Buoyancy-force-driven transitions in flow structures are examined numerically by a finite volume method. The specific problem considered is a steady, hydrodynamically and thermally fully developed flow in a square channel with streamwise curvature, spanwise rotation and wall heating or cooling. The buoyancy force is found to significantly affect the flow structures and heat transfer. Several previously unknown phenomena are revealed. Results include both transitions in how structures and effects of the flow transitions on flow resistance and heat transfer.