The only other time researchers saw low oxygen levels in recent times was during the 2016 El Niño event, which kept the ocean and freshwater from their normal tidal circulation. This blocked the river mouth with sand and created a stagnant pool that killed wildlife, including dozens of leopard sharks.

Chula Vista now joins a growing list of cities in the South Bay and San Diego County to declare a state of emergency over the Tijuana River sewage crisis.

Chula Vista leaders approved a local state of emergency due to the Tijuana sewage overflow on Tuesday night. It authorizes the city's manager, mayor and councilmembers to work with both local, federal and Mexican authorities to improve conditions in the Tijuana River.

The resolution is largely symbolic, calling on the White House and other top government officials to fast-track more spending for solutions

Chula Vista has joined other cities in declaring a state of emergency over the Tijuana River sewage crisis. The city council voted unanimously to approve a resolution introduced by Mayor John McCann Tuesday evening.