I was surprised to find out that Trillium Lake is fairly young, having only been formed in 1960 by the damming of Mud Creek. The lake is deepest at the southwest corner where the dam is located (20+ feet deep) but the majority is less than 7′ deep. This is one of my favorite places on Mt. Hood not only because of its beauty but because it’s a very quiet place despite having lots of campground space.
There is a great trail around the perimeter of the lake. Because the land around the lake is very marshy, much of the trail is on elevated wood platforms. This is a well-built and maintained trail, but beware- there are a few places where the wooden path has been reclaimed by the lake, but nothing that can’t be jumped over.
These photos are from August 10th: