In October the stream flow on Boulder Creek will be 20 to 35 cubic feet per second (It is 24cfs as I write this) and the water temperature will be 42 to 52 degrees. On sunny days the water will start out cold and warm up fast. At 10:00 in the morning the stream temperature can be 43 degrees and by 2:00 in the afternoon it can be up to 48 degrees. The mornings can be downright slow fishing, but the afternoons can be fantastic!
You will see small BWO mayflies hatching in the early afternoons on the creek...and many of the trout will be eating them off the surface, but the most productive method is still going to be the dry/dropper rig using a #16 caddis dry (Elk Hair Caddis, Stimulator or Clown Shoe Caddis) with a #18 weighted nymph hanging off the bend of the dry about 12 inches. Use a BWO Tungsten Micro-Mayfly nymph or a Poison Tung.
The brown trout in Boulder Creek will begin spawning the second week of October and will go at it approximately three weeks. Sometime around the 6th or 7th of the month you will see the trout messing around with each other and acting funny. You will not have to adjust the way you are fishing, just move slower and try to make longer casts upstream, because the browns will be holding in very shallow water over loose gravel.
By mid October the leaves will start falling in earnest. The leaves will clutter up the pools and eddies and create an annoying obstacle. Many will be bright yellow in color and will make seeing your dry fly or indicator very difficult. They will continually get snagged on your flies as well. This is very irritating, especially during breezy days when there are a lot of new leaves being added to the mix. To overcome this I use a Clown Shoe Caddis dry fly tied with a fluorescent cerise tuft of McFly yarn on the top. You will be able to pick out this color easily amongst all the yellow leaves.
And don't forget to eat some Creek Apples!