U.S. Environmental Protection AgencyThe independent federal agency, established in 1970, that regulates federal environmental matters and oversees the implementation of federal environmental…
Unconsolidated shore wetlandsIncludes all wetland habitats having three characteristics: (1) unconsolidated substrates with less than 75% areal cover of stones, boulders, or bedrock…
UnderflowThe movement of groundwater through the soil or a subsurface stratum, or under a structure; specifically, the water flowing beneath the bed of a stream…
UnderstoryThe trees and other woody species growing under a relatively continuous cover of branches and foliage formed by the overstory trees.
United States Code (USC)A compilation of the general and permanent federal laws of the United States. It is divided into 51 titles that represent broad areas subject to federal…
Unrest (episode)Usually non-eruptive volcanic activity (e.g., ground deformation, steam plumes, degassing) that may be interpreted as a precursor to an eruption.
Unstable slopesSlopes considered unstable due to their incline (or critical angle of repose), applied to slopes made of unconsolidated material.
UplandThe portion of the landscape above the valley floor or stream.
UplightingLight directed upward at greater than 90° above nadir, generally upward into the sky.
Upper-air soundingAn upper-air observation of the vertical profile of an atmospheric variable such as temperature or wind.
UpwarpA broad anticline with gently sloping limbs formed as a result of differential uplift.
Utility scale facilitiesFacilities that generate large amounts of electricity that is delivered to many users through transmission and distribution systems.