A summary discussion of the knowledge available to describe the geohydrologic setting of the northern portion of the Colombia Plateau is presented in this chapter. Particular attention is focused on the Hanford Site portion of the Pasco Basin. The principal topics addressed involve the characteristics and known or suspected dynamics of the surface-water and groundwater flow regimes. Alternative flow system conceptualizations are presented where data are inconclusive or sent. Discussions are carried out recognizing information strengths and weaknesses.