It shows high levels of Anatoxin A, a type of poison produced by algae blooms.
The California Water Board is urging people to stay out of the water, saying the toxin can make people and animals sick.
RELATED: Toxic algal blooms found in Merced River, recreation closures in place
The water samples were taken after the sudden deaths of a Mariposa County family and their dog.
Authorities said they are still investigating how the family died. Officials have not said if the toxins were a factor in their deaths.
RELATED: Harmful algae bloom found at San Luis Reservoir in Merced County
Campgrounds and recreational areas along the river between Briceburg and Bagby are still closed.
The closure will stay in effect through at least September 17th.