This is Lake Helene (10,643′) at the base of Notchtop Mountain (12,129′) in Rocky Mountain National Park. The lake is a 2.9 mile long hike from the Bear Lake parking lot. It is considered a moderate trail, but it does climb about 1,200′ from the parking area to the lake.
You can do this hike as an out and back hike, but I suggest making a day of it and going all the way to Fern Lake, adding another 5.7 miles onto the trek. It is mostly down hill from Lake Helene and once you reach the Fern Lake area you can catch a bus back to the Bear Lake parking lot. It is a great way to spend a day in the wilderness.