1. The successful operation of hydraulic structures erected on foundations which were previously considered unsuitable for the construction of large dams shows that with a properly functioning control of seepage under field conditions bolder engineering solutions are fully justified. 2. In the erection of dams on foundations unstable to washing a combination of wisely selected engineering measures can insure dependable operating conditions. 3. In planning dams on rivers which carry large amounts of sediment one should consider beforehand the possibility of the forming of a natural upstream apron by sedimentation on the bottom of the reservoir and formations on the upstream side of the structures. By so doing, the cost of seepage control installation can be reduced.