As the mitigation banking industry continues to expand and diversify, additional avenues to create wetland and stream mitigation credits are needed. The removal of dams that impound reservoirs to restore the natural hydrologic regime and geochemical processes of a watercourse is often a desirable option for the overall public good.

To address this, the U.S. Army Corps of Engineers (USACE) submitted a regulatory guidance letter for the “Determination of Compensatory Mitigation Credits for the Removal of Obsolete Dams and Other Structures from Rivers and Streams” on Sept. 25.

Dam Numbers Daunting

According to the National Inventory of Dams (NID), in 2016 there were 90,580 dams in the U.S. There are also approximately 1 to 2 million small dams in the U.S. that do not meet the criteria necessary to be included in the NID.

The list below provides additional details of the NID listed dams:

  • 27,380 (30%): high or significant hazard potential
  • 45,402 (50%): dam is less than 25 feet tall
  • 58,148 (64%): privately owned
  • 48,516 (54%): built before 1970

As you can see, there are literally tens of thousands of old, relatively small, privately owned dams with a hazard potential considered high or significant. By removing the impoundments that are no longer serving their intended purpose or need, stream and wetland credits could be generated through the mitigation banking process.

Short-Term Headaches, Long-Term Benefits

The guidance recognizes that dam removal will cause short-term adverse environmental effects and long-term beneficial environmental effects. These short-term adverse effects include: downstream deposition of released sediment from behind the dam, impacts to aquatic fauna, etc.

Also, sediment may contain toxins or pollutants that, if released downstream during construction, can pose a serious risk to aquatic fauna and human health. The use of best management practices is imperative to reduce any negative effects of the dam/obstruction removal.

Dams themselves also create negative effects to the impounded streams that include, but are not limited to, the following:

  • Chemical pollution from low dissolved oxygen in dam releases.
  • Thermal pollution in dam releases from limnetic stratification.
  • Barriers to migration for aquatic fauna and destruction of habitat.
  • Sediment sequestration and regulated discharges result in degradation of the pattern, profile, and dimensions of the receiving river reaches.

To this, the guidance states that mitigation credit determination should be based on the long-term net gains in functions that result from the restoration activity as opposed to short-term adverse effects.

But what about the potential loss of established wetland habitat due to the dam removal? The guidance states that losses of wetlands that occur as a result of stream restoration through the removal of obsolete dams and other structures should not require compensatory mitigation – if those actions result in net increases in aquatic resource function.

The objective of the Clean Water Act (CWA) is to “restore and maintain the chemical, physical, and biological integrity of the Nation’s waters (33 U.S.C. 1251(a)).” Often times, dam removal is the best option to achieve this goal.

Dam Removal and Mitigation Credits

The USACE has provided this guidance to assist in making informed decisions about the potential credit generation following dam or obstruction removal. The guidance considers two (2) areas for mitigation credit production through the removal of obsolete dams:

  • Areas of the river or stream channel that physically respond to the removal of the obsolete dam or other structure through improvements in stream geomorphology, stream hydrology and sediment transport processes.
  • Restoration, enhancement, or protection/preservation of riparian areas as adjacent riparian areas are critical to the ecological functioning of streams.

As with most things, this process is not as simple as removing the dam, generating credits and making money.

Construction in previously inundated areas provides a number of challenges. Decades of deposited fine sediment can range from ankle-deep to several feet, is unconsolidated, and can completely bury large equipment. Allowing adequate time to dry is critical, but this mud will still prove to be challenging during construction activities.

The construction of new impoundments for recreation, energy, drinking water supply, livestock, flood retention and other uses will certainly continue and should as needed. In doing so, we should also consider the responsible removal of impoundments that no longer serve their intended use to maximize the utility of the nation’s aquatic resources and the objectives of the CWA.

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About the author

Austin Russell
Austin Russell
Austin serves as an Environmental Specialist for RS&H’s Transportation-Infrastructure Practice.