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Dolores River Home Hydrology Geomorphology DRAMS Team 2023 Dam Release
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.
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).
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 was diverted in some years to keep water flowing through the diversion at all times.
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.
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.
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.
From previous research, we have developed an understanding of how changes in hydrology have impacted vegetation on the Dolores River. For the DRAMS monitoring project, we sought to expand our understanding of vegetation dynamics, particularly in response to the lower flows now commonly seen on the Dolores River.
Additionally, the Dolores River Monitoring and Recommendations Team sought input from DRAMS to increase understanding of how different flow levels impact native fish and their habitat. Vegetation monitoring occurred at 6 sites (see Figure X – map of DRAMS vegetation study areas).
To assess changes in riparian vegetation in response to different flow levels, we began monitoring the geomorphic features in the river corridor. Based on position relative to the river channel and their dominant vegetation types, we divided each of the monitoring sites into zones controlled by both geomorphology and vegetation tolerances (Figure A).
Four of the zones have characteristic vegetation types, that changed in similar ways across the sites over the course of the monitoring period. Each zone is described in detail below:
Willow cover increased at most sites post-flood.
In many areas, stream banks are covered in dense growths of coyote willow, with some giant reed grass (Phragmites), and alder in the upper sites (e.g. Dove Creek) (Figure B).
Vegetation encroachment on the channel margin is a key component in the channel narrowing and habitat simplification that is impacting native fish.
After the 2023 high flows, willow cover in this zone increased by 20-30% at most of the sites.
Vegetation cover increased and bare cobble decreased over time.
Cobble bars are important high water fish spawning habitat that is located along river banks or on islands in mid-channel. Historically they were kept open and bare of vegetation by high scouring flows during spring floods (Figure C).
At the sites where we have sufficient data on this zone, cover by non-woody plants (sedges, grasses, other small weedy plants) increased 15-30% from 2022-25. Bare ground/cobble area decreased by 20-30%.
The number of woody plant stems (mostly willow) increased gradually each year, from around 5-10 stems/m2 in 2022 to ~20 stems/m2 by 2025.
No change in privet, tamarisk and other shrub cover over time in this zone.
Increase in cottonwood cover post-flood in this zone.
This area is higher above the river channel, and hosts a mix of native and non-native shrubs, dominated by New Mexico privet and non-native tamarisk (Figure D).
We saw little to no change in the abundance of privet and other shrub species during the study.
However, at two sites we also find cottonwood trees growing in the upper floodplain zone (Figure E). See next bullet
Dry years led to declines in cottonwood health and coverage, but coverage of cottonwoods increased by 10-20% after 2023 high flows, and the number of apparently “dead” saplings declined as they re-sprouted following the flood.
Dryland shrub cover increased on the upper floodplain during dry years, but significantly decreased after high flood flows.
These species thrive on higher and drier areas located mainly outside of the main floodplain, but at several sites they are also moving onto the upper floodplain during the increasingly frequent periods of drought and low stream flows. Main shrub species are sagebrush, rabbitbrush and saltbush. (Figure F)
During dry, low flow periods, upland shrub species cover increased significantly. For example, at Big Gypsum they comprised over 25% of total site cover by 2022 after 3+ years of low flows.
However, a single high flow year in 2023 caused big declines in these inundation intolerant species – by 2024 at Big Gypsum, they only covered about 5% of the site. (Figure G)
For more detailed findings of the vegetation monitoring effort, visit the 2023 Flood page.