Waterlines: Tamarisk and our changing riverbanks | PostIndependent.com
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Waterlines: Tamarisk and our changing riverbanks

Gigi Richard
WATER LINES
Free Press Weekly Columnist

It’s a beautiful sunny afternoon and you’re on the Riverfront Trail, pedaling your bicycle along the Colorado River, and suddenly there are orange caution signs and the pavement turns to gravel. What happened? Is the path being improved, or new utilities being installed? Probably not. Most of the time, when you see gouges in the river bank leaving the paved bike path unsupported, or even missing, the Colorado River is responsible.

Flooding rivers have a lot of power to do an amazing amount of work. A flooded river can move gravel, and larger stones, reshaping the riverbanks. The work that rivers do is beneficial, such as maintaining spawning gravels and side channels for native fish to flourish, and depositing nutrients on the floodplain. However, sometimes that swift current, chewing away at the banks, undermines man-made structures that are located in the floodplain, like bike paths.

Back in the late 1800s, Westerners were frustrated with losing valuable farm and ranch land to fast moving rivers and decided to do something about it. A common strategy for protecting a river’s bank from erosion was to dump junk along the bank – large chunks of concrete, rubble, bricks, and old cars. Another strategy was to plant tamarisk, an invasive plant, along the streams to help stabilize the riverbanks.

Tamarisk, a remarkable plant, has colonized many rivers in the Southwest and has become a nuisance. The list of tamarisk’s vices is a long one. Dense tamarisk stands have stabilized the banks and locked these rivers into a single-thread channel, an unnaturally stable position. Tamarisk’s deep taproot allows a thicket of tamarisk to extend farther from the river than native riparian forest, which may take more water from the river. It forms a monoculture, out-competing native plants and leaving behind a salty duff on the ground that’s not conducive to the growth of other plants. It resists all attempts to remove it, being resilient in the face of floods, droughts and fires.

In the last five years, efforts to remove tamarisk have ramped up as restoration of our river corridors and preserving water have become higher priorities. But what does the removal of this bank stabilizing plant mean for our managed river systems? If we remove tamarisk will the rivers erode their banks with renewed vigor?

The answer is not so straightforward, and like many questions related to the functioning of natural systems, “it depends.” The bank material, the removal method, the vagaries of weather, and the presence of upstream dams all affect whether tamarisk removal will result in increased bank erosion or not. Some studies have shown dramatic channel change following tamarisk removal, for example on the Rio Puerco, New Mexico, and other studies show little, such as in the Canyon de Chelly in Arizona.

Ongoing research, conducted by Colorado Mesa University and the Tamarisk Coalition, into this question will help us understand some of the complexity of the interaction between tamarisk removal, and the potential destabilizing response of the river. Over the last two years, the Colorado River channel has been surveyed prior to tamarisk removal at three sites in the Grand Valley. The river will be resurveyed, after the next high spring snowmelt, and changes in the channel will be studied. In addition, historic aerial photographs are being analyzed to understand how the river has moved historically in areas where tamarisk was removed and where riparian vegetation remains.

As population grows, demands on our water supply will increase. We strive to support healthy watershed and riparian areas and tamarisk removal efforts will continue. Better understanding of how rivers respond to tamarisk removal will be useful in designing effective riparian restoration and tamarisk removal efforts. So, when you come across gravel sections along the bike path, you can appreciate the hardworking Colorado River, doing its job, moving sediment and eroding its banks, whether tamarisk removal played a role, or not.

Gigi Richard, Ph.D., is a Professor of Geosciences at Colorado Mesa University. For more information on the Tamarisk Coalition and their current projects visit, http://www.tamariskcoalition.org/programs/projects-monitoring


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