by Vincent Sortman
|Degradaed Urban Stream|
Milllions of dollars are spent every year on stream restoration projects. Typically the goals of these projects are to reduce sediment and nutrient (nitrogen and phosphorus) loadings in the streams and ultimately the receiving water body. In urban watersheds, sediments are usually generated by channel erosion caused by high intensity runoff from impervious surfaces. Nutrients and other pollutants such as hydrocarbons are carried directly to streams via storm drain systems. Research has shown that higher denitrification rates occur with greater surface area to water volume ratios. High surface area to water volume takes place when stormwater flows are spread out onto the floodplain.
Unfortunately most stream restorations are designed to keep water in the channel rather than allowing it to access the floodplain. The most prevalent stream restoration design technique is based on the concept of bankfull discharge. Stream channels designed using this technique will convey water and sediment up to the bankfull discharge which is typically around a 1.5-year recurrence interval discharge. This means in a normal year it is possible that no flows ever reach the floodplain. These channels are very efficient at moving water, sediments, and pollutants downstream. While channels designed using the bankfull technique can be very stable and provide aquatic habitat, they do not meet the goals of reducing sediment and nutrient transport.
In order to accomplish these goals, channels need to be designed in such a manner that nearly all storm flows access the floodplain where nutrients can be processed and sediments deposited. If this doesn’t sound like a natural stream system that may be because beavers were nearly extirpated from the landscape. Beavers were ubiquitous in North America and their dams produced a much different fluvial setting than we are accustom with today.
Beaver dams are a natural analog for a stream system that maximizes the ratio of surface contact to water volume. They pass baseflow unhindered but retard and spread out storm flows onto the floodplain. Not only does this allow for the accumulation of sediments and nutrients on the floodplain but it also raises the local groundwater table and hydrates the floodplain. This creates a very advantageous environment for hydrophytic flora and fauna, which is the type of vegetation that should dominate the floodplain.
|Regenerative Stream Restoration|
Many studies have verified the benefits of beaver ponds. Pollock et al (2004) assessed the current and historic distribution of beaver ponds and other coho salmon rearing habitat in the Stillaguamish River, Washington, and found that the greatest reduction in smolt production capacity originated from the extensive loss of beaver ponds.
While we want to mimic beaver dams to achieve their water quality and habitat attributes we do not want to constantly maintain woody debris structures. For this reason the “dams” we design for the regenerative restoration approach are parabolic-shaped gravel/cobble weirs across the channel and into the floodplain. Because the parabolas are long and shallow the shear stress through them remains quite low even during large flow events. Not only do these weirs act as beaver dams to pond water and spread out storm flows but they also create riffles. Similar to natural riffles, the riffle/weirs provide habitat for macro-invertebrates and promote hyporheic flow. Hyporheic flow (shallow, interstitial flow) is important for stream ecology as it has a strong influence on biogeochemical cycling and stream-water temperatures.
The regenerative stream restoration technique is well adapted for impaired streams in developed watersheds. The usual impairment in urban stream is deep incision and an associated lack of aquatic habitat.. An incised channel not only produces large amounts of sediment through bank erosion but it also lowers the local groundwater elevation. This essentially dries-out the floodplain and creates an environment for upland and invasive plant species. Installing riffle/weirs and creating a low flow channel (rather than a bankfull channel) raises the local groundwater elevation and rehydrates the floodplain. The moist regime allows for the gradual return of hydrophytes thus regenerating the floodplain to a more natural condition. Because the riffle/weirs create a very low energy environment it is possible to add large woody debris to the channel without the concern of it washing downstream. Establishing a series of short riffles and deep pools with large woody debris restores aquatic habitat to the stream. Among other benefits, the deep pools provide summer low flow refugia which is often absent from impaired urban streams. While different landscapes may require different approaches to stream restoration, the regenerative approach is quite useful in urban systems as a tool for restoring the ecosystem functions of the stream channel and floodplain.
Another benefit of the regenerative stream restoration approach is the elimination of typical stream restoration construction activities. Because the regenerative approach raises baseflow water surface elevations to the top of bank and establishes a low energy environment it is not necessary to grade channel banks and install bank protection and flow diversion structures.
Stream restoration has become a well-funded and justifiable ecosystem restoration ambition for many organizations. Unfortunately the favored stream restoration technique of a bankfull channel does not achieve the goals of reducing sediment and nutrient loads. The regenerative approach to stream restoration, where appropriate, creates a baseflow channel near top of bank that maximizes the streams connection to the floodplain. This then produces an array of ecological benefits:
- Traps Sediment
- Processes Nutrients and Pollutants
- Raises Groundwater Elevation/Hydrates the Floodplain
- Creates Habitat for Native Floodplain Vegetation including RTE species
- Increases Aquatic Habitat
- Provides Summer Low Flow Refugia
- Attenuates Stormwater
- Mimics Natural Beaver Dam Complex