 When tectonic activity first led to the uplift of the Sierra , the river formed as a steep stream eroding into the range 's western flank , carrying sediments that would later help form the floor of the Central Valley . A rich riparian zone around the Merced once supported millions of migratory birds along the Pacific Flyway , and the river had one of the southernmost runs of salmon in North America . Indigenous people , including the Miwok and Paiute tribes , lived along the river for thousands of years , thriving on the bountiful flora and fauna supported by the river and its diverse lower course , aided by fertile soils eroded from the mountains . 
