The Nooksack, along with all other Puget Sound rivers, has a wild steelhead population that is listed as threatened under the ESA. So no directed fishing season exists for these runs. Fishing is allowed during December and January for the hatchery steelhead run, but that run is so poor that it barely returns enough fish back to the Kendall Ck hatchery to meet broodstock needs for its egg take.
There are a very few summer steelhead in the Nooksack, mainly in the SF, and the area they inhabit, along with wild spring chinook salmon, is closed to fishing.
The upshot is that the Nooksack River is open to fishing for steelhead, but not during the times and places when a decent number of wild fish are around.