Hyporheic Zone

Australia: The Land Where Time Began

Hyporheic Zone

This is the zone of saturated sediments that range from fine-grained muds to river sand and the coarsest of gravels on the beds of many rivers, that extend to the sides beyond the banks. This zone is absent from streams and parts of streams with beds that are bedrock or beds of very fine grained sediment. Sand, gravel or cobbles form the beds of at least some parts of most rivers, where water exchange can take place horizontally and vertically, and the direction of flow can differ in different parts of the channel of the same river. The exchange can take place across gravel bars and aquifers.

This zone forms the interface between the riverine and groundwater systems, exchange of water and minerals taking place between the river, the groundwater below the bed and alluvial aquifers along the sides of the river. The zone is an ecotone (an area where the biota of the the different systems involved in the exchange merge) . A place where the conditions differ from the systems involved and where some species can evolve specialisations that allow them to flourish in such a place. Ecotones also occur on land at places where adjacent ecosystems meet.

The quality of water in all the participating systems is affected by the biological processes that occur in this zone. The bad news is that most human activity connected with water use, such as extraction of gravel, which causes large amounts of silt to clog the interstitial spaces, and the inadvertent side effects of other activities such as the addition of pollutants, and excessive fertiliser in runoff, and the excrement of grazing animals, increasing salinity, and even the water pumped from alluvial aquifers that finds its way into the stream all have a deleterious effect on this important zone.

Microbial and chemical processes take place in this zone that affect the water quality of the river. An example is the mineralising of organic matter, which make it available for plants such as algae in the river. Water upwelling from the hyporheic zone is of a different quality and can change processes occurring in the river, such as algal growth and changes in the invertebrate faunas which is probably the factor that changes the breakdown of leaf litter. The effects on changes in algal growth can be seen in streams in the Flinders Ranges, where there is rich algal growth at the point of upwelling which tapers off downstream of the upwelling point as the nutrient concentration decreases downstream of the same point.

The amount of water in the hyporheic zone may be several times the amount of water in the river above it on large alluvial flood plains, extending for several kilometres on either side of the stream if fine sediment is not present that can reduce the porosity.

Changes in the surface discharge, watertable depth and varying permeability can change the boundaries of the hyporheic zone. At times when there is a low flow rate in the river, most water is moving from the deeper groundwater and alluvial aquifers. When flow rate of the river increases the water from the river can flood the hyporheic zone. The beneficial effects of this downwelling water is its oxygen content, nutrients and organic matter and small invertebrates. It can also contain deletarious material such as pollutants, silt which blocks pore space and acid run-off. Water percolating down through the sediment is treated by the biofilm on the particles, usually improving the quality of the water. They are kept in check by the grazing invertebrates in the zone. There is a point at which decreasing particle size increases the surface area available for biofilm colonisation while retaining the necessary porosity for water to filter through, if the particles of silt are too fine insufficient water gets through to keep the necessary flow of water moving through the sediment, and in the other direction, as the particle size increases the amount of water flow increases but the surface area available to the biofilm is reduced.

The invertebrate fauna of the hyporheic zone has members that live permanently in the zone and have evolved characteristics similar to those of species living permanently in caves where no light penetrates, such as the loss of eyes and the lengthening of sensory appendages. Some are believed to give clues to climates of the past. The zone is also believed to be a refuge area for invertebrates from the surface in times when the stream stops flowing, as in drought.