In what has virtually grown into an annual event, Silverwood Lake was the site on an algae bloom.
In response, the California Department of Water Resources instituted temporary restrictions at the popular recreational venue, including banning the use of boats, watercraft and fishing. On Tuesday, July 14, the lake was close to allow for the infusion of copper sulfate into the water to manage the algal bloom.
The California Department of Water Resources’ action was taken to head off what would otherwise have become a severe “danger” level at the lake in the San Bernardino Mountains, south of Hesperia.
Algal blooms are a recurring problem because warm water, calm conditions, and excess nutrients allow blue-green algae, known scientifically as cyanobacteria, to multiply rapidly. While blue-green algae is a natural part of the lake’s ecosystem, hot summer conditions incubate fast expansion of the phenomenon, which can be accompanied by the release of toxins harmful to humans and animals. Visually, a bloom resembles surface mats or streaks, which has been likened to spilt paint, and can also manifest as foam or “scum.” In addition, when algal blooms move into what is labeled the “harmful range,” they can produce toxic compounds and various unpleasant odors, resulting from the breakdown of metabolic matter and the release of volatile organics, ranging in description as “fishy…septic… [and] gasoline-like.” This can impact water quality and present complication at downstream water treatment plants.
The July 15 treatment was taken to resolve the current issue and head off further blooms later this summer and into the early fall.
The introduction of copper sulfate into Silverwood Lake has happened before.
On August 11, 2025, the California Department of Water Resources and the management of the Silverwood Lake State Recreation Area issued danger and warning advisories for Cleghorn Beach and Sawpit Beach, which banned swimming, wading and the consumption of fish from the lake, and generally prohibited contact with the algae, and prohibited drinking or cooking with water draw from the lake or eating shellfish from the area. In addition, pets were not allowed into the water.
In the summer 2024, a toxic algal bloom warning was issued at Silverwood Lake, entailing swimming closures and copper sulfate treatments.
Similar blooms disrupted recreation at the lake and required closures in the summer and early fall of 2023, 2021, and 2019.