Trees avoid land erosion because they act as stoppage by clinging to soil. Lands with many trees/forests face very little erosion while deforested land tend to erode very rapidly.
Heavily forested areas like Western Washington also tend to receive a significant amount of rainfall, because trees make the air more humid by evaporating water from their leaves.
Thus, we can see that deforestation in Washington would lead to higher rates of soil erosion, and less rainfall.
