The Hyporheic Zone
by John Gross
The hyporheic zone is the saturated area beneath a stream—the area in which water flows beneath the ground—but influences and is influenced by surface waters of a stream. The word “hyporheic” (pronounced hi-poe-REE-ick) is Greek, and means below the river or below the stream. The extent of this area varies with the porosity of the substrate over which the stream is flowing. Gravel beds, being porous, will have a more extensive hyporheic zone than streams with clay or bedrock beds. Generally groundwater in the hyporheic zone is on its way to becoming surface water, that is, contributing to the volume of water in the stream.
These areas are important to trout and whitefish that require cool water. Water coming from the ground to the surface is generally quite a bit cooler than surface water during the summer. So areas of groundwater discharge, springs for example, are “refugia”— places where fish and other aquatic life can find refuge from warmer temperatures elsewhere. Hyporheic zones also contain a surprising array of aquatic insects and other organisms. In fact, the hyporheic communities may be more biologically diverse than populations in the rocks along the streambed. Because hyporheic zones receive no sunlight, all of the organic matter that supports these organisms must come in with the water as it flows in and out the hyporheic zone. A living “film” on the surfaces of the sub-stream gravel captures these nutrients, especially nitrogen and carbon. This film is grazed by larger organisms, and in turn, even larger ones. Because hyporheic zones increase the productivity of streams and decrease pollution, they are important habitats for maintaining water quality and replenishing food in cold water fish refugia.