The collection and timely evaluation of  data are primary prerequisites for determining the conditions of dams. Portable instruments, such as hydrophones, have been useful in the investigation of deficiencies in dams and have been used effectively to locate leaks. Hydrophones have been particularly useful in measuring leakage through the concrete facings of rockfall embankments where the disruption of the slab joints, or cracks in panels, were primary sources of leakage. The test involves the comparison between background sound intensity with intensity measured in the vicinity of leaks.  The hydrophone can be lowered off the side of a boat located over the points of the suspected leakage.