The San Luis Rey River is a river in the southern part of the U.S. state of California. It begins in the Cleveland National Forest, near a mountain called Palomar Mountain, in several forks. It flows south into a reservoir, called "Lake Henshaw". It then turns west and flows to its mouth at the Pacific Ocean. Altogether, it is about long. Its watershed covers an area of 562 square miles. The lower part of the river is polluted, but the water in the upper part of the river can be used for drinking water.