The region is mostly mountainous, characterized by steep river valleys. Elevations in the Chetco River watershed range from sea level to at the summit of Pearsoll Peak. Precipitation averages between per year, with October through June being the wettest months. Seventy percent of surface runoff is collected from rain, and 30 percent from rain on snow. Twenty-five separate wetlands totaling have been identified in the watershed. Temperatures average between , although the Brookings effect (or Chetco effect; similar to a foehn wind) often brings localized hot weather to the Brookings area. The increase in temperature is caused by the geography of the region; cool air funnels down the Chetco River valley from the Siskiyou and Coast ranges, gradually heating up before eventually reaching Brookings as a warm wind. The mountains also shield the area from cool marine layers. Partially as a result of this phenomenon, Brookings recorded its highest temperature ever, , on July 8, 2008.