I've seen guys fishing for salmon as far up as the Old Cummer Bridge.
https://maps.app.goo.gl/eBWtELD5T6S4odjw6Also now that the river no longer flows thru the Keating channel I think you may have some luck at the new mouth of the Don.
Check the regs for any restrictions. The mouth of the Don is part of the Zone 20 where as the Don itself is part of Zone 16.
Oh and if you really want to have some fun fish Centre Island. Pike, Bass and Salmon this time of year.