Prior to large scale timber harvest starting in the mid-1800s most of the Big River Basin supported mature coniferous forest, though original stands exist only in small areas today. Currently, redwood forests dominate the basin, but give way to Douglas-fir and oak woodlands in the upper elevations. Redwood in the Big River Basin typically occurs with Douglas-fir as a stand component, rather than occurring in pure stands. The Coastal Subbasin has the highest percentage of area in redwood-Douglas-fir stands (91%) and the Inland Subbasin has the least (68%).
Douglas-fir does occupy some pure stands and, in an inverse ecological trend to redwood, the range is from none in the Coastal Subbasin to 13% of the area in the Inland Subbasin. In the Coastal and Middle subbasins the redwood-Douglas-fir type is predominant, but in the Inland Subbasin, redwood occupies the lower portion of the gulches and changes to drier species such as Douglas-fir and the oaks and grasslands up slope. Overall, hardwoods occupy about 20% of the basin and grasslands about 4%. Blueblossom (Ceanothus spp.) and pampas grass are found in the Coastal and Middle subbasins and are usually a result of landscape disturbances.
Small sized trees that average 12-24 inches diameter at breast height (dbh) cover 62% of the basin. Stands that average greater than 24-inch dbh trees cover 31.3% of the area, pole-sized trees cover 5.5%, and sapling-sized trees cover 0.9%. The Coastal Subbasin has the most acres of stands that average greater than 24-inch dbh trees, which may be a result of higher year-round precipitation. Most of the basin has a crown canopy density of over 80%.