Grand Bassa has a flat coastline. A narrow coastal plain extends inland from the seashore, and the land gradually rises to the hilly hinterland of the County. High elevation regions have forest of evergreen and deciduous trees, including ironwood and mahogany.

The County has several major rivers. These are St. John, Farmingtohn, Merchin, New Cess, Ilor, Timbo, and Benson River. Consequently, the shore is broken by estuaries, tidal creeks, and rocky capes. The coastal region does not rise more than 60 to 70 meters, excepting occasional small hills. This situation makes several settlements near the sea—including Buchanan City—vulnerable to inundation and erosion. Moreover, Benson River and Merclin River overflow the banks in some areas during the rainy season. As a result, many villages remain inaccessible during certain periods of the year due to the deteriorated roads.