The Rio Mostazal is a freestone river flowing from east to west through the Coquimbo Region of northern-central Chile. Originating in the elevated eastern reaches of the region, the river descends through a landscape shaped by the characteristically dry climate and rugged valleys of this part of Chile. As it travels westward, the Rio Mostazal eventually transitions into the Rapel River (Coquimbo), which in turn feeds into the Embalse La Paloma, a large reservoir that serves as a critical freshwater resource for the region. The river exists within a broader hydrographic network that includes other notable waterways of the Coquimbo Region, making it one of several rivers that sustain life and agriculture in an otherwise arid environment.