Spanish army officer Gabriel Moraga was one of the first Europeans to explore California's Great Central Valley (1806 -1808). As the first explorer, he became the source of many of the place names in the region, including San Joaquin, Sacramento, the Merced River, which he named El Río de Nuestra Señora de la Merced (The River of Our Lady of Mercy), the Kings River, which he named Río de los Santos Reyes (River of the Holy Kings) and Merced, Mariposa, and Calaveras. He named the Calaveras River after skulls found there. Mariposa was named from the swarms of butterflies.
Moraga led armies of Spanish soldiers through the valley and displaced local Native American groups.