Many ducks, geese and other wildlife have turned up dead at the Fairmount Park lake.
One resident who visits the lake almost every day started to notice the problem earlier this month.
"I mean, just two weeks ago it was beautiful. We loved coming here," said Jennifer Taylor, who held back tears over the loss of so many animals. "Now I can't even sleep at night because all I think about is all my little friends that have died already."
She's counted dozens of dead animals but most of them are ducks that spend their time in the water. An old video taken by Taylor shows abundant wildlife flocking towards the water, a sharp contrast to the lake right now.
"I've been feeding them for years, and my husband found one of his favorite ducks face down dead about four or five days ago, and he came to me and screamed that they're all dead," Taylor said.
Taylor's husband described one of the ducks in the water appearing lethargic before finally dying.
"There was one mallard who was showing all the symptoms," said Jeff Wilson. "He couldn't keep his neck up and he just kept sliding into the muck, until he needed to breathe. Then he would struggle to get his head out, and then he would go back into the muck until he was finally dead."
The City of Riverside is aware of the problems at Fairmount Park but are still unsure what's causing so many animals to die.
"We are very concerned about the health of our wildlife," said Pamela Galera, the director of the city's Parks, Recreation and Community Services Department.
Galera says the California water board and Department of Fish and Wildlife are assisting by testing the water and air, and providing guidance on the birds' health.
The city turned on a fire hydrant to add fresh water to the lake, and workers have been assigned to look out for dead animals to clean up.
"Two of the ducks we always see, I found one of them dead on the ground, the female, and the male was gently tapping her trying to wake her up," Taylor added. "One of the fishermen by the lake says he's been doing that all day and he hasn't left her side."
For now, it remains unclear if the animals are dying due to triple-digit temperatures, a potential algae bloom or something else.
City officials said they should have the test results from the water in a few days.