Humpback whale Rio travels from San Francisco Bay into Sacramento River Delta

A humpback whale, identified by researchers as ‘Rio’ (CRC-11620), has completed an unusual journey from San Francisco Bay into the Sacramento-San Joaquin River Delta. First spotted in the bay on September 15, the whale traveled east through the Carquinez Strait and Suisun Bay before entering the Delta. The U.S. Coast Guard and The Marine Mammal Center have been monitoring the animal's movements.

Rio has been documented 15 times over the last 24 years in California and Mexico. While humpback whales are not adapted to freshwater, similar incursions have occurred in the past, such as the whale Humphrey in 1985. Experts are unsure if this detour was caused by illness, injury, or navigation errors.

As of recent reports, the whale was seen heading back west toward Port Chicago in Suisun Bay. The Marine Mammal Center advises onlookers, including boaters and kayakers, to maintain a distance of at least 100 feet to avoid influencing the whale’s direction.