In an article published in the Courier-Islander on June 08, 2012 (found here) BC Hydro spokesperson Stephen Watson says the diversion isn’t a barrier.
The current fish-way built by the DFO in the early 1980’s is only effective under certain flow conditions. As seen in this picture, when river flow levels are high, the spillway and undersluice flow attract the fish. This causes the “obstacle” to become a barrier. Three species of fish; steelhead, coho, and chinook; are prevented access to 42 km’s of anadromous water beyond this barrier. The 42 km’s are comprised of 27 km in the Salmon River and 15 km in Grilse Creek, of which the Salmon River Diversion is the only barrier.
In the article after stating the diversion isn’t a barrier, but an obstacle, Steven Watson goes on to admit “We agree, at times they may be delayed in moving upstream if flows are too high. ”
The current system to mitigate this event is for fish to be helped manually by volunteers and BC Hydro employees.