The Bureau of Land Management closed campgrounds and recreation areas along the river, between the towns of Briceburg and Bagby, Friday after receiving test results of water samples downstream from where the family died. Closure to the Merced River's recreation areas will stay in effect until Sept. 17.
Algal blooms can form in waterways that are shallow and warm.
"These algal blooms can produce toxins that can make people and pets extremely sick," Elizabeth Meyer-Shields, a BLM field manager, said in a statement. "We will continue to monitor for the algae's presence and look forward to when the public can safely recreate in the Merced River."
The bodies of John Gerrish, his wife, Ellen Chung, their 1-year-old daughter, Miju, and their dog were found on Aug. 17 on a hiking trail close to the river in the Sierra National Forest. A family friend had reported them missing.
A cause of death has not been determined, and investigators are considering whether toxic algae blooms or other hazards may have contributed to the deaths.
Toxicology reports are still pending, and investigators have ruled out any weapons being used or dangerous gases from a mine along the trail.
"This is a very unusual, unique situation," Kristie Mitchell, a spokesperson for the sheriff's office, said earlier this month. "There were no signs of trauma, no obvious cause of death. There was no suicide note."
A friend of Gerrish and Chung's told CBS affiliate KGPE-TV that the couple owned multiple rental properties in Mariposa. The friend also said Gerrish was from the United Kingdom and Chung from San Diego.
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