U.S. Department of the Interior

U.S. Geological Survey

Assessing Vegetation Response to Dam Operations 

using the Response Guild Approach 

Barbara E. Ralston, USGS in collaboration with 

David Merritt, USFS, Patrick Shafroth, USGS, Todd Chaudhry, NPS

Lori Makarick, NPS, Dustin Perkins, NPS
Overview

• Review general knowledge about 

riparian plant species downstream 

from Glen Canyon Dam

• Highlight known responses of vegetation to 

Glen Canyon Dam releases

• Explain monitoring using Response 

Guild Approach

• Identified Guilds for the Colorado River

• Linkages with National Parks in Upper 

Basin 

• Implementation of monitoring 

• Preliminary results from October 2012 

sampling trip

• Conceptual-frame based modeling

• Tamarisk Beetle Status
Inter-annual and seasonal 

variability in hydrology 

Flood timing 

Duration 

Magnitude 

Frequency

Surface water-ground 

water interactions –

Minimum discharge 

Daily range

Operations Location on the 

Landscape/Landscape condition

Drivers of Vegetation Change

Seed dispersal/germination

Scour, cover, occupancy space

Area of disturbance

Species selection/successional 

processes

Distance to water – upland, woody 

riparian species

Inundation duration – marsh species, 

woody riparian species
The Early Years

Stanton Photo 1890, Cardenas Creek, National Archives and Records Administration.

Mesquite

Predam flood 

stage 

2407m3/s
Plants along the Colorado River –

historic assemblage

Mesquite/Apache Plume

Seepwillow

Arrowweed

Tamarisk

Desert broom

Camelthorn

Coyote willow

Hydrologic gradient

Gooding's 

Willow/ 

Cottonwoods

Brickellia

Hackberry

Disturbance adapted

ephemeral

High Low

Low
Period I – Flood reduction and flow stabilization

Woody 

Vegetation 

Expansion

Marsh 

Development
Period II – Prolonged flooding, sediment reworking & 

export, vegetation removal & re-establishment

Vegetation 

Removal Marsh 

Development

Woody 

Vegetation 

Expansion
Period IV 2000-2012 – Reduced fluctuations, minimum 

annual delivery (drought) and equilization flows (MLFF, 

LSSF, HFE operations)

Continued vegetation expansion
113 Years Later with 40 Years of Regulation

Tamarisk mixed w/ 

Goodding’s willow

Postdam vegetation 

expansion

Predam flood 

stage 

2407m3/s

Repeat photograph 2003, Cardenas Creek 

(USGS Desert Laboratory Repeat Photography Collection).

Arrowweed, 

Baccharis/Coyote willow
Plants along the Colorado River –

post regulation

Disturbance adapted

Seepwillow

Arrowweed

Tamarisk

Desert broom

Common reed

Cattails

Horsetails

Camelthorn

Coyote willow

Mesquite/Apache Plume Brickellia

Hackberry

Goodding’s

Willow/ 

Cottonwoods

Hydrologic 

gradient

Horseweed

Low

High

Low
General vegetation response

 Riparian woody vegetation is expanding

 HFEs of present magnitude/duration do not remove 

woody vegetation

 Coarsening sandbars favor drought adapted vegetation 

shoreward migration of woody species

 Basin hydrology, daily fluctuations and maximum daily 

discharge affects woody vegetation expansion.

 Operations that scour sandbars followed by reduced 

flows  tamarisk colonization 

 At lower stage elevations, increased frequency of HFEs 

may favor clonal species that are burial adapted over seed 

production 
Riparian Vegetation-Flow Response Guilds 

Framework

Life history

Reproductive strategy

Morphology

Fluvial disturbance

Water balance 

Monitoring vegetation response to 

dam operations 

Groups of species that have shared life history traits and respond

similarly to physical variables (e.g., hydrologic regime, geologic

setting/substrate properties, ambient temperature)

Merritt, Scott, Poff, Auble, Lytle. 2010. Freshwater Biology 55:206-225
Acer negundo/

Prosopis glandulosa

Tamarix ramosissima

Phragmites australis

Juncus bufoniu

Selection and Adaptation
Life History Guilds

Traits Relevant Flow 

Component

 Longevity (life span)

 Annual, short- and long-lived perennial, 

biennial

 Frequency of extreme 

floods

 Anoxia

 Timing of floods

 Flow duration

Riparian Response 

Guilds
Reproductive Strategy Guilds

Traits Relevant Flow Component

 Vegetative-Sexual-Combined

 Timing in synchrony with flow 

component (dispersal, flower, fruit)

 Magnitude of extreme flow 

 Timing of high & low flow

 Rate of drawdown

 Duration of inundation

Riparian Response 

Guilds
Morphology Response Guilds

Traits Relevant Flow Component

 Growth form (e.g., herbaceous-woody, 

graminoid-shrub-tree)

 Rooting depth (phreatophytic)

 Root morphology (lateral-taproot, shallowdeep) 

 Size at maturity (canopy height, vegetation 

volume)

 Flow duration

 Magnitude and duration of 

low and high flow 

 Water table depth and 

inter- and intra-annual 

variability 

 Flow permanence, 

groundwater depth, flow 

variability

Riparian Response 

Guilds
Merritt and Bateman, Ecological Applications 2012
Green and Yampa rivers,

Colorado-Utah

Merritt, unpublished
Disturbance 

adapted, high 

canopy, longlived

Shallow 

rooted, 

mesic

herbaceous 

Ruderal perennials , 

mesic, 

herbaceous

Disturbanceadapted hydric, 

herbaceous 

perennials

Mesic, 

salt 

tolerant

Droughttolerant, 

upland 

species

hydric, 

perennial 

graminoids

Merritt, unpublished do not cite
Guilds Identified for Colorado River 

in Grand Canyon

• Growth rate

• Height at maturity

• Life span

• Resprout ability

• Anaerobic tolerance

• Drought tolerance

• Fire tolerance

• Salinity tolerance

• Vegetative reproduction

• Spread rate

• Root-depth

• Shade tolerance

• Moisture usage 

(inundation/duration)

• Fire tolerance

• Sexual reproduction

14 biological attributes 
Cottonwood/

willow

Juncus/mentha

Conyza – ruderal

Mesquite
Sampling Approach

Inactive Flood Plain (IF)

Active Flood Plain (AF)

Active Channel (AC)

3- 1m2 plots per zone – location surveyed 

3 transects per geomorphic feature

NAU Sandbar Sites - (Sampled in October 2012) 

Random Sites - TBD
RM 3, 16, 30, 32, 

41, 47, 51.5, 56

RM 63, 65, 87, 93, 

119, 122, 123, 

139, 145, 167, 

172, 194, 202, 220
Preliminary Results

 22 sites, 847 plots sampled

 73 species identified in Marble Canyon 

 10 (AF), 0 AC

 50 species Eastern Grand Canyon

 8 (AF), 0 AC

 44 in Western Grand Canyon

 7 (IF), 0 AC

Richness, diversity, frequency of guilds,

Cover, native/nonnative ratio
Developing a Conceptual Model for Vegetation 

Response

Open 

sandbar

Clonal Marsh 

Vegetation

Landscape characteristics

-Low elevation sandbar

-Accumulates silts & clays – return channel side 

Greater surface/gw dynamics

Obligate 

Woody 

riparian 

vegetation

Clonal/Perenni

al/Annual 

Marsh 

Vegetation

Facultative 

Woody 

riparian 

vegetation

Ecological States Operations that cause switches 

between states & rules of response

Open 

sandbar

Facultative 

Woody 

riparian 

vegetation

Clonal/Perenni

al/Annual 

Marsh 

Vegetation

Obligate 

Woody 

riparian 

vegetation
Reattachment bar

Map: L. Jamison 

Defoliated tamarisk

Native vegetation

Image: Tamarisk Coalition
Impacts & implications of beetle-induced 

tamarisk mortality

• Ecosystem patterns & processes

• Spatio-temporal scales

• Flora & fauna

• Nutrient dynamics, fire & hydrology

• Management implications

• Natural & cultural resources

• Recreation & visitor experience

• Monitoring, mitigation & rehabilitation

• Dam operations