I am not sure where the existing footbridge or the weir below Mather's weir are, can you be more specific?
Walked the beck again today and between Easterly Road and south parkway. It's easy on a map to get the impression foundry mill was up a slope/hillside because it's set back from the beck but the land is quite flat between the Beck and the Mill site.
Walking back up towards Roundhay the beck becomes the straightened version and despite the obvious 20th. Century walling you can see some old low courses of blackened stone helping keep the beck straight. What's left is part hidden by overgrowth on the bank top
There's only a bit here and there left and consequently the beck (and this was fascinating) is well on the way to reverting to a "windey" (as in wind from side to side) beck carving out curves either side that cause collapse to the footpath........
When it was originally straightened we don't know do we? But it could be well after mathers??
The weir is below the footbridge that connects Kentmere approach and the footpath at the end of the road over the beck and through to Grange Park Crescent.
You can stand on the footbridge today and the water is rushing over the weir. I posted some time ago that something had been done to this weir very recently between my walks over the last few years. It seems to have had an extra span of stone/wood placed in there.
Again as an "Oldie" sick of fighting technology rather that having it work for me, I took no pictures and unable to find Brunel's kind picture I can't upload what it looks like now but suffice to say it is a weir and it looks like it still serves a purpose.
If we can establish what that purpose was - and probably still is - then ipso facto it's not a weir for powering an undershot waterwheel.