The California Coast Ranges are relatively young mountain ranges being tectonically uplifted faster than the pace of erosion by several millimeters a year . These rapidly uplifted , unstable slopes produce natural landslides which have filled the channel for tens of thousands of years . Tree stump evidence from the banks near the estuary shows that the area around the mouth of the Eel dropped in the January 26 , 1700 , Cascadia earthquake . The basin which holds both the Eel and the Salt is underlain by the Eel river syncline , crossed by the Russ and Ferndale faults and affected by the Little Salmon Fault to the north . Both the San Andreas Fault system and the Cascadia Megathrust impact the geomorphology of the basin
Cascadia earthquake
1700 Cascadia earthquake
Salt River (California)