I know of two, possibly three points to access the Stillwater upstream of the singing bridge. In years past there have been problems with beaver dams blocking the upstream migration in the area around rt 190. The town of Sterling has an exemption from the trapping ban so this may not be as big of a problem anymore.
I'll try to get out there next weekend and see what the situation is. Of course, given that Quabbin access is currently closed, closure of the Wachusett may happen before the run