Like many rivers in the western United States, California's Eel River has two types of steelhead trout that swim upstream from the ocean to spawn. Today, these typically standoffish fish clans spawn in separate stretches of the river and at different times of year, but they are more closely related than are comparable groups in other western rivers. Scientists have long wondered why. Now they may have an answer.
A massive landslide from California's Nefus Peak may have blocked the Eel River with a 140-meter-tall dam, creating a lake and forcing steelhead trout to spawn downstream for several centuries until the dam eroded.
LINK (Via: Science)