We demonstrate theoretically that solute separation can be accomplished in pressure-driven flow through nanochannels due to solute-wall interactions. Such pressure-driven separation is efficient in identifying solutes with variable valences. This function complements exactly the electric field-driven separation (i.e., electrophoretic separation) in nanofluidic channels that works well for solutes differing in diffusivity. We also demonstrate the enhanced separation of solutes of either different valence or different diffusivity through the combination of a pressure-driven flow and an electric field-driven backflow in nanofluidic channels. This combined flow, however, has to be used with caution for solutes varying in both valence and diffusivity.