Locating and Assessing Wetlands
The examples used here are from the technical report
entitled A Wetland Protection
Plan for the Headwaters of Johnson Creek and the Middle Rouge River
- RPO-NPS-TM25.00. This study was performed with the
intention of providing community planners with information and locations
of valuable wetlands in their jurisdiction. In addition, it presents
steps that a community planner can take to ensure the continued
functioning of wetlands in their watershed. Community planners can
use this information to make informed decisions that will best protect
their wetland resources.
Wetland Mapping Procedure
This is the procedure that was used for mapping the Middle 1 Subwatershed
of the Rouge River. The location and size were determined by merging
three sources of information. One source was the National Wetland
Inventory ( 1979 ) based on aerial photography obtained in 1978.
The extent of hydric soils was estimated using soil survey information
prepared by the Natural Resources Conservation District ( NRCS)
for Washtenaw, Wayne, and Oakland counties. Hydric and wetland soil
types were identified using the Hydric Soils of the State of Michigan
prepared by the National Technical Committee for Hydric Soils. The
third source of information was the Michigan Resource Information
System (MIRIS) land use coverage previously prepared for the subwatershed.
Land use classification types that are correlated with wetland plant
communities were identified. This information was combined with
the hydric soils and NWI information to map the wetlands of the
Middle 1 Subwatershed. For purposes of this study, a wetland was
mapped when two or more of these data sources overlapped. This method
identified 557 wetlands in the Middle 1 Subwatershed.
The accuracy of this wetland mapping technique
is dependent on the accuracy of the maps upon which it is based.
Inaccuracies in aerial photo-interpretation, land use changes since
the date of aerial photography, and natural changes in wetland boundaries
could introduce error. While for some purposes it is crucial to
understand the exact boundary of a wetland, for a regional wetland
protection plan such as this it is more important to know that in
fact the wetland exists, has a certain area, and may or may not
be connected to a lake or stream. Perhaps more importantly, understanding
functions performed by a wetland is more valuable to wetland protection
than the exact boundary of the wetland.
View These Maps Online:
Hydrologic Soil Groups
Map Order Number: M-ewslhsg
View this map NOW! - EWSLHSG.PDF
Areas with Wetland Potential
Map Order Number: M-ewwetlu
View this map NOW! - EWWETLU.PDF
NWI, MIRIS Wetlands Maps
After identifying the valuable wetland resources
in the Middle 1 Subwatershed of the Rouge River, it was necessary
to assess the wetlands and determine their ability to perform specific
functions. These functions included:
- the provision of floral diversity and wildlife habitat
- food, breeding areas, and habitat for fish and herpetiles
- storage of flood water during wet weather events
- the conversion and abatement of nonpoint source pollutants
- protection of shoreline and stream bank erosion
- aesthetic and recreational opportunity and enjoyment
Many assessment methods have fairly elaborate computational
methodologies, as the evaluation of wetland functions is complex.
For a project with an emphasis on regional planning, more detailed
wetland assessment methods are inappropriate and labor intensive.
Therefore, an evaluation procedure that quickly and easily identified
wetlands important to the environmental quality of the Rouge River
was developed. The method used to assess wetlands in the Middle
1 Subwatershed, called the Rapid Assessment Method ( RAM), is based
on a method called the Indicator Valuation Method ( IVA). This is
founded on the principle that the presence or absence of specific
indicators reflect the degree to which a wetland performs a specific
function. Indicators having documented scientific associations with
wetland functions are used when such information is available. Hypothesized
relationships are also used, if there is a strong scientific basis
for the assumption. Once the wetland characteristics were established
using the RAM, they were used to determine the ability of a wetland
to provide the six functional benefits.
The RAM was developed in three major steps. First,
functions of wetlands valued by residents and stakeholders were
identified. Second, a field survey protocol designed to determine
which function(s) a wetland was performing was produced. Third,
wetland characteristics determined in the field were ranked according
to the ability of a wetland to perform the function of interest.
As with all wetland evaluation systems, there are
limitations of the RAM which should be understood. This evaluation
method was not designed for impact analysis, which should be done
on a long term site-specific basis. This method is not designed
to be used in legal proceedings where a detailed wetland investigation
is more appropriate. Finally, this method is not intended to be
used as a justification for adversely impacting wetlands that do
not provide a particular function or group of functions.
For the Middle 1 Subwatershed study, of the 557
wetlands identified, 199 were evaluated. There are four reasons
why the number of wetlands evaluated did not reflect the number
of wetlands identified:
- Wetlands identified as being separate wetlands in the office
were often found to be connected in the field, and therefore counted
as one wetland.
- Some wetlands no longer exist due to development or a change
in conditions since the coverages used to identify wetlands were
- Some areas identified by GIS methods were not actually wetlands,
but detention basins, ponds, reservoirs, or lakes.
- Many wetlands were inaccessible by public roads and therefore
could not be assessed on ground. Of the wetlands which were inaccessible,
the larger ones were evaluated from an airplane, while smaller
wetlands were not evaluated.
A detailed copy of the Rapid Assessment Method
for Evaluating Wetland Functions is available in the report. Click
here to view this report.
Rapid Assessment Method Results
Wetland performance was evaluated by tabulating the number of high,
medium, and low scores for each function. For example, there were
six questions used to determine the quality of a wetland for floral
diversity and wildlife habitat. One question asked how many vegetative
communities were present. Wetlands with three or more vegetative
communities received a "high" score, those with two received
a "medium" score, while those with only one community
received a "low" score. Answers to the other five questions
were also ranked, and the number of high, medium, and low answers
was tabulated. The resulting distribution of high, medium and low
answers were used to represent the ability of a wetland to perform
a function. Wetlands were ranked according to the number of high,
medium, and low responses they had for each function. This way,
the top wetlands for each function could be easily identified. A
wetland was considered to perform a function if it had a greater
number of high or medium scores than low scores.
Although a wetland may perform more than one function
extremely well, it is unlikely that it performs most or all functions
well, since some of the categories tend to be mutually exclusive.
For example, a typical wetland receiving high performance scores
for runoff attenuation is an emergent marsh with little open water
which is not permanently inundated and has no outlet. This same
wetland would receive low scores for the fishery habitat function,
as wetlands good for fish are permanently inundated, have open water,
and are connected to larger bodies of water such as lakes and rivers.
the results of this assessment, click here.
A Wetland Resource Protection
The Middle 1 Subwatershed Management Study has identified
that land development presents the single greatest risk to water
quality in the headwaters of the Rouge River. The Middle 1 Subwatershed
is unique among the subwatersheds of the Rouge River due to its
relatively low levels of development. However, the rate at which
this subwatershed is developing is increasing drastically. Because
of this, timely identification and protection of wetlands providing
ecological and economic services is more important than ever. Planning
and zoning in the subwatershed should be done in the most environmentally
responsible manner possible to achieve this protection.
This Management Study presents a management plan to
protect the wetlands of the Middle 1 Subwatershed. This plan is
intended to enhance and protect the functioning of wetlands in the
watershed. This goal can be accomplished by enacting protections
specific to the functions that each wetland has been determined
to perform. Furthermore, this plan outlines simple steps that a
community can take to protect and enhance wetland resources.
Protection measures include protecting plant diversity
and wildlife habitat; protecting aquatic habitat; protecting flood
water storage wetlands; protecting nonpoint source pollution abatement
areas; protecting shorelines and streambanks; and finally, protecting
aesthetic and recreational areas. You can find a more detailed description
of the protection plan in the report. (click
Some of the goals of the Middle 1 Subwatershed Study
that can be directly addressed through wetlands protection include:
- Develop a storm water management plan for compliance with the
Clean Water Act.
- Develop a plan for preventing, minimizing, and reducing pollutant
loading and flow variability which produce unacceptable impacts.
- Maximize opportunity for recreational uses by inhabitants of
- Enhance and preserve a healthy and diverse ecosystem compatible
with land uses within the Middle 1 Subwatershed.
- Develop a plan to minimize the negative impacts of excessive
flooding on adjacent property, including erosion.
- Protect downstream water resources.
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