Loch Lomond temporarily closes for water treatment

The Loch Lomond Recreation Area temporarily closed this week for water quality treatment.

The area closed Monday and will remain off limits until Saturday morning for precautionary algae growth management to ensure future water supply quality, said Chris Coburn, deputy director for the city of Santa Cruz Water Department.

“This planned closure allows for a proactive water quality treatment designed to preemptively manage algae growth and safeguard the aquatic ecosystem,” the city said in a press release. “By prioritizing the health of the reservoir now, the Santa Cruz Water Department is taking essential steps to maintain a reliable and high-quality water supply for the community.”

Coburn said the treatment is not a reaction to a problem, but the city’s way of trying to ward off conditions that naturally occur in a reservoir such as algae growth.

During the closure, the city applied a product called alum — or aluminum sulfate — to the water, which is a drinking water-approved chemical that prevents algae from growing, Coburn said. The city used this process in 2025 for the first time and saw positive effects, he added.

This is essential, as algae growth can lead to not only taste and odor problems, Coburn said, but potential toxins in the water.

“Last year we saw real diminished occurrence of the algae growth after we applied the product,” Coburn said. “So I’m optimistic that this alum is going to be a good tool in the toolbox to help us really control the growth of algae and prevent it from occurring in the first place.”

Loch Lomond is the city of Santa Cruz’s only surface water reservoir, holding 1.5 years of storage, Coburn said. The reservoir is a part of a larger network of surface water sources, such as rivers and streams, that the city uses to supply water to over 100,000 people in homes, businesses, schools and other public services.

An increase in unusual weather patterns has forced the city to rely on Loch Lomond heavier, as inconsistent rainfall has affected stream flow of the city’s rivers, creeks and streams. Therefore, Coburn said, managing water quality is as important as ever.

Now that the rainy season is approaching its end, the city decided it was the best time to treat the reservoir before the sunnier, warmer months, which drive algae growth.

“We’re closely monitoring water quality as we go through the application process, and then even outside of the work at Loch Lomond,” Coburn said. “It’s something that we work really hard to do and take very seriously the responsibility of providing safe and affordable water to our customers.”

Loch Lomond will reopen Saturday at 7 a.m. The recreation area offers boat rentals, fishing, hiking and picnicking, though swimming in the reservoir is prohibited.