From its origin at the Cowlitz Glacier, the Muddy Fork of the Cowlitz River rips down from Mount Rainier's southeastern slopes. Hundreds of thousands of years of flow has culminated in the creation of the Box Canyon, a 180-foot deep, 13-foot wide chasm through the stratovolcano's andosite bedrock.
From Stevens Canyon Road, visitors can easily pull over and get a quick glimpse of geological spectacle or jump onto a short, 0.4-mile loop that crosses over the canyon. While on this short walk, note the hard granite bedrock on the slopes adjacent to the trail and the scars and scratches that appear on the mostly smooth surface. Previous glaciation smoothed the bedrock, and debris stuck in the ice also scarred the rock over time as the glacier receded and advanced.
Interestingly, the Muddy Fork of the Cowlitz River and the Ohanapecosh River are the only two water catchments from Mount Rainier that don't eventually flow into the Puget Sound. Both rivers (eventually the Cowlitz River, as they combine) are tributaries of the Columbia River.