Photo: E. Palmquist 

Getting to know Colorado River 

riparian plant communities 

Emily Palmquist1 & Brad Butterfield2 

Glen Canyon Dam Adaptive Management Program 

Annual Reporting Meeting 

23 - 24 January 2024 

1 U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center 

2 Department of Biological Sciences, Northern Arizona University 
Why are riparian plant 

communities important? 

• Alter river shape and sediment dynamics 

• Supports migratory and resident animals 

• Traditional plant resources 

• Recreational impacts – positive and negative 

• Increase regional biodiversity 

• Mediate resources between rivers and uplands 

• Improve water quality 

Photo: E. Palmquist 
 

 

Species and 

characteristics 

determine function 

• Species have different roles in 

the environment 

• Roots, leaves, architecture, 

plant health form the basis for 

ecosystem function 

• ‘Riparian vegetation’ differs 

greatly depending on the 

species 

Photos: E.Palmquist 
Importance of Cover vs. Composition 

Goal 11. Riparian Vegetation. Maintain native vegetation 

and wildlife habitat, in various stages of maturity, such 

that they are diverse, healthy, productive, self-sustaining, 

and ecologically appropriate. 

• Cover 

• How much space plants take up 

• Productivity, abundance 

• Composition

• Characteristics (e.g. trees, grasses, root depth, clonality, etc) 

• Diversity 

• Who’s present, who’s missing? 

Photo: USGS GCMRC 
 

 

 

 

 

 

 

Photo: R.B. Stanton 1890 2016 Photo: Alan H. Fairley & Helen Fairley 

Plant communities 

are capable of large 

changes when dam 

operations change. 

• Little cover of riparian plants in predam era 

• Increase in cover of woody and herbaceous plants 

• But species differ over time 

• Period of riverine marshes associated with high fluctuating 

flows 

• Tamarisk beetle leading to tamarisk death 

• Losing Goodding’s willows 

Bedford and others, 2018; 

Brian, 1982; 

Carothers and others, 1976; 

Durning and others, 2021; 

Kearsley and Ayers, 1996; 

Palmquist and others, 2023; 

Ralston, 2010; 

Sankey and others, 2015; 

Stevens and Waring, 1986; 

Turner & Karpiscak, 1980 
Drivers of riparian plant communities 

Climate: Temperature, precipitation, humidity 

 River Hydrology: HFEs, daily fluctuations, seasonality 

 Geomorphology: Channel shape, landforms 

 Biotic Interactions: Competition, herbivory 

Riparian Plant Community: Cover, composition, diversity 
Mahoney and Rood, 1998; 

Poff and others, 1997; 

Butterfield and others, 2023; River Flows Bejarano and others, 2018; 

Flood timing, duration,

frequency, magnitude Drawdown/Rising rates and • Regeneration timing • Disturbance 

Volume 

• Riparian extent 

Variability • Species 

• Water reliability 

• Predictability 

• Flood attenuation 

• Erosion 

• High/low flow timing

• Water availability/stress during 

hot/cold periods 
 

 

 

 

 

Butterfield and others, 2020; 

Dean and others, 2019; 

Dean and Topping, 2024 ; 

Manners and others, 2014; 

Palmquist and others, 2023; 

Palmquist and others, in 

review; 

Sankey and others, 2023 

Plant – Sediment interactions 

• The amount of sand vs. rock along river shorelines 

influences which plants grow where 

• The kind and density of plants influences sediment

deposition 

• Tall, thick plant communities can reduce windblown sand 

• Relationships between sand and plants can drive river

ecosystem change 

Image credit: E. Palmquist & V. Leshyk 
 

 

What do we want this 

community to look like? 

• Plants are here to stay 

• The water is there, and plants are already using it 

• Already protected (proposed wilderness) 

• Already being used by resident and migratory wildlife 

Photo: USGS GCMRC Photo: USGS GCMRC 
 

 

 

What we have now 

Photos: E. Palmquist 

• Recorded over 300 species 

• Tiny annual forbs to tall trees 

• Native & nonnative species 

• Lots of species in sunflower 

family (Asteraceae) 

• Bloom in different seasons 

(spring, summer, fall) 

• Annual, perennial, biennial 

• Clonal or not 

Palmquist and others, 2018; 

Palmquist and others, 2023 
What we have now – grasses/forbs 

Photo: E. Palmquist 

Bromus rubens 

red brome 

Photo: E. Palmquist 

Photo: E. Palmquist 

Equisetum x 

ferrissii 

horsetail 

Sporobolus flexuosus 

Mesa dropseed 

Photo : E. Palmquist 

Cynodon dactylon 

Bermuda grass 

Palmquist and others, 2023 
What we have now – trees/shrubs 

Baccharis emoryi 

Emory’s baccharis 

Photos: E. Palmquist 

Photos: E. Palmquist Photos: E. Palmquist 

Baccharis sarothroides 

desertbroom 

Photos: E. Palmquist 

Brickellia longifolia 

Long-leaf brickellbush 

Palmquist and others, 2023 

Tamarix ramosissima x 

chinensis 

saltcedar 
 

Butterfield and others, 2023; 

Palmquist and others, 2023, What we have now – hydrological zones Butterfield and others, 2024, 

Palmquist and others, in review 

• Daily fluctuating flows, seasonal high and low flows, and HFEs form longitudinal strips of plant 

communities 

Image credit: Victor Leshyk 
 

 

 

What we have now -

composition 

• Distinct floristic groups associated with Marble 

Canyon, eastern Grand Canyon, western Grand 

Canyon 

• Diversity decreases with distance downstream 

• Shift from more shrubs to more grasses with 

distance downstream 

• Greater native cover and richness than nonnative 

Palmquist and others, 2018; 

Palmquist and others, 2023 

Graph from Palmquist and others, 2023. 
What we don’t have 

Photos: E. Palmquist 

• Cottonwoods and tree willows 

• But in tributaries 

• Only one species of shrub willow 

• Few sedges, rushes 

Palmquist and others, 2023 
 

 

How are we studying plant community 

responses to dam operations? 

• Annual monitoring (C.1) 

• Manipulative experiments (C.2) 

• Synthesis of multiple lines of evidence & predictive modeling (C.3) 

• Management decision support (C.4) 

Photo: USGS GCMRC Photo: E. Palmquist Photo: E. Palmquist Image: B. Butterfield 
Photos: USGS GCMRC 

C.1 We record 

patterns annually 

• Tracking status and trends

• Temporal shifts in metrics 

• Differences among geomorphic 

surfaces 

• e.g. Camping beaches are not 

representative 

• Trends in species of interest

• e.g. Baccharis species are 

increasing 

• Backbone for studying impacts of 

dam operations 

• Publications, Web Tool, SEIS, etc. 

• Impacts of dam operations vary 

throughout the canyon 

Palmquist and others, 2023 
 

 

 

 

 

C.1 Many factors interact with dam 

operations to impact riparian plant 

communities 

• Where you are matters: some species do better with high daily flows at 

night, others during the day! 

• Responses to dam operations depend on temperature, which increases

downstream 

• Amount of sand is almost as strong of an environmental driver as water

availability/inundation 

Sediment 

Dam 

Operations 

Plant Palmquist and others, in review 

Preliminary Information – Subject to 

Revision. Not for Citation or 

Distribution. 

Community 
 

 

 

Correlation is not causation 

Hydrograph has been fairly invariant over our recent monitoring period 

Many factors co-vary in space 

There are factors we do not or cannot measure that may be influential 

One solution: Experiments! We manipulate water availability in a 

controlled setting in ways that we cannot in the field 
C.2 Take

-homes from 

past experiments 

• Inundation tolerance is predictable from 

habitat preferences in the field 

• Drought tolerance is not, suggesting species-

specific responses to drawdowns in river stage 

• Clonal species (e.g. arrowweed), which are 

generally understudied in this region, behave 

much differently than the well-studied trees 

(e.g. cottonwoods) 

• Cottonwoods and tree willows are less 

tolerant of both inundation AND drought than 

common species in CRe 

Photo: E. Palmquist Image credit: Victor Leshyk 

Palmquist and others, 2022; 

Butterfield and others, 2024 
Baccharis emoryii Pluchea sericea 

Root Growth (log(g+1)) 

 

 

 

 

 

C.2 New for 2023: 

Daily fluctuations 

• Pilot experiment with three species 

simulating active channel and active 

floodplain habitats 

• Water level held constant, or fluctuating, 

with both day and night peak treatments 

• Null hypothesis: If daily fluctuations do 

not impact plant performance, then 

control and fluctuating conditions should 

produce similar responses 

• Preliminary results reject this null 

hypothesis, and provide some support 

for differences in responses to day/night 

timing 

Preliminary Information – Subject to Revision. 

Not for Citation or Distribution. 
 

Limitations of experiments 

• Greenhouse setting 

• Small plants 

• Need to assess relevance to 

natural systems and scales 

Photos: B. Butterfield 
 

 

C3. Predictive modeling 

and synthesis 

• Webtool, SEIS, etc. 

• Response to flow alterations 

• Rapid model prediction for many 

scenarios 

• Regional context 

• How does the CRe compare to other 

riparian plant communities across the 

region? 

• How might different hydrographs, 

outside those observed in the CRe, 

impact community composition? 

From: Butterfield and others, 2022 

Image credit: BOR 
C3. Predictive modeling 

and synthesis 

• Who do we have, who do we 

not have, and WHY? 

• Previous work (Butterfield and others, 

2022) suggests some intriguing patterns 

• Regional perspective, similar to 

Kennedy and others, 2016 

• Integration of multiple lines of 

evidence into our models 

• e.g. daily fluctuations 

• greenhouse experiments 

• field observations 

• regional analysis 

 

Climate niche model 

(Regional Data) says CRe is: 

Suitable Unsuitable 

Survey (Local Data) 

says CRe is: 

Unsuitable Suitable 

? 

? 

From: Kennedy and others, 2016 
 

 

Next Steps 

• Effects of daily fluctuations 

• Regional context 

• Greenhouse experiments 

• Plant traits 

• Impacts of plant communities on 

wildlife 

• Mechanistic models 

• Soil moisture dynamics 

• Sediment feedbacks 

• Flipping the script 

• Previously: Given these flows, what 

plant communities do we get? 

• Proactive: Given these plant community 

objectives, what flows would we need? 

Photos: J. Scott 
 

 

 

Plant community responses to 

dam operations are influenced 

by external changes 

• Tamarix die-back 

• Phragmites australis 

• Arrival of new nonnative species 

• Future flows uncertainty 

• LTEMP plant management 

Photo: E. Palmquist 
 

 

Many colleagues, collaborators, boat 

operators, students, and volunteers 

assisted with these projects -

Thank you! 

Photo: USGS GCMRC 

Further questions? 

Contact us: 

epalmquist@usgs.gov 

or Search “Emily Palmquist USGS” 

Bradley.butterfield@nau.edu 

Interested in the riparian vegetation program? 

Check out our websites: 

https://www.usgs.gov/centers/southwest-biologicalscience-center/science/overview-riparian-vegetationgrand-canyon 

https://www.usgs.gov/centers/southwest-biologicalscience-center/science/terrestrial-riparian-vegetationmonitoring-how 

or Search “GCMRC riparian vegetation” 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References 

Bedford, A., Sankey, T.T., Sankey, J.B., Durning, L.E., and Ralston, B.E., 2018, Remote sensing of tamarisk beetle (Diorhabda carinulata) impacts along 412 km of the Colorado River in the 

Grand Canyon, Arizona, USA: Ecological Indicators, v. 89, p. 365-375, https://doi.org/10.1016/j.ecolind.2018.02.026. 

Bejarano, M.D., Jansson, R., and Nilsson, C., 2018, The effects of hydropeaking on riverine plants—A review: Biological Reviews, v. 93, no. 1, p. 658-673, 

https://doi.org/10.1111/brv.12362. 

Butterfield, B.J., Palmquist, E.C., and Ralston, B.E., 2018, Hydrological regime and climate interactively shape riparian vegetation composition along the Colorado River, Grand Canyon: 

Applied Vegetation Science, v. 21, no. 4, p. 572-583, https://doi.org/10.1111/avsc.12390 

Butterfield, B.J., Grams, P.E., Durning, L.E., Hazel, J.E., Palmquist, E.C., Ralston, B.E., and Sankey, J.B., 2020, Associations between riparian plant morphological guilds and fluvial sediment 

dynamics along the regulated Colorado River in Grand Canyon: River Research and Applications, v. 36, no. 3, p. 410-421, https://doi.org/10.1002/rra.3589. 

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Diversity and Distributions, v. 27, no. 2, p. 377-388, https://doi.org/10.1111/ddi.13192. 

Butterfield, B.J., Palmquist, E.C., and Yackulic, C.B., 2023, The hydroclimate niche: A tool for predicting and managing riparian plant community responses to streamflow seasonality: River 

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Butterfield, B.J., and Palmquist, E.C., 2024, Inundation Tolerance, Rather than Drought Tolerance, Predicts Riparian Plant Distributions Along a Local Hydrologic Gradient: Wetlands, v. 44, 

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Manners, R.B., Schmidt, J.C., and Scott, M.L., 2014, Mechanisms of vegetation-induced channel narrowing of an unregulated canyon river—Results from a natural field-scale experiment: 

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Palmquist, E.C., Allan, G.J., Ogle, K., Whitham, T.G., Butterfield, B.J., and Shafroth, P.B., 2021, Riverine complexity and life history inform restoration in riparian environments in the 

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