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Riparian Vegetation | DRAMS | Four Corners Water Center

Riparian Vegetation

"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 of the correlation that different types of vegetation have with different river conditions. River corridors create a unique habitat for plants and animals due to different conditions such as increased water availability and biodiversity. Team members have focused their research on riparian vegetation along the Dolores at the Big Gypsum Valley study site.

Changes in vegetation density and composition on river banks and bars can lead to simplification of the channel and floodplain. 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 its observable lifetime in the Southwestern United States, the Dolores River has demonstrated two different components of drought. The first is a low summer flow style that is commonly found in un-dammed hydrologic systems. This style of drought can even cause rivers to run completely dry some years. The second style of drought is one with low or no spring peak flows caused by runoff being stored in upstream dams or catchments. It it 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 mimic one of these components 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 reach up to 700 CFS during peak growing season, when water resources were in highest demand. During summer droughts however, the diversion would either run dry or low.

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 the 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
The McPhee Dam, completed in 1984.

Vegetation change on the lower Dolores River

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

  1. Hypothesized vegetation composition from the pre-diversion period (pre-1889).
     
  2. 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.
     
  3. 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.

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 the garden bed, and low flows in the summer would permit seedling growth. Finally, June rains water the garden during peak tamarisk seed dispersal.

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 garden 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 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 as 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 Invasive Species map

On the interactive map below, click on the arrows on the top left to show the legend. Click on the corresponding colored areas within the map for information on each invasive species. Data source: DRRP.


Dolores River Revegetation map

On the interactive map below, the green shapes are where revegetation has occurred. Data source: DRRP.

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FCWC staff

Gigi Richard, Ph.D.
FCWC Director & Geosciences Instructor

Center of Southwest Studies, Room 265
970-247-6561
water@fortlewis.edu

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