The two forks join at the eastern boundary of the Rolling Plains forming the Brazos River. The Rolling Plains range from nearly level to rolling and possess moderate surface drainage. Saline soils are common, as are pockets of sand and clay loams. The Brazos enters the Cross Timbers and Prairies region of the state shortly after the confluence of the Double Mountain Fork and the Salt Fork. This area is characterized by a rugged topography and wide variances in geologic formations and soils.
Soils range from sands and sandy loams, to clay and alluvial loams.
Upon leaving the Cross Timbers and Prairies region, the Brazos River flows through the intermingled regions of the Blackland Prairie and the Post Oak Savannah. Here the topography changes from the ruggedness of the Cross Timbers and Prairies to gently rolling hills. Soils in the Blackland Prairie regions of this area are generally loamy to clayey and alluvial. In the Post Oak Savannah Region, soils typically range from sandy loams in the uplands to loam and clay alluvial soils in the bottomlands. The final length of the Brazos River flows through the low, wet, marshy coastal area of the Gulf Marshes and Prairies.