Riparian Vegetation

Dolores River Home Hydrology Geomorphology DRAMS Team

The term "Riparian corridor" refers to the unique plant communities that exist in areas near any natural body of water. Riparian corridors are important indicators of river health because native riparian plants (e.g. cottonwoods, willows, etc.) are intimately adapted to river flow and sediment conditions. River corridors create a unique habitat for plants and animals due to the increased availability of water, which leads to an increase in biodiversity. In the past, team members focused much of their research on riparian vegetation along the Dolores at the Big Gypsum Valley study site.

Changes in vegetation density and composition on riverbanks and bars can lead to simplification of the channel and floodplain, which in turn have major impacts on fish and wildlife. Monitoring vegetation change over time can help us learn what flow dynamics trigger vegetation expansion, what kinds of flow patterns might lead to vegetation decrease, and how drought years impact vegetation density on banks and bars.

Water flows through green vegetation surrounded by red cliffs and a blue sky. — Riparian tree establishment captured by Dr. Cynthia Dott.

The Role of Drought in Vegetation Establishment & Survival

In the recorded history of the Dolores River (e.g. written and photographic records) two different styles of drought have occurred. The first is a low summer flow style that is commonly found in un-dammed hydrologic systems in the Southwest. Because water was diverted from the Dolores River for agriculture starting in 1889, the summer drought pattern was exaggerated and often caused the river to run completely dry during mid- to late summer. The second style of drought is one with low or no spring peak flows caused by runoff being stored in upstream reservoirs for use in irrigated agriculture. It is unusual to be able to separate the impacts of these 2 components of drought, but the Dolores is unique: There are two different time periods that each exhibit one of these styles of drought, described below as Post Diversion (1889-1984), and Post Dam (1984-Present).

Post Diversion (1889-1984)

In 1889, one of the largest irrigation projects in the Southwest at the time was completed. The Montezuma Tunnel irrigation project cut through 5,200 ft of earth to deliver water to the Montezuma Valley. The Montezuma diversion would carry up to 700 CFS during peak growing season, when water resources were in highest demand. During summer droughts when water is most scarce, all of the flow in the river channel may be diverted to keep water flowing through the diversion at all times.

On old black and white photo of a tunnel with water surrounded by cliffs. — The Montezuma Tunnel, completed in 1889.

Post Dam (1984-Present)

In 1984, the completion of McPhee Dam had a marked effect on the hydrology and irrigation usage of the Dolores. The dam allows for storage of spring snowmelt flows, and a greater degree of control over summer irrigation. This style of irrigation creates droughts in the spring, when all the runoff is being stored in the dam, rather than flowing downstream as it would un-dammed.

Bird's eye view of a reservoir, dam, and river below — McPhee Dam, completed in 1984. Image from Mitch Tobin, The Water Desk

Vegetation change on the lower Dolores River

Schematic diagram of change over time in three generalized time periods.

Hypothesized vegetation composition from the pre-diversion period (pre-1889).
Observed vegetation composition from the post-diversion period (1950-1984). This period was characterized by frequent high peak flows and very low minimum flows which supported tamarisk recruitment.
Current vegetation composition from the post-dam period (1990-present). This period is characterized by low peak flows and higher minimum flows. Extensive channel narrowing and vegetation encroachment have occurred, dominated by willows (especially Salix, exigua, coyote willow), coupled with drying of the upper floodplain and loss of older cottonwoods. Tamarisk have also declined in this period due to active removal and introduction of the tamarisk leaf beetle.

Vegetation Post Diversion

During the Post-Diversion period (1889-1984), 92% of tamarisk within the Dolores River corridor were established. This was made possible by the unique hydrologic system that the Montezuma Tunnel created. High flows during spring runoff would scour and prepare a seed bed, and extremely low flows in the summer would provide ideal locations for seedling growth. Finally, years with high June rains watered the seed bed during peak tamarisk seed dispersal. For more information, read A tale of two rivers: Dam-induced hydrologic drought on the lower Dolores River and its impact on tamarisk establishment.

Vegetation Post Dam

During the Post-Dam period (1984-Present), almost NO tamarisk were established. However, willow was able to thrive due to lower peak flows (no scouring), and higher low flows that keep willow roots wet. The growth of the willow thickets and the hydrologic conditions that allow it ultimately lead to channel narrowing and loss of native fish habitat. The following photos show willow growth & encroachment (coyote willow or Salix exigua) on the banks of the Dolores River during the post-dam period.

The Dolores River Restoration Partnership

The Dolores River Restoration Partnership (DRRP) was founded in 2009, and has since worked to restore nearly 170 miles of the Dolores River riparian corridor following the removal of invasive tamarisk. The partnership has successfully removed about 70% of the tamarisk initially mapped and has subsequently focused their collective efforts on monitoring and maintaining past work, while strategically working on tamarisk and other invasives removal in hard-to-reach areas.

As part of the monitoring program, invasive species of concern are mapped, and that data is given to crews who will go into the field and remove or treat the species as determined by DRRP coordinators and the Bureau of Land Management.

Active revegetation of native trees, shrubs, forbs, and grasses is also part of restoration. Active revegetation often is conducted in areas with ideal soil, locations that would benefit from greater plant diversity, and areas where there is minimal cattle grazing to prevent predation. These sites can take several years to reach a level of restoration and ongoing stewardship is critical to the long-term health of the Dolores River corridor.

Dolores River Vegetation Maps

Shown in the link are two maps from the DRRP website: The Dolores River Invasive Species map and the Dolores River Revegetation map. These two interactive maps illustrate the location of various invasive vegetation species and areas of revegetation along the Dolores River.