Yellowstone National Park Report
Stream sediments are important locations of biogeochemical transformations upon which many stream ecosystem functions depend. Stream water is often exchanged between the stream channel and surrounding subsurface locations - this process is known as hyporheic exchange. While stream water is moving through the hyporheic zone, solutes and nutrients may undergo important chemical reactions that are not possible in the main stream channel. Further, because the hyporheic zone is composed of porous media (sand, sediment, alluvium, etc.), flow inherently slows down and the exchanging water has ample opportunity to interact with mineral grain surfaces and biofilms.
Gooseff, Michael N.
"Assessment of Hydrologic Transient Storage of Three Streams,"
University of Wyoming National Park Service Research Center Annual Report: Vol. 27
, Article 11.
Available at: http://repository.uwyo.edu/uwnpsrc_reports/vol27/iss1/11