Castle Lake: Located in the Mount St. Helens National Volcanic Monument, Castle Lake requires a 20-plus mile drive over rough logging roads, followed by a tough hike down a steep hillside to the lake. The key to finding the lake is to first find the Weyerhaeuser 3000 road. Would-be anglers not familiar with the area should get a St. Helens West hunting map, published by the Washington Forest Protection Association. Those who put forth the effort have a chance to catch dandy-size rainbow trout; fish up to 10 pounds have been reported. This is a self-sustaining fishery, with no fish planted, so selective gear rules are in effect, along with a one-fish, 16-inch minimum size limit. The lake is open year-round, but the road in is usually blocked by snow until at least May.
I have fished Castle lake and will concur with the previous posting that there hare lots of 12"-15". However, if you fish leeches or damsel nymphs deep and slow, even with full sun on the lake, you will have the chance to hook into some of the "monsters" of Castle Lake. The biggest ones I have caught are only in the 5 to 6 lb range. but understand that if the lake is full of 12" to 15" year after year, they do grow up. The bigger ones have just figured out that they don't need to come up for the little bugs when there is bigger stuff to eat deeper. The north and east sides of the lake seem to be the most productive fished from a float tube. cast to the shore and strip slowly as the fly drops in depth. the deepest part of the lake is about 100' so don't worry about gong too deep but you might have to let your line sink for a while. I've found a lot around 30-40' on the drop off from shore.
unfortunatly the other posting is also correct about the hike in and more specifically about the hike out. you can take the elk trails(shortcut) down as gravity is working with you, and you can be at the lake in about 20-30 minutes from the trailhead. Just remember what you take DOWN you'll also take back UP! So be ready for a longer return.
Chances are, if you make the trip, that there will probably be only a few other people, if that, on the lake.
This is not a trip that anyone goes on to catch a few hours of fishing and you'll maybe make it once more this year if you do well so DO: BRING YOUR CAMERA AND TAKE LOTS OF PICTURES, LIGHT GEAR. BOTH DRY FLY AND WET, LOTS OF FLIES, THE FISH HIT LIKE FRIEGHT TRAINS AND IF YOU FISH WITH LIGHT TIPPETS LIKE ME, YOU'LL PROBABLY BUST OFF A FEW.
Very seldom anymore will you find a pristine, crystal clear lake with sooo many truely native rainbows and sooo few people to spoil it. Keep only what you will eat and release all others. And have one hell of a great time. Wish I was going with you.
Have fun!
Don-on the fly