The rivers Wye and Severn begin less than 4 kilometres apart on the side of Plynlimon.
They flow down adjacent sides of the mountain and as the Severn approaches Llanidloes, the Wye is still just 4 kilometres away on the other side of the hills.
The line of high ground is part of the watershed, that divides the water that flows to the Wye (on the other side), from the water that flows towards the camera and into the Severn.
And those hills make a big difference.
Over the hill, the Wye turns south and takes a fairly direct route towards the Bristol Channel. Meanwhile the Severn heads north-east, away from the sea, and makes a huge loop through Shropshire, Worcestershire and Gloucestershire before widening into the Bristol Channel.
So the two rivers finally meet at the end, but arrive there by very different routes.
Pull out a road map and follow each river.
We will be continuing our downstream journey in the New Year.
Meanwhile, there will be a map in your inbox this week. (If you haven’t yet signed up, fill out the form in the sidebar.)