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Intentionally introduced to the western United States in the 1800s, tamarisk is a bully of a neighbor that replaces native species with a dense monoculture that no native herbivores care to eat

The San Rafael River is only 90 miles long, originating at the confluence of three creeks emanating from the Green River in the Wasatch Plateau, two miles upstream of the Labyrinth Canyon Wilderness. This is red rock canyon country in Utah, rugged and sublimely scenic. It is a wonder that the San Rafael, which dwindles to a shallow creek during summer and fall, could have carved such deep canyons.

Approximately 15 miles downstream of the confluence is Little Grand Canyon, about 10 miles long, and at the Wedge Overlook, 1,000 to 1,200 feet deep. The overlook provides not only a fine view of the river below but also a panoramic view of Sid's Mountain Wilderness to the south and Mexican Mountain Wilderness to the east. The eastern end of the Little Grand Canyon opens to the Historic Swinging Bridge, built in 1937 to allow mining and cattle trucks to cross the San Rafael River at the Buckhorn Draw. Primitive campgrounds at Wedge overlook, Swinging Bridge and along Buckhorn Draw make this an adventurer’s destination.

The San Rafael River enters Mexican Mountain Wilderness at Swinging Bridge. From there, Mexican Mountain Road runs between the river and an escarpment of tall cliffs for 30 miles. This is a rough road, definitely 4WD-HC, but it is worth the time and jostling, for it leads to Mexican Mountain and three spectacular slot canyons: Lockhart, Upper Black Box and Lower Black Box. A slot canyon is particularly deep and narrow—for example, both Upper and Lower Black Box are miles long, and in some sections, each is 400 feet deep and other sections only 25 feet wide. In both slot canyons, the water is so deep that most of the passage is achieved by swimming or drifting in an inner tube. Upper Black Box is usually entered by rappelling vertical walls 60 or 80 feet tall. I don't do that. I have only peered into Lockhart and Upper Black Boxes—both provided awesome views and opportunities for photos of Mexican Mountain looming high above a deep and narrow slot canyon.

With all the pinnacles, canyons and cliffs to appreciate, it is easy to overlook the slow and silent drama gripping the plant community in the red rock canyons of the San Rafael River. In the early to mid 1800s, multiple species of tamarisk were introduced to the western United States, and today six species can be found on or around the Colorado Plateau. The most common tamarisk species is probably Tamarix chinensis (synonym ramosissima) from China. Tamarisk was purposely introduced to the southwest for its abilities to thrive in a dry climate and colonize and stabilize soils that no other plants could tolerate.

The problem with tamarisk is that it is a bully of a neighbor, replacing native species, such as cottonwoods, willows and rabbitbrush, until the streams and rivers are lined with a dense and virtually impenetrable monoculture that no native herbivores care to eat. Although it has the growth form of a shrub, tamarisk is technically a tree, and dense stands turn into denser stands of deadwood, transforming the plant community and creating a fire hazard.Ìý Each plant can produce between 500,000 and 600,000 seeds per year, so when a fire comes, the dead branches spread the fire quickly, killing most plants. When the next rains come, an enormous bank of tamarisk seeds are waiting; tamarisk becomes more numerous with each fire.

Four or five decades ago I saw stretches of the Green River lined with stately cottonwoods that were inviting to campers, picknickers and fishers. Since then, tamarisk has moved in and changed that pleasantly shaded riverbank to a dense, sharp, scratchy thicket, profoundly unpleasant to fight through. In addition, tamarisk colonized the river's edge, trapping sediments and narrowing the channel. Some strands of cottonwoods, increasingly isolated from the water, have died. Narrowing the river channel changes its ecology for a variety of fish species. Tamarisk has many pretty flowers, but the only other civil thing that can be said for tamarisk is that it is very nearly the perfect weed: accumulates deadwood, is flammable and inedible, and has deep roots and high seed production.Ìý

When tamarisk invaded national parks and monuments and state parks, state and federally employed ecologists initiated control measures. Dinosaur National Monument, Arches National Park, Saguaro National Park, Mojave Trails National Park, Canyonlands National Park, Glen Canyon and Lake Mead National Recreation Areas all initiated programs to manage the perfect weed. They were joined by programs in the Colorado, Virgin, Dolores, Green and San Juan Rivers. It isn't easy to remove the perfect weed from a landscape. Fire, herbicides, chainsaws and bulldozers have all been tried, and although they can diminish the population of tamarisk, it always returns. Tamarisk is in the Little Grand Canyon and along the San Rafael River to Upper Black Box and below the Lower Black Box to the Green River. It is hard to find a stream or river in the southwest that is not being slowly claimed by tamarisk.

A new tool for the managers of public lands is being applied now. When tamarisk was introduced to North America, it escaped the herbivores that had evolved to eat its leaves and roots. But now, closely related species referred to as "tamarisk beetle" are being introduced to tamarisk thickets—including some in the downstream portions of the San Rafael River. Introductions by managers evoke both hope and dread.Ìý

Some introductions have been wonderful successes; others have been disastrous. So far, the managers have not seen any proclivity for the tamarisk beetle to eat anything other than tamarisk. Experienced managers do not use the word eradicate, but a realistic goal is to reduce tamarisk to a minor species in an otherwise healthy community of native species.Ìý

Jeff Mitton is a professor emeritus in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. His column, "Natural Selections," is also printed in the Boulder Daily Camera.

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