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Lifesaver or pollutant?


Dysfunctional stream channel and riparian zone in the Brazos River Basin. Riparian zone vegetation has been removed and replaced with a single species of upland grass, notice the erosion and the clear line of dead grass after a high-flow event submerged the grass.

by Tiffany Malzahn

That’s not just dirt at the bottom of the Brazos River.

The loose sand, clay, and silt on the bottom of streams and rivers is called sediment, and it provides habitat for freshwater mussels, aquatic plants, and other organisms that live at the lowest levels of a body of water.

Things go swimmingly until there’s a disturbance.

When the surrounding landscape is disturbed, the sediment balance in streams and lakes can become unbalanced. The sediment can become a pollutant that smothers aquatic organisms and their habitat. That disturbance can also make the water a cloudy brown or sometimes red. It can also reduce the depth of stream channels, increasing the potential for future flooding.

So how do we prevent disturbances?

First, we must identify what causes it. One of the most prevalent sources of sediment pollution is from erosion from the river’s streambanks, also known as the riparian zone, according to a 2006 article from Proceedings of the Eighth Federal Interagency Sedimentation Conference.

The riparian zone is the vegetated area between the water and higher ground. In the riparian zone, water, soil and vegetation interact and the plants that grow in this zone are distinctly different from neighboring plants that grow naturally on higher ground.

But how do riparian zone plants prevent erosion and sediment pollution?

Functional stream channel and riparian zone in the Brazos River Basin. Notice the lack of erosion in the channel and the variety of vegetation types.

  1. Most riparian plants have extensive root systems, or high rootmass. This high rootmass creates tension in the soil of streambanks that increase the stream banks resistance to erosion. When multiple types of riparian plants are present, it has been documented to increase resistance to erosion by 20,000 times when compared to the resistance to erosion of a streambank without vegetation, according to an article in Bulletin of the Geological Society of America.
  2. Healthy, dense riparian vegetation creates resistance and slows the flow of water along stream banks. The decrease in velocity has a twofold impact on the stream channel. First it decreases the shear stress on the streambank reducing the degree of erosion that might occur. Secondly the slower flow of water increases the chances for the silt and debris suspended in the water to deposit itself, in the riparian zone, maintaining stable streambanks, according to the National Research Council.
  3. Woody debris that comes to rest in a stream through natural processes can also impact erosion and sediment buildup along streams. When a fallen tree comes to rest along a stream bank, much like living riparian vegetation, it creates resistance that slows the flow of water on the stream banks and increases the buildup of the riparian bank. The newly deposited sediment around woody debris, then provides a location for new riparian vegetation to grow, thus further strengthening the stream bank.
  4. In addition to reducing erosion, a dense riparian zone reduces sediment pollution by filtering sediments out of stormwater runoff from upland areas before it reaches the streams.

It is important to note that dense and diverse riparian zones provide the best protection against erosion and sediment pollution. A tree every few hundred feet does little to create resistance and slow the water’s flow. A riparian zone with a mix of trees, shrubs, herbaceous vegetation and grasses, sedges, and rushes will provide the full spectrum of natural protection.

Taken by BRA Aquatic Scientists during a riparian zone assessment on the Brazos River near Richmond in October 2019. The photo depicts approximately 2 feet of additional sediment collected in the riparian zone since the monitoring location was established in 2013. (original survey marked is yellow circle at the bottom of the hole)

Riparian grasses, sedges, and rushes are very good at trapping suspended sediments carried in the stream flow and provide resistance during periods of low flows, according to the National Research Council. While trees and shrubs are most effective at creating resistance during high flow events.

The Brazos River is a treasured, beautiful waterway that provides scenic views to landowners and an important natural resource for humans, plants and animals of the state. Protecting and caring for its lifeline by protecting its riparian banks will not only safeguard the river-side owners’ property but will ensure the future health of this waterway. And educating one another is just one small step to help in that effort.

If you’re a landowner that would like to begin restoring riparian zones on your property, there is help available. You can call on the expertise of Texas A&M AgriLife Extension Service, Texas A&M Forest Service, the U.S. Department of Agriculture – Natural Resources Conservation Service, and Texas Parks and Wildlife Department personnel to help you develop a restoration plan for your property.

Brazos River Authority Environmental and Compliance Manager Tiffany Malzahn




Citations:

National Research Council (2002). Riparian Areas: Functions and Strategies for Management. Washington, DC: The National Academies Press. Retrieved from: https://doi.org/10.17226/10327.

Simon, A. & N. Pollen. (2006, April). A Model of Streambank Stability Incorporating Hydraulic Erosion and the Effects of Riparian Vegetation. Proceedings of the Eighth Federal Interagency Sedimentation Conference, Reno, NV. Retrieved from: https://pubs.usgs.gov/misc/FISC_1947-2006/pdf/1st-7thFISCs-CD/8thFISC/Poster_Simon.pdf.

Smith, D.G. (1976). Effects of vegetation on lateral migration of anastomised channels of a glacier meltwater river. Bulletin of the Geological Society of America 87, 857-860. Retrieved from: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/87/6/857/198325/Effect-of-vegetation-on-lateral-migration-of?redirectedFrom=fulltext.