The start of the Mersey, where the Tame and the Goyt converge, is under Stockport, more or less. I didn’t know it would be so easy to find when I chose to track it down. I guess I could track those two rivers as far upstream as possible to find the farthest point from which water flows into the estuary.
Update I think this is the source of the Tame. And the Goyt starts here.