Channelization and concrete lining of the Rio Puerto Nuevo and its tributaries in the San Juan Metropolitan area has been proposed to control flooding in low lying areas adjacent to the stream. Concern about the effect of these channel modifications on the ground-water system prompted the U.S. Geological Survey in cooperation with the U.S. Army Corps of Engineers to conduct an investigation of surface-water and ground-water interactions in the Rio Puerto Nuevo basin in 1988. A principal objective of this investigation was to determine the potential effect of channelization of the Rio Puerto Nuevo on ground-water levels.