A local hiker told me that a land berm was built along the lower edge of this lake several decades ago to retain more of the water - to attract ducks for hunters. Hunting is no longer allowed here, but the enhanced pool appears to be well-loved by the local avian life. Several hundred ducks floated along the top of the water, occasionally flapping their wings, or diving down. Among them were grebes, coots, and mallard ducks.
Vernal Pools are shallow bodies of water that occur in depressions in grassland and woodland areas. These pools are filled by water from winter storms then dry up during the spring or early summer. Most plant and animal species that occur in vernal pools are endemic (occur only there) and highly specialized or adapted to vernal pool habitats, and some are listed as endangered species. (San Diego Wildfires Education Project)