Dam Removal for Improvement of Stream Functions

In this follow up on the previous post discussing dam removal projects as a source of stream mitigation credit, we are going to look at how dam removal projects and stream restoration projects can both be evaluated to show ecological uplift. Increasingly, the regulatory and academic communities are viewing stream restoration projects through the lens of functional improvement rather than older metrics such as visual stream stability. This post examines how dam removal projects can also be viewed through this increasingly common assessment and evaluation process.

While there have been a few different functional and semi-functional assessment methods, two of the most widely used are from Fischenich (2006) and Harman (2012). For our discussion, Harman’s functional groups provide a starting point for understanding these approaches. Harman describes five different functional groups:

  • hydrology (how water is transported from the watershed to the channel)
  • hydraulics (how water interacts with the channel and surrounding area)
  • geomorphology (how diverse bed forms are created through the transport of wood and sediment in the channel)
  • physicochemical (how nutrients are processed and how temperature and oxygen are regulated)
  • biology (biodiversity of the system and the aquatic and riparian organisms).

So does every dam removal project positively affect each of these functional categories in the same way? Obviously not, since each project is distinct from the next. However, we can make some general characterizations and look at a specific example to see how one project fits into this framework and what that means for stream mitigation credit.

Dam removal has the potential to create functional uplift in all of the groups described above. By removing dams on in-line ponds, natural hydrologic cycles can be returned to systems that have long been altered. Dams also drastically alter both the hydraulic and geomorphic functions of a system. Run of the river dams can create hydraulic jumps at high flows that scour the downstream channel and induce bank erosion, while also blocking the natural movement of sediment and woody debris that create and maintain diverse bed features. The last two functional groups, physicochemical and biological features, are impaired in many dammed river systems because dams can cause increased temperatures at the top of their impoundments and oxygen depleted waters at the bottom of the impoundments, and can become a sink for nutrients, creating eutrophic conditions. Both of these effects can harm biological communities in addition to the physical barriers dams present for aquatic organism passage.

Let’s look at a specific project to see how it addresses functional uplift. The example project is the McCabe Golf Course Dam, which was located on Richland Creek in Nashville, TN. The dam was not high hazard, did not have endangered species associated with it, and was on an urban waterway. It was approximately 4 feet tall, 50 feet wide, made of concrete, built in the 1970s, and backwatered approximately 800 feet of channel. You can see a short video below on the project.

While this was a successful project, this was not a ‘special’ dam; there are many like it in communities all over the country, and in fact there are three similar dams right upstream of it. As a common dam it is analogous to many traditional stream mitigation projects, which occur on small streams that have been degraded by urbanization and agricultural practices. Very few stream restoration projects occur on high value streams with critical habitat for endangered species. So in this way, many stream restoration projects are also not ‘special’. This is important to note, because like stream projects, we should be targeting as many of these dams as possible for removal, and mitigation can be a driver for these projects.

Did the McCabe Golf Course Dam removal project provide functional uplift in all of these categories? When it comes to hydrologic function, since the dam was a run of the river dam and had no available freeboard, the removal of the dam did not change the way the water got to the stream and the way the water moved downstream.

Hydraulic functions were improved, but not how they would be in a traditional stream restoration. Since we did not excavate a floodplain on the formerly backwatered channel, the channel may still be moderately entrenched, but the flows through the old impounded area now fluctuate in concert with the rest of the stream system. The “new” channel has room for bankfull flows to spread out before they reach the banks of the “old” channel. Before the dam removal, when Richland Creek had high flows, the banks were immediately subjected to the stress from these flows because the impoundment already filled the entire channel. It takes larger flows for this to occur now.

The most significant functional improvements to this system were geomorphologic: the banks of the channel that were formerly underwater are now naturally vegetating, which widens the riparian corridor and increases overall bank stability. Another major geomorphic uplift is in increased bedform diversity. Before the removal, there was only an 800-foot long pool. After the removal, there is a diverse set of riffles, runs, pools, and glides. These are all stream features that were nonexistent before the dam was removed. To support these features, the natural sediment transport regime has been reestablished for this section of the creek. This promotes uplift in the former impoundment and downstream of the former dam.

For physicochemical functions, direct evidence of this uplift is less conclusive without measured data. However, there is anecdotal evidence such as the reduction of foul smelling anaerobic muck that was found along the edges of the old impoundment and no sporadic floating algal mats during the summer months.

Lastly, biological functions experienced several areas of uplift, the most obvious being the removal of a barrier to aquatic organism passage, which directly benefits the local populations. There is also an increase in habitat niches with the new bedform diversity that the stream has now. Direct sampling of the biological community is the only way to conclusively show positive changes to the fish or macroinvertebrate communities. This should be a goal of many dam removal projects, but was not a part of the monitoring plan for this project.

This quick assessment illustrates that you can assess functional uplift for dam removal projects using the same method that you would for stream restoration projects. Not only can you use the same method, in many cases dam removal projects may show more functional uplift than some traditional stream restoration projects. Additionally, this project was able to provide this uplift with a minimal area of disturbance from construction, a short construction period, without taking out any trees, and in an urban setting, which are all common criticisms or difficulties faced by stream restoration projects in locations such as these. Since this is seen through the same lens that many stream restoration projects are when assessing their viability for generating stream mitigation credit, we should consider dam removal as another tool to generate stream mitigation credit by achieving functional uplift across many categories.

Fischenich, J.C., 2006. Functional Objectives for Stream Restoration, EMRRP Technical Notes Collection
(ERDC TN-EMRRP-SR-52), US Army Engineer Research and Devel¬opment Center, Vicksburg, Mississippi.

Harman, W., R. Starr, M. Carter, K. Tweedy, M. Clemmons, K. Suggs, C. Miller. 2012. A Function-Based
Framework for Stream Assessment and Restoration Projects. US Environmen¬tal Protection Agency, Office of Wetlands, Oceans, and Watersheds, Washington, DC EPA 843-K-12-006.