The rodent is apparently enjoying life in Paso Robles on the central coast. Plant workers saw the capybara consuming vegetation and swimming in the water storage pond before making its way toward the Salinas River late last month.
The California Department of Fish and Game estimates the capybara is about 100 to 120 pounds and may be the same one seen at a ranch about a mile from the treatment plant two years ago.
The animal, the largest rodent in the world, is native to South America. Wildlife officials say it was probably an illegal pet that escaped or was released.
Capybaras are not dangerous, but they can grow to more than four feet long and weigh 140 pounds. In the wild, the rodents usually only live about four years, but in captivity they can live twice as long.
Right now, there are no plans to capture the animal.
The Associated Press contributed to this report.