The hydrogeomorphic approach to functional assessment of riparian wetlands: evaluating impacts and mitigation on river floodplains in the USA

1. The 'hydrogeomorphic' approach to functional assessment of wetlands (HGM ) was developed as a synthetic mechanism for compensatory mitigation of wet lands lost or damaged by human activities. The HGM approach is based on: (a ) classification of wetlands by geomorphic origin and hydrographic regime ( b) assessment models that associate variables as indicators of function, an d (c) comparison to reference wetlands that represent the range of conditio ns that may be expected in a particular region, in this paper, we apply HGM to riparian wetlands of alluvial rivers. 2. In the HGM classification, riverine wetlands ape characterized by format ive fluvial processes that occur mainly on flood plains. The dominant water sources are overbank flooding from the channel or subsurface hyporheic flo ws. Examples of riverine wetlands in the U.S.A. are: bottomland hardwood fo rests that typify the low gradient, fine texture substratum of the south-ea stern coastal plain and the alluvial flood plains that typify the high grad ient, coarse texture substratum of western montane rivers. 3. Assessment (logic) models for each of fourteen alluvial wetland function s are described. Each model is a composite of two to seven wetland variable s that are independently scored in relation to a reference data set develop ed for alluvial rivers in the western U.S.A. Scores are summarized by a 'fu nctional capacity index' (FCI), which is multiplied by the area of the proj ect site to produce a dimensionless 'functional capacity unit' (FCU). When HGM is properly used, compensatory mitigation is based on the FCUs lost tha t must be returned to the riverine landscape under statutory authority. 4. The HGM approach also provides a framework for long-term monitoring of m itigation success or failure and, if failing, a focus on topical remediatio n. 5. We conclude that HGM is a robust and easy method for protecting riparian wetlands, which are critically important components of alluvial river land scapes.