Llyn Clywedog

The Clywedog Reservoir is a reservoir near Llanidloes, Wales on the head-waters of the River Severn. The construction of the reservoir was enabled by an Act of Parliament which asserted that "At certain times the flow of water in the river is inadequate ... unless that flow were regulated so as to ensure that at those times water in addition to the natural flow will flow down the river." Its primary purpose was to regulate the flow in the River Severn to mitigate flooding and provide support for drinking water abstractions for the West Midlands. It was completed in 1967 and is situated near the B4518 road north of Llanidloes, Powys.

It also regulates the flow of water in the River Severn by releasing water into the river channel during low flow periods and re-filling during the wetter winter months. This enables major water abstractions to be made from the River Severn to supply the West Midlands with 50 million litres of drinking water.