Upper Des Plains River Basin
Isolated Depressions
In the Upper Des Plaines Basin, wetlands are classified as isolated if they are located outside
of the mapped ten-year floodplain. Their hydrology is driven by direct precipitation and associated
runoff, with additional subsurface flow under certain geologic settings.
Isolated depressions that have a natural channel outlet are further classified as open depressions,
while those without an outlet are classified as closed. The primary functional difference between the
open and closed subclasses is that because of the channel outlet, open depressions have the ability to
regularly export materials downstream. Open isolated depressions are commonly found along headwater streams.
Depressions can also be sub classified as herbaceous or shrubby, based on the percent cover of shrubs
(primarily Cephalanthus occidentalis). Sites with less than or equal to 30 percent tree/shrub
cover are classified as herbaceous, sites with greater 30 percent shrub cover are classified as shrubby.
Herbaceous depressions consist primarily of low marsh or sedge meadow communities, or both.
Floodplain Depressions
Floodplain depressions are distinguished from isolated depressions in that they are located within the mapped
ten-year floodplain. Therefore, floodwater also plays a regular role in the site’s hydrologic regime. Because
of their location within the floodplain these sites are able to regularly export materials downstream, and also
have the capacity to mitigate flooding in upland areas. Floodplain depressions can also be sub classified as
herbaceous or forested. Sites with greater than 30 percent cover of trees are classified as forested, sites with
less than or equal to 30 percent trees are classified as herbaceous. Herbaceous floodplain depressions will commonly
have low marsh vegetation communities that are dominated by Typha spp. In general, the herbaceous floodplain
systems tend to be less vegetatively diverse than their isolated counterparts.
Guidebooks
Lin, J.P. (2006). "A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of
Depressional Wetlands in the Upper Des Plaines River Basin,"
ERDC/EL TR-06-4,
U.S. Army Engineer Research and Development Center, Vicksburg, MS.
Back to HGM Approach
Web Date: October 1997
Updated: April 2008
|
