On Fridays, we have been looking at some of the characteristics of rivers, particularly those seen on the River Severn close to its source on Plynlimon.
In the first few miles, the river begins to cut a valley.
The valley is soon rather deep, as if the river is trying to get down to sea level as fast as possible.
There are lots of near vertical drops with waterfalls…
By the time the river reaches its first town of Llanidloes, 11 miles from the source, it has descended from 610m above sea level to only 160 metres above. That’s a drop of 450 metres in only 11 miles.
There’s only another 160 metres left to descend to reach sea level, but the river takes another 209 miles to do that.
If I put that information onto a scale drawing, you get this:
I’ve only plotted and joined three points – the source, the river’s height and distance downstream at Llanidloes and the point where it enters the sea at the Bristol Channel.
You could use a different scale and do it in a lot more detail, but the overall shape would still show the river descending steeply at first, losing most of its height early on, followed by a much more gentle slope for the rest of the distance to the sea.
This steep then gentle long profile is typical of many rivers. Obviously the starting point will depend on the height of the land, and different rock types on route will cause sudden drops and changes in slope, but many rivers have a long profile something like that of the Severn.
So far, we’ve been looking at features of a river in the steep section near the source. We will now be turning our attention to the river in the area of more gentle slopes downstream. Join me again next Wednesday.