he San Luis Rey River is a river in northern San Diego County, California. The river's headwaters are in the Palomar Mountain Range and Cleveland National Forest, near Palomar Mountain and the Santa Rosa Mountains. The river mouth, on the Pacific Ocean, is in northern Oceanside. Length: 69 mi Discharge: 36.3 ft³/s Source: Lake Henshaw Mouth: Pacific Ocean Country: United States Basin size: 557 sq mi (1,440 km2)