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Created by:
Abby Rokosch
Jessen Book Siobhan Fennessy
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As mitigation projects
grow in number, the overall landscape composition is beginning
to change. "Classic"
wetland types have been destroyed, and new wetlands have been
created in their stead. However, rather than creating wetlands
identical to those that were destroyed, engineers often choose
the easiest means of fulfilling the requirements of the mitigation
permit they were granted. Along these lines, wetlands are often
created with no other purpose than retaining water. Gwin et
al. (1999) explored this phenomenon as they described the
wetland composition of the Portland, Oregon area. In their study,
Gwin et al. used the HGM method of classifying wetlands
to describe 45 naturally occurring wetlands (NOWs), and 51 mitigation
wetlands (MWs). By quantifying the number of each wetland type
(again, along the lines of the HGM method), Gwin et al.
were able to display shifts in wetland community composition.
Interestingly, Gwin et al.'s study revealed a large number of MWs that defied classical definition. Hasty,
or possibly even uneducated (?) mitigation design methods have lead to the creation of a whole new set of "Atypical"
wetland classifications, unlike anything natural (see below).
Descriptions of the regional, atypical hydrogeomorphic
(HGM) classes for wetlands in the Portland, Oregon metropolitan
area. Definitions based on principles developed by Brinson (1993)
and Smith et al. (1995). (Reprinted from, Gwin et al.,
1999).
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HGM Class
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Geomorphic Setting
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Water Source
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Hydrodynamics
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| Depression-in-riverine-setting |
Topographic depression located alongside a stream or river channel. Depression was formed through
excavation adjacent to the stream and is separated from the stream channel by a berm. Closed contours allow the
accumulation and retention of surface waters. Inlet/outlets may consist of culverts, spill dams, or open channels
between the wetland and tributaries and between the wetland and the stream. Inlets/outlets to the stream are located
above normal stream level, preventing interchange of water between the wetland and the stream except during storm
episodes and high flow conditions |
Water is primarily from precipitation, groundwater discharge, and overland surface flow from adjacent
uplands. Surface waters are often delivered through tributary culverts and storm drains. During the rainy season,
subsurface hydraulic connections and overbank flow from the stream during flooding events are also primary sources. |
At base water
levels, surface water in the wetland are stabilized at or
below the elevation of outlets or water control structures
and are isolated from the stream. During flooding events,
surface flow from tributaries may merge with streamflow
as the tributaries deliver water through the wetland to
the stream and when the stream overtops its banks during
extreme events. After flooding events, surface waters return
to the stream channel until water levels in the wetland
fall below the level of inlets/outlets. |
| In-stream-depression |
Topographic depression located within a stream or river channel. Depression formed through excavation
and/or damming of a portion of the stream channel. The inlet and outlet are the stream channel or culverts containing
the stream channel leading into or out of the wetland. |
Primary water
source is the stream channel. Wetland also receives water
from tributaries (can be through culverts and storm drains),
overland surface flow, groundwater discharge, and precipitation. |
Unidirectional surface flows dominate most of the year. At low water, water levels are regulated
by the water-control structure or edge of the excavation. This prevents surface waters from exiting the wetland
downstream through the stream channel and maintains standing water within the wetland, which promotes stagnant
conditions. |
| Depression-in-slope-setting |
Topographic depression placed on sloping land where there is a discharge of ground water to the
surface. Depression has been formed through excavation on the hillside or at its base. A berm may be placed on
the lower edge of the depression to impound water. Closed contours allow accumulation of surface waters and maintenance
of ponded conditions. Inlets/outlets can include culverts and storm drains. |
Primary water source is groundwater or overland flow from the slope wetland located immediately
up-gradient from the depression. Wetland also receives water from precipitation and surface flows, often delivered
through culverts. |
Ground water is at the surface during the rainy season and moves into the wetland laterally through
the soil during the dry season. The closed contours of the depression promote accumulation of water and maintenance
of ponded conditions, stabilizing water levels. |
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