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Vertical: Hyporheic zone (pg 2 of 7)

Even in streams without substantial contribution of groundwater there is an important exchange of water between the stream and the sediments underneath the stream. This area is known as the hyporheic zone.

The hyporheic zone runs parallel to the channel, sometimes spreading out laterally for hundreds of yards or even miles, depending on size of river, breadth of floodplain, and porosity of sediments. Oxygenated river water enters the hyporheic zone by downwelling and seeps through the sediments below the streambed, occasionally returning to the stream channel by upwellings in the forms of springs.

Scientists only recently have realized the importance of this zone. Its suitability for organisms depends on interstices, or the spaces between particles of sediment. Owing to the way river sediments sort, particles get smaller as depth increases. So most habitat lies no more than a couple of feet beneath the channel.

But in large streams with porous substrates, the hyporheic zone reaches deep below the streambed. Researchers have found stoneflies more than 30 feet deep in the floodplain of the Flathead River in Montana, and more than a mile from the channel. They concluded the biomass of the hyporheic zone may exceed that of the river channel.


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