I've wondered about it too, I have seen fish rise in it. The other one I have been curious about is the walterville canal. regarding it the regulations say "closed: Walterville canal downstream from the salmon weir and in the bypass canal from the Walterville salmon weir upstream to the Mckenzie river. Which is a little confusing. Perhaps it would help if I knew where the salmon weir was.
There is some good fishing in there. Castmasters and spinners work great. I like to first sneak up on the bank and present either a small lure or a grasshopper right along the side of the bank and let it dead drift down maybe a foot from the bank. That kind of fishing usually means a lot of walking as the fish spook easily. There are some fat cutthroat, lots of little bows, and the steelhead or two. During the summer you can stop at the Blueberry farms on Greenwood drive and load up on pick your own. If it's real hot than just jump in the canal!
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.
I caught a ton of cutts and one fat rainbow in there last September. I was unemployed and went there just about every day. I caught them all on one secret lure I can't bring myself to share. The cutts ranged from ten to sixteen inches and the one rainbow was a fat seventeen incher. Then by the end of September they quit biting. They were in pre spawn mode but weren't dark yet and I'd bet that had something to do with the bite.