For most of the twentieth century, California treated the beaver as a nuisance. Trappers had already removed nearly all of them by 1850, and the animals that remained were often relocated or destroyed, because their dams flooded roads and pastures. By the 1940s many wildlife managers had concluded that beavers had never really belonged in much of the state.

That conclusion turned out to rest on thin evidence. Researchers working through fur-trade ledgers, mission records and old newspaper accounts found beavers described in rivers from the Oregon border to the edge of the Mojave. The animals had not been absent. They had been removed so early that nobody alive remembered them.

The distinction matters, because a beaver dam is not simply an obstruction. Water that slows behind a dam sinks into the ground instead of racing downstream, which raises the water table and keeps side channels wet into late summer. Those channels give young salmon cool places to grow, and the moisture they hold can slow a fire moving across a valley floor. After two recent Northern California wildfires, researchers found green vegetation standing around beaver ponds while the surrounding hillsides had burned.

None of this makes beavers easy to live with. A dam built in the wrong culvert still floods a road, and the consequence lands on a county budget rather than on a river. What has changed is the first response. California now runs a restoration program for the species, and a landowner facing a flooded field can request a flow device, a pipe set through the dam that lowers the pond without removing the animals that built it.