Environmental Review Toolkit
Water, Wetlands, and Wildlife

Wetlands and Aquatic Resources


Paul Garrett, Natural and Cultural Resources Team,
Headquarters, Federal Highway Administration
Presented at The Wetlands Symposium, New Orleans,
hosted by the USACE, April, 1995

Seventeen wetland projects established in fourteen states under Section 404 requirements as compensatory mitigation for impacts to wetlands caused by highway construction were evaluated (Crabtree, et al, 1992). Qualitative descriptions of procedures and site conditions, and comparisons with reference wetlands using the Wetland Evaluation Technique (WET)(Adamus, et al, 1986) and the Hollands-Magee (Hollands and Magee, 1986) functional assessment procedures were used to determine success in replacing or replicating wetland functions. Restoration, creation, and enhancement projects for both coastal and inland wetlands were evaluated. One project was an advance bank. This paper is a summary and analysis of the results.

The mitigation projects evaluated were completed between 1981 and 1988. The average age of the mitigation sites at evaluation was approximately 3.6 years. A total of approximately 444 acres of wetlands had been impacted by fifteen highway projects. The median wetland area impacted was approximately 8 acres; the maximum was approximately 273 acres; the minimum was zero acres (bank). Impacts by wetlands types were palustrine forested, 275 acres; palustrine shrub/scrub, 66 acres; palustrine emergent, 70 acres. Approximately 250 acres were developed as compensatory mitigation; of this total, an estimated 85 acres of existing wetlands were enhanced or modified. Creation of new wetland from upland accounted for 54 percent; restoration, 14 percent; enhancement of existing wetlands (conversion of one wetland type to another to enhance specific functions, such as habitat), 34 percent. At least 151 acres of additional existing wetland and uplands associated with wetlands functions were preserved.

On four projects, existing wetlands were converted from one wetland type to another, for instance, from palustrine emergent, palustrine shrub/scrub, or palustrine forested to open water, submergent wetlands. These projects were considered enhancement or creation projects. Eight projects used old or newly developed material sources (borrow pits) as all or part of the mitigation site.

All mitigation projects studied resulted in recognizable wetlands. Compensatory mitigation goals were characterized by managers as partially met at fourteen sites, and as completely met at three sites. Seven projects showed a net loss of area, five, a net gain, and four had gains balancing losses. One project was an advance bank and was not included in the area calculations.

To evaluate functional replacement and success of mitigation projects, direct comparisons of functional indicators were made between the mitigation sites and respective reference wetlands using the Hollands-Magee and WET methods. Reference sites were either the impacted area in a pre impact state, or another existing wetland site with functions or conditions similar to the mitigation objectives. For comparisons using WET, comparisons between mitigation and reference sites were considered negative (a loss of functional capacity) if the functional index was higher for the reference wetland than for the mitigation site, i.e., High> Moderate>Low. Comparison was considered positive (a gain in functional capacity) if the functional index was higher for the mitigation site than for the reference wetland.

The majority of projects evaluated were not functionally equivalent to wetlands impacted. On most projects, area-for-area replacement and functional equivalency were not identified objectives of the 404 permit conditions. Mitigation objectives were identified on the basis of locally determined resource needs or objectives, or site potential. Habitat was the most commonly identified functional objective, followed by hydrologic functions. Ownership and availability appeared to be major considerations in determining mitigation sites used and mitigation objectives.

Comparisons of habitat-related functions (Wildlife Diversity for Breeding, Migration, or Wintering) used in WET resulted in 27 negatives (47%), four positives (7%), and 26 no change (46%), out of a possible 57. The Production Export function of WET was not included. Comparisons of water quality-related functions of WET resulted in 12 negatives (21%), nine positives (16%) and 36 (63%)no change out of 57 comparisons made. Hydrology-related functions of WET (Flood Flow Alteration) resulted in 4 negatives, 1 positive, 14 no change.

Similarly, comparisons of the Biological Function of Hollands-Magee resulted in 10 negative comparisons (0.55), six positives (0.33), and two no change out of a possible 18. Water quality-related functions resulted in 12 negatives (0.66), four positives (0.22), two no change, of a possible 18. Hydrology-related functions resulted in 23 negatives (0.64), 10 positives (0.27), three no change out of a possible 36. For comparisons of overall site conditions made using both methods, agreement was relatively good (13 out of 18 agreed, five disagreed).

Apparent reasons that projects did not meet all functional criteria or resulted in an apparent loss of functional capacity or conditions include 1) changes in other environmental management activities or conditions which affected the mitigation project, 2) failure to carry out the mitigation plans, 3) projects inadequately or improperly designed to meet objectives, 4) objectives not adequately defined, 5) mitigation plans and objectives not consistent with site potential, and 6) insufficient time for mitigation projects to develop mature site characteristics.

Because the reference sites were not equivalent to pre impact wetland site condition in all cases, and because pre impact conditions were not always thoroughly described and documented, direct comparisons of pre impact and post mitigation functional conditions are difficult to interpret in a landscape context. The lack of pre and post project inventory and documentation could affect the results of comparisons. The functional assessment methodologies used were sufficiently sensitive to detect differences in functional indicators and conditions between reference and mitigation wetlands.

Interpretations of these results are clouded by limitations of the methods used and other possible factors. Structured methodologies such as WET 2.0 and Hollands-MaGee offer the advantage of being objective, in the sense that bias of the user is at least reduced if not eliminated, and consistent, in the sense that the same criteria are applied in the same way to replacement and reference wetlands. However, biases in the methods themselves are carried through the analyses. Both of these methods used geographically-limited, although extensive, literature-based data to build the input questions and scale the outputs. These biases are not readily apparent in application of the methodologies, and cannot be easily eliminated. The methods are general in nature, and are not oriented toward special case management situations, which might require local or specific knowledge to correctly interpret environmental conditions and indicators. Nor do they take into account special management objectives around which mitigation may be designed, implemented, and managed. In many cases, mitigation objectives are not defined as the entire set of wetland functions, but are oriented toward specific needs or requirements of the locale or managing agencies.

Many of the specific goals and objectives identified in the project coordination/permit review process were met by the compensation projects, and in all cases functional wetlands did result. Overall, the results indicate that these mitigation projects did not meet no-net-loss criteria as far as functional equivalency or area are concerned. In some cases, a net loss of wetland acreage occurred due to conversion of one wetland type to another, or enhancement of an existing wetland. Special project objectives were not given priority, and no function was weighted or considered more important than others in the analysis.

The results of mitigation efforts on these projects, which were developed and implemented under the 404 permit process, indicate the difficulty in implementing compensatory mitigation of wetlands on a site-specific basis to achieve no net loss of wetland functions. A constant problem is the availability of a suitable site for wetland creation or establishment. If a site has a high potential for wetland development, it is most likely already wetland, or has been wetland and has been drained or converted to some other land use or cover type.

Use of these methods as done in this study is qualitative, providing a relative measure of results. Direct comparison of site-specific characteristics, such as water quality (clarity, pH, nutrient concentration), biotic (plant cover, species composition, or biomass), and physical (water depth and duration of flooding) characteristics could be a useful approach requiring less inference about functional capacity and mitigation results, and could be of greater value in developing watershed management strategies and plans. This suggestion assumes that critical wetland characteristics or functional indicators, such as depth, hydroperiod, or plant cover, have been identified or agreed upon for the desired functions. The use of reference wetlands as a current baseline is a valuable technique and provides a check against continuing changes in general wetlands conditions due to climatological, watershed, or other factors.


Adamus, P. R., Clairain, E. J., Jr., Smith, R. D., and Young, R. E. 1987. Wetland Evaluation Technique (WET): Volume II. Office of Research and Development, Federal Highway Administration, U. S. Department of Transportation, Washington, D. C. Available from: National Technical Information Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield, VA, 22161.

Crabtree, Allen, E. Day, A. Garlo, and G. Stevens. 1992. Evaluation of Wetland Mitigation Measures, Vol. I. Report No. FHWA-RD-90-083. Federal Highway Administration, Office of Engineering and Highway Operations, R&D, 6300 Georgetown Pike, McLean, Va, 22101-2296. Available from: National Technical Information Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield, Va, 22161.

Hollands, G. G., and D. W. Magee. 1986. A Method for Assessing the Functions of Wetlands, in, Proceedings of the National Wetland Assessment Symposium. Association of State Wetland Managers, Chester, Vt.

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