Earth Notes: Zion’s Troubled Cottonwoods

Sep 16, 2015

It’s an iconic southwestern scene: the glimmer of green or yellow cottonwood leaves fluttering against the backdrop of Zion Canyon’s tremendous red- and cream-colored cliffs. Along southwest Utah’s Virgin River, groves of cottonwood trees please the eye, offer very welcome shade, and provide habitat for numerous types of animals.

The iconic and troubled cottonwood trees of Zion National Park.

But Zion’s cottonwoods are in trouble. Big as they are, the cottonwood trees along the river aren’t particularly long lived. Worse yet, as they die they are not being replaced by younger trees.

The story of the Virgin River shows how western riparian areas have been altered by humans, even in places protected from most impacts. Decades ago the National Park Service channelized the river to protect the canyon road and buildings. That has produced a deeper channel, and a stream that no longer wanders through the canyon bottom in response to floods.

Cottonwood seeds can sprout in wet sand by the river, but there they’re invariably wiped out by floods. On higher land in the canyon bottom, big cottonwoods can grow because their roots go deep enough to reach water, but seedlings can’t find the regular moisture they require.

Even if they could, the canyon’s large population of deer is an ever-present threat to the survival of sapling trees.

The Park Service has drawn up plans to re-engineer parts of the canyon infrastructure, but hasn’t received funding to do the work. Saving the seemingly natural canyon experience that people love now may require more very human intervention.