With California's wild Coho salmon populations down to 1% of their former numbers, there's growing evidence that beavers – long reviled as a pest of the waterways – are essential to restore the species, writes Maria Finn. In the process, they raise water tables, recharge aquifers and improve water quality. What's not to love?Beavers are the single most important factor in determining whether Coho salmon persist in California. They work night and day, don't need to be paid, and are incredible engineers.