Boulder - Grand Pass and Thunder Lake Trail

The Boulder - Grand Pass and Thunder Lake Trail is a 23.3 km trail that starts in Allenspark, Boulder County, Colorado. Based on our data, the hike is graded as Difficult. For information on how we grade trails, please read measuring the difficulty of a hiking trail on hiiker. Also, check our latest community posts for trail updates. This hike can be completed in approx 6 hrs 33 mins. Caution is advised on trail times as this depends on multiple variables. For more info read about how we calculate hike time.