Ask Lookout: The San Lorenzo River water level was high in July. Is that normal?

It hasn’t rained in months, so high water levels in the San Lorenzo River probably seem bizarre in the dead of summer.

In July, the City of Santa Cruz posted an alert that the river was running high and that low-lying areas immediately around the river could see minor flooding, but this never happened.

Those high water levels aren’t due to an excess of stormwater running off into the river, which empties into Monterey Bay near the Santa Cruz Beach Boardwalk. They are actually normal for this time of year, caused by a slow accumulation of water. Waves that come from the south push sand against the mouth of the river, which combines with low water flows in the summer to create a sand bank, closing off the river. That phenomenon is called “shoaling” and can cause the river to fill up and eventually overflow, since the water no longer has anywhere to go.

The Santa Cruz Harbor’s jetty also plays a role in trapping water in the river. Sand that used to flow downshore began getting trapped by the jetty and slowly piling up at the rivermouth and nearby beaches.

A major project that the city completed last summer aims to help with this problem. The San Lorenzo River Lagoon Culvert Project seeks to set up a water height-control system for the lower portion of the river where it empties out to the ocean. That includes installing two long pipes that run from the trestle bridge to the river’s mouth. One pipe is designed to move saltier water out of the river while another larger pipe assists with moving a greater amount of water out of the river lagoon to prevent flooding.

The infrastructure is supposed to prevent flooding, first and foremost, but as water flows are still on the higher end, the culvert system is not able to expel the necessary amount of water to prevent minor flooding. That means that manual breaching — using heavy machinery to dig a canal from the lagoon to the ocean to release excess water — could still be necessary. The project is intended to reduce the need to do so, even if it doesn’t quite fully eliminate it.

City public works spokesperson Ashley Hussey told Lookout that there is currently no breach scheduled, but that manual breaches could take place any time until the winter, which is when the shoaling stops. Waves typically come from the northeast in the winter, which does not create the same sand bar that the summer southerly swells do, meaning that the mechanism is intended to be operational only in the summer.

Hussey said that as water flows decrease in the late summer and early fall, the city will make necessary adjustments to the culvert and work with design engineers to ensure the system can move enough water to maintain a water elevation of 5 feet in the lagoon.

However, Hussey added that the city has seen a reduction in the number of breaches necessary, which means the culvert project has largely done its job so far. The city has had to breach the lagoon only three times this year, all in July, and three times in 2024, during the fall. That compares to nine times in 2023, before the culvert was fully functional.

The project is also supposed to help with wildlife protection. A lagoon breach can contaminate the mostly freshwater habitat of the river with the ocean’s saltwater, changing the environment for endangered species that rely on a freshwater ecosystem.

The city’s project page says that data collection focusing on how effective the culvert is at preserving the habitats is ongoing, but early findings appear to show that it is reducing the salinity of the river lagoon, thus improving the fish habitat.

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