Valley confinement describes the degree to which bounding topographic features (such as hillslopes, alluvial fans, glacial moraines, and river terraces) limit the lateral extent of the valley floor and the floodplain along a river. Valleys can be broadly classified as confined or unconfined, with corresponding differences in their appearance, vegetation, ground water exchange rates, topographic gradient, and stream characteristics. Unconfined valleys are generally less extensive than confined valleys in montane environments, but host a diverse array of terrestrial and aquatic organisms and provide disproportionately important ecosystem functions (e.g., hyporheic exchange, pool-riffle channel morphology, suitable grain sizes for spawning, enhanced riparian habitat). Consequently, identifying the location and abundance of each valley type is increasingly recognized as an important aspect of ecosystem management.
The Valley Confinement Algorithm (VCA) is a GIS based program that uses NHDPlus data to delineate unconfined valley bottoms. This webpage provides access to the VCA ArcGIS Toolbox script and documentation describing the program.