From the Wild Basin parking area, head out west on the trail on the north side of North St. Vrain Creek, called the "Wild Basin Trail." The trail passes Copeland Falls after .3 mile and continues another 1.2 mile to an intersection. This first section of trail has little elevation gain on a wide and heavily used trail. At 1.5 mile from the TH, continue on the "hiker only" trail that heads on up to Thunder Lake (right fork), now gaining more elevation. In another 1.3 miles a trail joins in from the left. (That trail leads up to Ouzel and Bluebird Lakes.) Just beyond this intersection is one of the designated backcountry campsites called the North St. Vrain, because it's located near that creek. It has two tent sites, each of which can accommodate a couple tents and there's a backcountry toilet. It's located in a section of lodgepole forest. Another .2 mile farther is the Siskin campsite. It has one tent site and no toilet. From the prior trail intersection for Ouzel Lake, an additional 1.3 miles brings you to yet another trail intersection. The right fork goes up to Lion Lake. Take the left fork here for Thunder Lake, which will be another 1.8 miles. There is another backcountry camping site here that accommodates horses. There are three sites and a backcountry toilet. The trail all the way to Thunder Lake is well-used, easy-to-follow and fairly gradual considering it gains over 2,000 feet in elevation over nearly six miles. At Thunder Lake, there is a maintained patrol cabin. The overall location is quite scenic with Mt. Alice and Pilot Mountain dominating the view to the NW. The following link will take you to a RMNP website that provides most of the information you will need to plan a backpack trip into this area.