The Mostazal River (Spanish: Río Mostazal) is a watercourse located in the Limarí Province of the Coquimbo Region, Chile. It is a significant component of the Limarí River hydrographic basin, situated in the semi-arid northern-central part of the country.
The river originates in the high Andes mountain range, near the international border with Argentina. It flows generally in a westward direction through the commune of Monte Patria. The Mostazal River is a primary tributary of the Rapel River (a different watercourse from the larger Rapel River located in the O'Higgins Region), which eventually joins the Grande River to form the Limarí River.
The hydrological regime of the Mostazal River is primarily nival, meaning its discharge is largely determined by the melting of snow accumulated in the mountains during the winter months. Consequently, its highest flow rates typically occur during the late spring and early summer. Because it is located in a transversal valley within a semi-arid climate zone, the river is a critical resource for local agriculture. Its waters are used extensively for the irrigation of vineyards, fruit orchards, and other crops that characterize the economy of the Monte Patria area.
Due to its dependence on snowmelt and the fluctuating precipitation patterns of the region, the Mostazal River is subject to significant seasonal and interannual variability. Management of its waters is a central concern for local agricultural communities and regional water authorities.