Planning to fish the High Sierra requires some knowledge about snowpack and terrain. Keep in mind that year's with either big or late snow will likely delay the ice-out of high elevation lakes as well as the opening of the trails. This year (2013), the thin snowpack allowed most of the high elevation lakes to be fishable in late May and early June. Above an angler wades the rocky shoreline of lower Conness Lake at an elevation of 10,500 ft.
Although not native to the Twenty Lakes Basin in Inyo National Forest, several of the lakes hold California Golden Trout. Compared to a species like brook trout, I have found the goldens way more challenging to catch on the fly. Fly selection is part of that challenge, but locating them without spooking them can be half the battle.
Without the help of a float tube or pontoon boat, it's tough to get flies to where the fish are holding. Look for deep drops near the shore or find rock ledges. After ice-out, the fish can move in and out of the deeper parts of the lake and will cruise along the transition areas (e.g., rock ledges).
Another productive zone of high elevation lakes can be the in-flows and out-flows to and from the lake. Fishing these zones will depend on whether the lake level and snowpack are big enough to create the flow.
Several of the lakes in Twenty Lakes Basin have this amazing green / jade color. I believe its a result of the run-off from the Conness Glacier. The glacier is situated at about 11,548 feet and can be seen from Saddlebag Lake to the east. Conness Glacier is the largest glacier in the Sierra Nevada north of Tioga Pass.
I think Jared Smith summed up the experience best in his book Fishin' Trails "Imagine a place where there is literally zero chance of a car alarm going off within ear-shot. Sounds great right? Now add to that the fact that this place is absolutely loaded with some of the feistest and most beautifully-colored trout on earth. Now you have a mere glimpse of what backcountry fishing is all about."