The North Cascades mountains rise nearly vertically from the floor of the canyon and surround it on both sides. Directly upstream of the dam, Ruby Creek flows into the reservoir, forming the Ruby Arm of Ross Lake. Directly downstream, Diablo Dam forms Diablo Lake, much smaller than Ross Lake. Other creeks that flow into Ross Lake include Little Beaver Creek and Big Beaver Creek from the west, and Lightning Creek from the east. North Cascades Highway, also known as Washington State Route 20, follows the Skagit River to where it turns away from the river at Ross Dam and follows Ruby Creek instead.