In an unexpected reversal for the species, a surge of Chinook salmon is swimming up Bay Area creeks following autumnal rain.

In previous years, salmon populations struggled from drought and dry channels. The fish responded to “usually large storms” in late October by rushing up the region’s channels, swimming into areas they have never been seen before, according to Joe Sullivan, Fisheries program manager with the East Bay Regional Park District.

“What we’re seeing is their amazing resiliency to drought and climate change,” Sullivan said in an email. “We haven’t seen this number of salmon trying to spawn in East Bay streams for more than 20 years.”

Chinook salmon, also known as king salmon, are red-hued fish with distinct black spots on their backs. Weighing up to 30 pounds, king salmon are the largest of the Pacific salmon species. After living in salt water for three years, the fish return to freshwater rivers to spawn and die.

In recent years, Chinook populations have collapsed at an astonishing rate, according to a Mercury News article. The current surge is unlikely to end well if rainfall ceases.

The salmon population was previously presumed to be gone indefinitely from Hayward’s San Lorenzo Creek. However, dozens were spotted swimming in the creek’s urban flood control channel, according to the article.

“We get a couple big storms during the right time of year and here they come!” Sullivan said in the email. “This tells us that salmon have evolved to be resilient to longer periods of drought and if we give them access to spawning habitat they can persist.”

Last year, only one fish was reported in lower Alameda Creek. Now, there are many salmon in a small pool created by a concrete barrier near BART piers. However, if storms do not continue the fish may be trapped in the waters, unable to reach the breeding grounds.

Historically, nearly all Bay Area streams had breeding salmon populations. The fish would remain in the waters, as they instinctively return to the same stream they were born in, Sullivan noted.

The decline of the salmon population began in the middle of the 20th century when dams, diversions and flood control channels were being constructed in the creeks, according to Sullivan.

“This led to salmon being unable to access historical spawning and rearing habitat upstream,” Sullivan said in the email.

However, compared to other salmon species, Chinooks have an advantage, the article notes. Baby Chinooks can swim safely to the ocean in the spring, preventing them from perishing in summer’s dry rivers.

Moving forward, the South Bay Clean Creeks Coalition plans to begin monitoring fish populations. Those who see salmon in the lower Alameda Creek Flood Control Channel are asked to report the sightings to the Alameda Creek Alliance.