Stream flow
Volcanic rocks of southern Guam are not very permeable. They allow little rainfall infiltration. This causes a slow flow of ground water and fast flow of rain water. Heavy rains result in high runoff and can cause flooding. Hydrologists have calculated that the bulk of water flowing in streams and rivers of southern Guam comes from rainfall, and between 20 and 50 percent is supplied by groundwater.
The amount of water flowing in streams in southern Guam is highly variable. Streams may flood because of ample rainfall during the wet season, or dry out completely during the low season. Hydrologists follow the changes of flow in Guam's streams throughout the year. Data for selected rivers is shown in the table below. To see more data for southern Guam rivers please visit the USGS site dedicated to stream flow on Guam.
Western side of southern Guam
River |
Length |
Area drained |
Max. flow |
Min. flow |
Avg. flow |
Aplacho |
1.743 mi |
0.50 sq. mi. |
1690 ft3/sec |
0.12 ft3/sec |
n/a |
La Sa Fua |
2.879 mi |
1.06 sq. mi. |
1440 ft3/sec |
0.12 ft3/sec |
4.39 ft3/sec |
Umatac |
1.856 mi |
2.11 sq. mi |
7460 ft3/sec |
0.10 ft3/sec |
8.59 ft3/sec |
Central part of southern Guam
River |
Length |
Area drained |
Max. flow |
Min. flow |
Avg. flow |
Imong |
1.297 mi |
1.95 sq. mi. |
6100 ft3/sec |
0.36 ft3/sec |
9.52 ft3/sec |
Almagosa |
1.364 mi |
1.32 sq. mi. |
2650 ft3/sec |
0.05 ft3/sec |
5.98 ft3/sec |
Maulap |
1.515 mi |
1.15 sq. mi |
2420 ft3/sec |
0.31 ft3/sec |
5.02 ft3/sec |
Eastern side of southern Guam
River |
Length |
Area drained |
Max. flow |
Min. flow |
Avg. flow |
Ugum |
7.121 mi |
5.76 sq. mi. |
14700 ft3/sec |
2.7 ft3/sec |
24.2 ft3/sec |
Ylig |
7.453 mi |
6.48 sq. mi. |
4900 ft3/sec |
0.07 ft3/sec |
27.2 ft3/sec |
Pago |
6.251 mi |
5.67 sq. mi |
17300 ft3/sec |
0.00 ft3/sec |
26.1 ft3/sec |
Flow data for selected southern Guam streams (source: USGS)