions), and tends to be of higher volume. This, together with the topographic effect, can result in heavy rainfall falling on a near-saturated upper catchment during autumn and winter.
The River Severn shows a classic transition from incised, steep valleys in the headwaters around the Rivers Vyrnwy, Tanat and Morda, to wide floodplain valleys in the lower reaches. Generally the bankfull water width and depth increases downstream while the water surface slope decreases, and bed gradient decreases. Near the source the channel is 1m wide, whilst nearer Tewkesbury the width is 70m.
The River Severn's sinuosity, which is influenced by geology, gradient and human modification of the floodplain, generally decreases downstream.
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