The next bridge after the Ashlu turn off is the Elaho bridge at about 28 km. The Elaho is the larger river that flows from the west of the Squamish and yes you travel directly west to follow this river rather than traveling up the west side of the Squamish. You can only go about 42 km on the Elaho spur before you get stopped by a wash out, if you don't have a lifted 4x4. The canyons up there are spectacular and the fishing is more difficult to access. There are no salmon or anadromous fish up there, but I'm sure there are plenty of residents.
As Ralph mentions you will have no trouble catching fish in poor visibility water but be prepared to accidentally floss and snag quite a few fish, particularly if you need to use a sinking line to get the fly down in front of the fish. In shallower water a floating line will help. I tend to change up to short floating a chartreuse or chartreuse/black jig. Without the swing there are a lot less snags/foul hooks and chartreuse seems to be more visible than pink.