The Tuolumne River originates in Tuolumne Meadows, the largest sub-alpine meadow in the Sierra Nevada. The river flows through tall grasses, hosting native Rainbow Trout, Brook Trout, and Brown Trout. The high-altitude environment shapes the behavior of these fish, which often seek slow or backwater areas to conserve energy. The river's ecosystem differs from lower-elevation streams, with much fewer aquatic insects. Only artificial lures or flies with barbless hooks are permitted in Yosemite National Park. Evening hatches of pale morning duns, yellow stoneflies, and caddisflies occur from June to early October.