Almanac note · Outdoors
Green water or lake scum deserves a pause
California's HABs portal and report map show reported harmful algal blooms in freshwater, estuarine, and marine environments, with reporting and health guidance.
Green water, surface scum, mats, or an odd smell at a lake, river, reservoir, or slow-moving shore area is worth a pause. It may be harmless algae, but some blooms can become harmful algal blooms.
California’s HABs portal is the main state page for this topic. The report map shows locations of reported harmful algal blooms in freshwater, estuarine, and marine environments. Reports can come from online submissions, partner agencies, and water monitoring data.
Use the map before a lake day when the water looks questionable, then look for signs at the actual site. A map can lag behind field conditions, and an unmarked spot is not a promise that everything is fine. When in doubt, give the water space, keep pets away from scummy areas, and use the park, county, or water agency notice for the place you are visiting.
Where to see it
California Harmful Algal Blooms portal and HAB Reports Map.
Official sources
Official source trail
Reviewed July 4, 2026
California Porch explains the path. The official source is still the place to confirm the current rule, fee, form, map, deadline, or office decision.
Use the official page before you spend money, file paperwork, rely on a deadline, or change a property.
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