The fish in the San Juan feed mostly on tiny midges so most of the time you are fishing extremely small (size 20 to 24) midge imitations. It is quite challenging to land such large fish on such tiny flies and light tippet but it can be done, and it is a lot of fun and great sport.
I took a day trip up to the Juan yesterday. It was cold but sunny and not too windy so it was pretty pleasant out on the water. My Simms G4 Goretex waders kept me dry and (relatively) warm.
I started out near the dam and didn't have much luck. No one around me was really catching anything. The fish were not feeding actively.
I moved further downstream to the "Kiddie Pool." In the upper run the fish were feeding aggressively. I landed 11 fish in a relatively short period of time - all of them rainbows. The fish were hitting on a size 24 midge with a black body and a flashy material on the head. I fished the flies below a Thingamabobber Strike Indicator. If you do a lot of nymph fishing than be sure to check out the Thingamabobber - it floats high, is sensitive to strikes and is easy to adjust on your leader.
I decided to head down to the Texas Hole for some streamer fishing after that. The Texas Hole is a huge cut that is probably the most popular spot on the whole river. Fishermen line up side to side on the hole and fishing guides bring their clients to drift the hole on boats. The Texas Hole is usually a pretty happening place.
The reason that I like streamer fishing on the Texas Hole is because I can get by with thicker tippet. Also, it is much easier to get a solid hook set with the bigger hook. For these two reasons I have a better chance to land a true trophy trout. I have hooked into several huge (25 inch plus) trout on the San Juan, and it is incredibly tough to land them on 6x tippet and size 22 flies. It can be done... but luck had better be on your side.
I didn't really have much luck streamer fishing on the Texas Hole this day. I only got one fish in my Brodin Net and it wasn't very big. Nonetheless, it was a fun day on the river. The San Juan is always an adventure.