San Luis Rey Watershed

This is the last part in the series of results from our water monitoring lab. If you haven’t read our watershed report, head over here and check it out. In this final entry in our series, we take a look at the San Luis Rey Watershed.


Photo Credit Shannon Switzer,

The San Luis Rey Watershed splashes down from its headwaters in the Palomar and Hot Springs Mountains before hitting the ocean at the shores in Oceanside. The San Luis Rey watershed lies in the northern reaches of San Diego County. The watershed stretches across 560 square miles, making it the third largest in San Diego County. The San Luis Rey Watershed may be big, but its human population is one of the smallest. Unlike many of its southern neighbors, the San Luis Rey Watershed contains very little urban landscape. Vacant/open space takes up half the watershed, while agricultural (cattle grazing, nurseries, citrus and avocado groves) and residential each account for about 15 percent of the watershed. Local jurisdictions occurring within the watershed include the cities of Oceanside, Vista, and Escondido, and the counties of San Diego and Riverside. The territories of six federally recognized  Tribal Indian Reservations also overlap with this watershed.

Our water quality index scored this watershed as “Fair.” Pollution sources in this watershed include residential and commercial activity, but this watershed also contains a significant amount of agricultural activity, a source of pollution that is more challenging for regulators and local governments to abate. Excess nutrients (nitrate and ammonia) are driving this low index score. I suspect that past and current agricultural fertilizers are contributing to this pollution.

Oncorhynchus_mykissLike many other rivers in southern California, the San Luis Rey River suffers from habitat degradation. Historically, this river was home to abundant Steelhead Trout. Like it’s salmon cousin, steelheads are born in freshwater streams and migrate out to sea. The steelhead return to their place of birth to spawn future generations. Dams and hydromodification have blocked these fish from their ancestral home.

We partner with the Golden State Flycasters to monitor the San Luis Rey River. Coastkeeper provides equipment and analysis, and the Golden State Flycasters do the rest. In addition to supporting flyfishing throught the county, the flycasters are working hard to protect and restore habitat that sustains healthy fish populations.

Thanks for following along as we explored watershed health in the five part Watershed Report series. We hope you’ll continue to pay attention to the health of our waters and explore volunteer and advocacy opportunities with San Diego Coastkeeper.