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By Sara Thornton

On September 25, 2018, the U.S. Army Corps of Engineers (USACE) issued Regulatory Guidance Letter (RGL) 18-01, “Determination of Compensatory Mitigation Credits for the Removal of Obsolete Dams and Other Structures from Rivers and Streams.”

This guidance presents factors that USACE district engineers should consider when determining the amount of mitigation credit generated from the removal of obsolete dams or other structures to restore rivers and streams.  RGL 18-01 is the result of a collaboration of USACE with other federal agencies, including the Environmental Protection Agency, and recognizes that mitigation credit should be assessed for restoring historic stream functions through the removal of certain structures, such as obsolete dams.  These mitigation credits are tied not only to restoring the natural stream flow regime, but also in restoring aquatic habitat.

Section 404 of the Clean Water Act grants USACE authority to issue permits for the discharge of dredged or fill material to waters of the United States.  Often compensatory mitigation is a condition of these permits to offset unavoidable impacts to waters of the United States. Such mitigation can be provided through preservation, enhancement, and restoration activities that improve the physical, chemical and biological processes of waters of the United States.

RGL 18-01 focuses on how compensatory mitigation could be achieved through stream restoration that seeks to return a stream to its natural/historic flow regime. This type of stream restoration through the removal of dams addressed by RGL 18-01 may be performed by mitigation banks and in-lieu fee programs or be conducted as permittee-responsible mitigation.  Specific considerations for determining how much mitigation credit should be assessed for such mitigation activities include: (1) the area of the river or stream channel that physically responds to the removal of the obsolete dam or other structure; (2) restoration, enhancement, or protection/preservation of riparian areas; (3) contribution to recovery of endangered and/or threatened species; (4) contribution to recovery of diadromous fish; (5) improvements to stream habitat, including water quality; and (6) distance to the next in-stream obstruction that may affect restoration success.  Another significant consideration includes whether long-term protection can be provided to the restored area in light of the fact that streambeds are often publicly-owned.

RGL 18-01 makes clear that mitigation credits will be determined on a case-by-case basis after evaluating a mitigation proposal involving the removal of an obsolete dam or other structure due to the significant variability in the potential benefits of such restoration activities based on the factors to be considered.


Sara Thornton is a Principal in the Firm’s Water Practice Group and Compliance and Enforcement Practice Group.  If you have any questions related to this article, please contact Sara at 512.322.5876 or sthornton@lglawfirm.com.

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