Steelhead in San Mateo Creek spawn primarily in the winter, when heavy storm runoff from the Santa Ana and Santa Margarita Mountains raises the creek's flow enough to breach the sandbar at San Onofre State Park. In the few weeks or months following when there is still a substantial flow in the creek, the steelhead spawn and then the adults return to sea. In the dry months following the streams in the lowlands shrink to small pools that still support amphibians and small fish including steelhead fry which need to spend about one year in freshwater. Recently, steelhead have been sighted in the creek in greater numbers, bringing attention to coastal San Diego and Orange County streams.