Problems of flow involving fresh water overlying saline water in aquifers and methods for their analysis are reviewed. Equations amenable to mathematical solution generally involve the idealization of treating the fresh water as a distinct zone separated from the underlying saline water by a sharp interface. The criterion for interface stability is derived in this study and its practical significance is described. A new solution describing up-coming of saline water below horizontal tile drains is derived and used to calculate approximate optimum depth of drain placement. A procedure for estimating performance of collecting wells is outlined.