Arroyo de en Medio (Spanish for "in the middle creek") is a 2.5-mile-long (4.0 km) coastal stream in western San Mateo County, California. Arroyo de en Medio rises on the western slopes of Montara Mountain and discharges to the Pacific Ocean at Half Moon Bay at the location of the unincorporated community of Miramar at Miramar Beach. The watershed of Arroyo de en Medio consists of relatively permeable sandy soils capable of significant recharge to its aquifers, which supply considerable potable water to the local area.