Middle Boulder Lake is a lake in Klamath National Forest, Trinity Alps Wilderness, CA.
Middle Boulder Lake is lightly used, unlike it’s larger and prettier sibling, East Boulder. The access trail is about 2 miles long; the lake lies at the foot of a big sloping meadow. In summer the lake warms rapidly, and weed banks grow in the shallows. The water gets murky, too. There is a large campsite on the southeast, next to the small stream which flows into the lake. Brook trout inhabit the lake, but appear to suffer heat stress later in the summer.
Middle Boulder sits in an interesting spot, geologically. The orange ridge to the east of the lake is mostly peridotite, a basaltic seafloor rock; while the cliffs and peaks to the south are formed from a later granitic intrusion. The contact between the two formations runs right under the lake, and the small stream flowing into the lake appears to follow (mostly) the zone of weakened rock at the contact. In the first picture, above, the reddish ridge on the left is peridotite, and the gray, more tree-friendly slope to the right is granitic.