My name is Seren and today I'm investigating the UK's longest rivers. This is the River Severn, and at 354km long, it's the longest river in the UK. It starts here in Wales, where we call it Afon ...

But first, let’s zoom out. I’m on the River Severn at this point here on the map… But the river actually starts up in the Cambrian Mountains of mid-Wales here, and runs all the way to into ...