The main photo shows the reservoir on World Photography Day, and the first PiP is of the reservoir in February 2014. It's possible to see that today's water levels are significantly lower following a long period of drought and unprecedented high temperatures in July and August, despite significant rainfall here at the beginning of the week. The other 2 PiPs show the Afon Clywedog, taken on August 19, which feeds the reservoir and lies to the west.

The dam is 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 does this 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.

The concrete buttress dam is the tallest concrete dam in the UK, with a height of 72 metres (236 ft) and a length of 230 metres (750 ft). When at capacity the reservoir contains approximately 50,000 megalitres of water.

It took me a bit of time to find the photo of the dam from 2014 in my archives. I must try a better system.