Hydrologic conditions of southern and northern Guam are quite different. In southern Guam, volcanic rocks are relatively impermeable and support a well developed surface drainage network. In northern Guam, permeable limestone allows virtually all rainfall to infiltrate the rock and recharge the aquifer contained within.

This atlas focuses on the northern Guam. There, the high permeability of limestone and the ubiquitous presence of closed depressions on the plateau surface preclude stream flow. Some small ephemeral streams form during intense rainfalls on the flanks of the volcanic outcrops that comprise Mt. Santa Rosa and Mataguac Hill, but these quickly sink at the contacts with the surrounding limestone. Freshwater discharge from the karst plateau of northern Guam is almost entirely from coastal seeps and springs. The only surface streams occur at the southern end of the plateau, adjacent to the Pago-Adelup Fault, where the Chaot, Hågatña, Fonte, and Pago rivers flow across the clay-rich and less-permeable argillaceous limestone and capture some of the groundwater.