That region of a river which typically starts at the benthic
zone and goes downward until groundwater from the river is no longer
included in the substrate. This is a relatively new area for ecological
study. It is strongly suspected that this poorly oxygenated zone serves
as a reservoir for stream life, particularly macro invertebrates, at
times of drought, flood, and other environmental stress. Channelization
destroys this zone, along with the benthic zone, as well.