My Aquifer > Groundwater Basics > Is an aquifer an underground river?
Ground water has to squeeze through pore spaces of rock and sediment to move through an aquifer (the porosity of such aquifers make them good filters for natural purification - see question #8). Because it takes effort to force water through tiny pores, ground water loses energy as it flows, leading to a decrease in hydraulic head in the direction of flow. Larger pore spaces usually have higher permeability, produce less energy loss, and therefore allow water to move more rapidly. For this reason, ground water can move rapidly over large distances in aquifers whose pore spaces are large (like the lower Portneuf River aquifer) or where porosity arises from interconnected fractures. Ground water moves very rapidly in fractured rock aquifers like the basalts of the eastern Snake River Plain. In such cases, the spread of contaminants can be difficult or impossible to prevent.