Upper Thunder Mountain Lake lies at roughly 6400', just east of the Cascade crest, and therefore drains towards Icicle Creek and Leavenworth. Most of its modest shoreline consists of granitic slabs or large granitic boulders that have arrived from the surrounding slopes. The steep, rocky shores don't lend themselves to beaches, because there is little sand or gravel size sediment and even if there were, it would be lost rapidly to the bottom of the lake.
But at the north end, where the bottom is more gradual, a beach has formed. More accurately, a small stream delta has formed, but there is sufficient wave action to have reworked it into a beach. A series of berms mark the progressive fall of the lake over the summer.
AERIAL VIEW The source of the sediment is a small basin that yields grus (the granular remains of weathered granite) that the seasonal stream can easily carry to the lake. Most of the action probably happens in the early summer by water flowing from the melting snowpack or perhaps in occasional heavy rains. I suspect that lighter rains probably soak pretty fast into the porous soil and yield little surface flow to transport material, but this is not exactly a system I'm familiar with.
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