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Restoring Health of Dolores & San Miguel Rivers

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The San Miguel River is threatened by the invasion of tamarisk.


After eight years of tough battle, an ambitious effort led by The Nature Conservancy to remove invasive tamarisk along Colorado’s San Miguel River has reached an important milestone — 120 miles of the river are now tamarisk-free.

Tamarisk is a major threat to river systems across the West, choking out native plants and drinking up precious water resources. The impact of this aggressive competitor is symptomatic of the poor health of much of the Colorado River system, where damming and excessive water use by humans have compromised these rivers abilities’ to function naturally.

But the successful removal of tamarisk from this long stretch of the San Miguel means that native cottonwoods, willows and grasses are now returning to the river's banks.

Fighting Tamarisk, One Weed at a Time

What makes tamarisk such a threat to nature and people in the West?

This small, tree-like species (which is also known as "salt cedar") forms dense thickets that choke out native plants and consume massive quantities of water. Tamarisk roots extend 100 feet into the soil and can drink up more than 200 gallons of water a day, draining precious water resources in a region where every drop counts.

Tamarisk’s dense growth harms wildlife by blocking entrance to the water. And the plant is a nuisance to boaters and fishermen because it narrows streams and chokes out campsites.

“The impact of these woody invasives is huge — they rob waterways of their health and make recreational access cumbersome,” says Peter Mueller, director of the Conservancy’s North San Juan Mountain Program in Colorado.

But, he adds, “When you get rid of this wicked tree, all of a sudden you can see the light, and you can see the river again.”

Removal isn’t easy though. If tamarisk isn’t killed within a few minutes of cutting, it begins to re-grow. That’s why the Conservancy relied on the help of dozens of volunteers to clear tamarisk from the banks of the San Miguel.

A Secret Weapon: The Tamarisk Beetle

Aiding the effort is the spread of the tamarisk beetle, introduced into the West in the 1990s as a biological control agent. The U.S. Department of Agriculture imported tamarisk beetles from Eurasia, where they keep tamarisk in check, and after years of quarantine and testing, released them in Utah, Wyoming and Colorado.

“These beetles are one of the most tested biological agents we have and there’s little risk of them harming other plants,” says Mueller.

Over the last three years, the beetles have defoliated a majority of the tamarisk on a 60-mile reach of the Colorado River. From the release site in Utah, the insects have now moved into Colorado and the Dolores River watershed.

A Model for Success

The San Miguel is one of the last naturally functioning rivers in the West — and one of its most pristine. A tributary of the Dolores River, it descends rapidly from the spectacular, snow-capped San Juan Mountains, through forests and grasslands into the red sandstone semi-desert of the Colorado Plateau. 

This river system supports healthy riparian and aquatic communities harboring more than 60 rare or imperiled plant and animal species.

Now that the San Miguel is on the road to recovery, the partnership will turn its focus to the Dolores — a more difficult river to tame because damming has altered its flow and flood timing, a condition that favors tamarisk and other exotic species.

Restoring the health of the Dolores will require not only tamarisk removal, but improved water management and planting of native species.

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