The source of the River Severn is high on the side of a mountain (Plynlimon) in the middle of Wales.
The river begins in a boggy bit of open moorland but after about 1 kilometre it crosses into the Hafren Forest.
For the next 5 kilometres the river is surrounded by trees, mostly conifers, clinging to the slopes of the valley sides. The Severn’s tributary rivers also pass through the forest.
Less than 4 kilometres from the source of the River Severn is the source of another of Britain’s major rivers – the River Wye. It also begins high on the mountain in a similar bit of boggy open moorland but as it descends there is no forest to pass through. The River Wye and its tributaries flow down over a virtually treeless landscape, covered in low growing moorland plants and grazed by sheep.
Both rivers have a network of gauging stations and one of the ways that the data is used is to compare how each river responds to rain.
When it rains on the Hafron Forest, the trees catch the rain. If the rain doesn’t last long the water might just evaporate off the trees with hardly any finding its way into the river. If the rain is very heavy, or lasts for a long time, it will eventually drip through the trees and trickle down towards the river, but that is going to happen a lot more slowly than over the other side of the hill, where there are only low growing moorland plants to slow the flow of water into the River Wye.
And why do we care?
Trees slow the progress of water to the river and thus planting and maintaining tree cover is important for preventing flooding downstream. Yet trees are also a crop – 29 000 cubic metres of wood is harvested from the Hafren forest each year.
Comparing the response to rainfall in these two rivers helps us to work out the best way to manage the landscape so that trees are harvested but also flooding prevented.