Severn Stories

The River Severn starts as a small stream in the Welsh mountains.  Here, the rock is impervious, which means that it does not absorb water and sheds large amounts of rain.  This is why the river channel fills rapidly after heavy rainfall.
However, it also subsides quickly and for most of the year the river is quite low. This enables plants like this purple-loosestrife to grow in the shallows near the river banks.