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Debits are the units of measure for quantifying the unavoidable adverse impacts that construction / development activity has on a wetland, river, stream, shoreline, or buffer. Debits are offset by the purchase of credits – the unit of measure for quantifying the beneficial impacts of compensatory wetland mitigation activities, including wetland restoration, establishment, enhancement, and preservation projects.

The Washington Department of Ecology has developed a tool, called the Credit/Debit Method, to calculate whether proposed wetland mitigation adequately replaces the functions and values lost when wetlands are impacted. This method generates a score for a wetland for each of three wetland functions valuable to society (improving water quality, reducing flooding and erosion, and providing habitat for plants and animals), and this score is based on three aspects of each function (potential of the wetland site to provide the function, potential of the landscape to maintain each function at the site scale, and value each function has for society). Debits are calculated for the wetland that will be impacted by multiplying its (pre-impact) score for each function by the size (area) of the impact, while credits are calculated by multiplying the area of mitigation by the increase in each function score that can be expected when the mitigation site is finished. Before debits and credits can be balanced, the Credit/Debit Method requires adjustments to be made to account for the loss of functions during the time it takes a mitigation site to fully develop its functions (called temporal loss), as well as for the possible risk that the mitigation project will not fully succeed.

Information on calculating debits

Information on purchasing credits

 

For more information about King County’s Mitigation Reserves Program, please contact Megan Webb, Program Manager (in the Water & Land Resources Division’s Rural and Regional Services Section).