Depending on when you visit the river mouth, the river may or may not be flowing out to the bay. The interplay between sand moved by ocean currents and the river creates a seasonal sandbar at the rivermouth. When we get enough runoff from the rain, high river flows come downstream and break through the sandbar, and the river drains visibly into the ocean.

When the river’s flow is too little to push the sand out of the way, the sandbar disconnects the river from the ocean and creates a lagoon. Fresh water is still flowing into the sea, its just below ground through the beach sand! When this happens, the water in the lagoon is called brackish; it’s a mix of saltwater from Monterey Bay and freshwater from the San Lorenzo Watershed. The lagoon can even convert to 100% freshwater over time as water flows down through the river.

Fish Need the Lagoon

The lagoon is critical habitat for many species of fish. Anadromous fish like the threatened steelhead trout and endangered coho salmon start their lives in freshwater, travel to the ocean, and return to the freshwater to breed. Ocean species like starry flounder and topsmelt use the lagoon as a nursery. Juvenile steelhead and coho salmon rely on seasonal freshwater and brackish water conditions in lagoons to make the transition from freshwater to saltwater. When lagoon conditions are optimal, plentiful food helps them grow quickly, which results in greater ocean survival while a large habitat area allows them to evade predators.

Another endangered species relying on lagoon conditions is the very small Tidewater Goby. These fish, less than two inches in length, can breed year-round, but rely on the calm summer conditions in the lagoon for maintaining sufficient population numbers to respond to rapidly changing lagoon conditions. While this species is fairly tolerant of a wide range of water quality conditions, the population size may suffer if sandbars fail to form, or if closed lagoons are artificially breached and rapidly drained.

Plants Along the River

The plants along the river today are a mix of native and non-native species. Some of the most common native species include the arroyo willow, the California blackberry, and the coffeeberry. Can you spot some? This is what they look like:

Many of the plant species that are most easy to see are non-native. This is the case for the towering eucalyptus trees that line the path on the east side of the river. The eucalyptus trees were intentionally planted to help stabilize the steep bank and to reduce sedimentation in the river. Riprap, or more simply put large loose rock, was used to stabilize the less steep portion of this reach of the San Lorenzo. The rock became habitat for weedy and non-native species. Other non-native species include fennel, pampas grass, and kikuyu grass.

Look for the Birds

While most human visitors come to Santa Cruz during the summer months, lots of birds spend their winter on the banks of the San Lorenzo River. Look for Buffleheads, Common Goldeneyes, and Common Mergansers. Towards the trestle bridge you may see shorebirds like Yellowlegs, Dowitchers, Peeps, and the Semipalmated Plover as they pass through Santa Cruz on their migration routes.

Spotted Sandpipers can be found along the bluffs of the river mouth, but if it’s not mating season, don’t expect to see these handsome birds in pairs as they are not very social birds. Seafaring birds like gulls are present all winter but are most numerous in their flocks during the month of January. Birds also like to nest in the eucalyptus, and throughout different seasons you can find Double-crested Cormorants, Peregrine Falcon, and even the occasional Osprey hanging around the river just like you are! Click on the bird names below to go to eNature.com and listen to their sounds:
Double-crested Cormorant
Peregrine Falcon
Osprey

The River’s Future

Imagine seeing the river mouth before urban development, when this important habitat was teeming with fish, birds and native plant species. You’ll still see some of these here today, but with increased attention on beneficial management practices imagine what could be. The Coastal Watershed Council (CWC) advocates for the preservation and protection of coastal watersheds through the establishment of community-based watershed stewardship programs. They along with the San Lorenzo River Alliance are working to change how people think about and can use the San Lorenzo River.

Take the Self-Guided Mobile Tour

This piece is part of the San Lorenzo River Tour by the Coastal Watershed Council. Download the free app with many tours of the Santa Cruz area and beyond.