Green River, Utah.  Kayaking in early-morning reflections along Bonita Bend on the Green River in Canyonlands National Park, Utah. For 125 miles below the town of Green River, Utah, the Green River meanders slowly south towards its confluence with the Colorado River. Several miles after combining its flow with the Colorado, the river enters the roiling rapids of Cataract Canyon. Although others explored these canyons prior to the Powell Expedition of 1869, Major John Wesley Powell led the first well-documented scientific exploration of the canyons of the Colorado.