In some places, other, sometimes more productive, aquifers underlie those mapped. In other places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped. Local aquifers are not shown because of the scale of the map. Limit of continental glaciation. North of this line, glacial sand and gravel aquifers overlie bedrock aquifers in many places.
Basaltic rocks form most of the volcanic-rock aquifers mapped.
These flows cover extensive areas in the northwestern United States and Hawaii. In places, the basaltic-rock aquifers are extremely thick. For example, those of the Columbia Plateau aquifer system in Washington are more than 2,544 meters thick in places, and those of the Snake River Plain aquifer system in Idaho are locally more than 800 meters thick. In most places, however, the thickness of these aquifers is 100 meters or less. In Idaho, the basaltic-rock aquifers are extremely permeable, and numerous large springs discharge several tens of cubic meters per second from them.