From cruising google maps, I think the salmon weir they are talking about is west of walterville, off camp creek rd, just a few hundred yards from where the canal rejoins the Mckenzie.
Map here, and the bypass canal is the small stream that cuts SSE to the Mckenzie. I am guessing it is designed to keep salmon from taking a shortcut up the canal to the dam. That would mean that just the few hundred yards below the weir, and the few hundred yard bypass canal are off limits, the rest of it is open to catch and release, artificial flies and lures only, as is leaburg canal above the powerhouse. At least that's my interpretation.
Michael