How does rain affect salmon?

Salmon spawning

With hardly a drop falling from the sky this January, the welcome downpour California experienced in December is starting to feel like a distant memory. But when incredible amounts of rain did fall last month, it got some third-graders in Ms. Gee’s class wondering: how would all that water affect the salmon traveling up the Central Valley’s rivers on their fall migration? They asked our biologists if the rain could help the salmon, or make it harder for them to find their way.

It turns out that rain can have mixed effects on salmon. Rain brings many benefits: early in the fall, it helps cool down the water to temperatures that salmon prefer, and spring rains stir up dirt in the water, which helps young salmon hide from predators. Rain does help wash the chemical “scent” of the watershed downstream, making it easier for returning adult salmon to navigate back to the streams where they were born. Rain also raises the level of rivers, which can help salmon travel into locations that they otherwise can’t access during other times of the year. For example, the Eel River in northern California is often too shallow for salmon to enter from the ocean until the first storm of the season. The river can even stop flowing before it reaches the the ocean due to a lack of rain.

However, rain’s ability to “open up” waterways for salmon is not always a good thing. Last month, many salmon became stranded in dead-end canals of the Yolo Bypass near Sacramento because high water levels after rain made it easier for the fish to enter the canal system. Many of the fish became trapped and died without spawning once the water levels dropped. Too much rain can also lead to very high flows that scour riverbanks and destroy salmon nests, or create silt that suffocates their eggs. And rainwater can also wash chemicals or metals from roads and driveways into rivers that are toxic to fish. Despite these drawbacks, more rain generally means there is more water to go around for both salmon and people – so here’s hoping for more of it!

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