RE:Seattle trip 10/11-10/13
If you have the choice, I'd recommend the fall. Mid - late September is ideal - we usually have good weather, the flows in the Yakima drop (usually in the first week of September) and the river becomes wadable. It's great to fish it at this time - just bring your small caddis and mayfly patterns (#16-#20) and drive down the Yakima Canyon Road (it's about 2 hrs from Seattle). Lots of access from the road, and it's 90% public land. Wonderful desert county and LOTS of water to fish. The weekends get a bit crowded, but the weekdays you'll have most of the river to yourself. There are plenty of great campgrounds along the river, or you can stay in a motel in Ellensburg - about 10 minutes drive from the Canyon entrance.
Fishing on the Yakima at that time of year is self-explanatory (if you're an experienced trout fisherman)- the pools are well defined when the water level is that low - you'll be wading 3/4 across a 150' wide river to fish the deep pools under the willows along the far bank. The fish key on small nymphs and drys and I know of people scoring on everything from low-water emergers to your basic #18 Parachute Adams (probably the best searching fly that time of year).
The summer steelhead fishing is at it's best this time of year, too. The rivers are low and clear and this is when floating lines, skated muddlers & riffle-hitched speys really come into their own. The Snoqualmie and Skykomish are both great destinations for this. On bright days, the best action is (really) early mornings and evenings.
If you're thinking about a trip down to the Deschutes, this is also a good time to go there. I'll defer to Andre about that one, tho!
Give a holler on the board if you're headed out this way - most of the hardcore steelheaders really focus on fishing at that time of year and you'll definitely be able to join an experienced fisherman (or two) for a trip if you're here a week.
Hope this helps,