Bearing in mind their average scull probably draws about 6" on a bad day.....
Streetmap.co.uk- search results for 278882,694995
Ignore the arrow, it's just there to make sure everyting I'm about to talk about is visible!
You can see the railway crossing the river north of the station. There are two road bridges upriver of the rail bridge. The first of these is the "new" bridge carrying the A9. At high tide there is plenty of water to get a rib through both it and the rail bridge. Note the gothic script describing the furthest upriver bridge. This is the one you see on postcards (and the location of a famous battle for the historians amongst you). At low tide you could probably "white water raft" it in an Avon Redstart, high tide I think
could be done with outboard fully down.
The weir is somewhere around the island NNE of Jn 10 of the Motorway above Old Mills farm (there's a clue in the name!
) . If you zoom down a level the 1:25K shows it better.