The lower trunk and roots of a tree in a stream, exposed by undercutting or uprooting.
Such large woody debris, (or, in riparian management-speak, LWD) is an essential part of a stream or creek's ecosystem-- and not just because a rootwad provides shelter and habitat for aquatic life: juvenile fish will use the roots to hide from predators, mature fish will use them for shade, and the pools behind them to lay eggs, and aquatic invertebrates find the roots a useful substrate to attach themselves to. (Many fishermen know that rootwads are where the fish are hiding.) Insects, birds, and small mammals may use the roots above the waterline as well. In addition, the mass of roots collect detritus for the food web, impedes the flow of a stream-- deflecting the current in such a way that stream bank erosion is diminished, pools are created, and sedimentation downstream is reduced.
Rootwads also provide better resistance to flooding in creeks or streams (and protection for critters living there during high water)-- thus, some creek managers involved in riparian restoration will actually add rootwads by anchoring the tree trunks (or bole, in Army Corps of Engineers terminology) into the stream bank. (It used to be standard practice to remove large woody debris like rootwads from streams, until research discovered this exacerbated erosion problems). Managers consider rootwads temporary or interim restorations: as organic material, it will eventually decompose, so the useful lifespan of a placed rootwad is estimated between five and fifteen years. One advantage of using them-- they are cost-effective: you can typically use a local materials (i.e., a tree), as opposed to trucking in boulders or logs for other "flow deflection structures."
Rootwads are one of many streambed restoration tools, and rarely used alone. They also are typically not used in areas where the stream bank is mostly sand (no way to keep the rootwad in place), or where recreational activity (such as boating or swimming) is a priority, or where there are downstream structures in need of protection. Revegetation typically accompanies the addition of a rootwad.
Brian Coad and Don E. McAllister. Dictionary of Ichthyology. 18 May 2004. < http://www.briancoad.com/Dictionary/R.htm> (25 August 2004)
Columbia River Inter-Tribal Fish Commission. Protecting and Restoring Watersheds: A Tribal Approach to Salmon Recovery.
<http://www.critfc.org/oldsite/handbook/glossary.html> (25 August 2004)
Jason Remich. (2002) Ohio Stream Management Guide No. 21 Large Woody Debris in Streams. Ohio Department of Natural Resources. < http://www.dnr.state.oh.us/water/pubs/fs_st/stfs21.htm> (25 August 2004)
Mike Roell, Mike Leach, and Jamey Decosk. "Streamside Trees and In-stream Woody Debris Characteristics." Riparian Reviews. Volume 3, Number 1 Spring 1998. <http://www.snr.missouri.edu/ream/vol3no1.html#Stream> (26 August 2004)
Sylte, T.L., and Fischenich, J.C. (2000). "Rootwad composites for streambank stabilization and habitat enhancement," EMRRP Technical Notes Collection (ERDC TN-EMRPP-SR-21), U.S. Army Engineer Research and Development Center, Vicksburg, MS. < http://www.wes.army.mil/el/emrrp/pdf/sr21.pdf> (25 August 2004)