Project A: Streamflow, Sediment, and Vegetation Dynamics; 

Implications for Geomorphic Change in the Colorado River and 

its Tributaries

David J Dean and David J Topping, U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff, AZ

• Project Elements and Objectives

• A.1. Stream Gaging and Hydrologic Analyses

• A.3. Sediment Transport and Budgeting

• Cooperators: USGS AZ WSC 

• LTEMP Resource goals: Sediment
Presentation Outline

1. Brief Discussion of Drivers of Geomorphic Change 

in Rivers 

2. Moenkopi Wash and Little Colorado River

a. Streamflow, Sediment, and Vegetation 

Dynamics → Geomorphic Change

b. Implications for Sediment Delivery to 

Colorado River

3. Colorado River

a. Effects of possible low flows (SEIS) on 

vegetation encroachment and geomorphic 

change

Moenkopi Wash

Little Colorado River (LCR)

Colorado River
1) General Discussion of Drivers of Geomorphic 

Change in Rivers 
Gregory

Moenkopi Wash

1941

2015

L

H

N

USGS unknown photographer

Dean

Moenkopi Wash, AZ,

ooking upstream from 

wy 89 bridge 

ear Cameron, AZ

preliminary data, do not cite
Little Colorado River

How Does

This Happen?

How Does

This water and 

sediment 

movement 

downstream?

Gregory

1914

2016

Little Colorado River, AZ

Looking Downstream from 

Highway 89 bridge 

near Cameron, AZ 

Dean and Topping, 2019, GSA Bulletin

H.C. Gregory
Water Sediment

Channel Size 

and Shape

Amount

Size

Timing

Flood

Duration

Vegetation

Flood

Magnitude

Transport

Flood

Frequency

Base 

Flows

Timing

Biogeomorphic Feedbacks

preliminary data, do not cite
2) How does this apply to Moenkopi Wash and the Little Colorado River?
Moenkopi Wash

Upper Gages

Lower Gages
Moenkopi Wash 

Hydrologic Change

1st Decline

Early 1940s

2nd Decline

Mid 1950s

Changes in Annual Peak Discharge

preliminary data, do not cite
1st Decline

Early 1940s

2nd Decline

Mid 1950s

Moenkopi Wash 

Hydrologic Change

The mid 1950s decline in discharge unique

1. Decline in annual peak Q

2. 2 extended periods of only small floods

3. Decline in number of floods

What did this mean for channel change? 

preliminary data, do not cite
Moenkopi Wash

Physical Changes

1933/1934

Large bare 

sandy point bars

1941

Channel Width 20-40 m wide 

preliminary data, do not cite
Moenkopi Wash

Physical Changes

ins

Initial decrease in discharge

No measurable changes in 

channel width

Vegetation encroachment beg

Aerial Photos

Wide alluvial 

valleys

Narrow 

alluvial 

valleys

preliminary data, do not cite

1952

Sparse 

Vegetation

Sparse 

Vegetation
Den

Veg

Den

Veg

corri

%

se 

etati

se 

etati

dor

2nd decline in discharge

Dense vegetation throughout river 

Reductions in channel width by ~30

Aerial Photos

Wide alluvial 

valleys

Narrow 

alluvial 

valleys

preliminary data, do not cite

1968

on

on

Moenkopi Wash

Physical Changes 
Moenkopi Wash

Physical Changes

Recent decades

No sizeable floods, additional narrowing

Narrowing = long-term loss in channel capaci

Wide alluvial 

valleys

Narrow 

alluvial 

valleys

Cross section-near Tuba City gage

preliminary data, 

do not cite

2013

ty
Moenkopi Wash

Effects of Physical Changes 

on Water Conveyance

1959

r 

preliminary data, do not cite

Loss of channel capacity=

Overbank flooding at lowe

discharge

2022
Moenkopi Wash

Effects of Physical Changes 

on Water Conveyance

1) Overbank Flooding at lower discharges

2) Vegetation slows water down

3) Water gets stored in floodplain 

depressions

4) Results in 

1) modification of hydrograph shape

2) flood attenuation

2022

preliminary data, do not cite
Moenkopi Wash

Effects of Physical Changes on Water Conveyance

Modification of hydrograph shape

Flood 

Start

Flood 

End

Flood midpoint

Asymmetry<<0.5

Asym≈0.5 Asym>0.5

3rd largest flood

Ever measured 

In Moenkopi Wash

preliminary data, do not cite
Moenkopi Wash

Effects of Physical Changes on Water Conveyance

Modification of hydrograph shape

Flood Attenuation

Flood 

Start

Flood 

End

Flood midpoint

Asymmetry<<0.5

Attenuation

of peak 

by ~2/3

preliminary data, do not cite
Moenkopi Wash

Effects of Physical Changes on Water Conveyance

Modification of hydrograph shape

Flood 

Start

Flood 

End

Flood midpoint

Asymmetry<<0.5

Paired Floods That Passed both 

“at Moenkopi” and “near Cameron” stream gages

Average Asym = 0.49

Average Asym= 0.62

Average Peak Atten = 50%

preliminary data, do not cite

Moenkopi Wash

Effects of Physical Changes on Water Conveyance

Modification of hydrograph shape

Flood Attenuation
Moenkopi Wash

Effects of Physical Changes on Sediment Routing

Decline in sediment transport during Floods

Narrow, vegetated channel an inefficient

conveyor of sediment

preliminary data, do not cite
Moenkopi Wash

Effects of Physical Changes on Sediment Routing

Decline in sediment transport during Floods

Narrow, vegetated channel an inefficient

conveyor of sediment

Pasta=Sediment

Water = Flood water

Strainer=

Vegetation

The Strainer Effect

preliminary data, do not cite
Moenkopi Wash

Effects of Physical Changes on Sediment Routing

Decline in sediment transport during Floods

Narrow, vegetated channel an inefficient

conveyor of sediment

Pasta=Sediment

Water = Flood water

Strainer=

Vegetation

The Strainer Effect

~1,000 ft3/s

Declines in transport because of sediment trapping by veg

preliminary data, do not cite
Moenkopi Wash

Why Does Moenkopi Wash Matter?

Little Colorado River near Cameron

(downstream from Moenkopi Wash)

4 of 6 largest concentration

Samples from Moenkopi

Before 1959

Biogeomorphic feedbacks within Moenkopi 

Wash are reducing sediment loads to Little 

Colorado River

preliminary data, do not cite
Physical Changes within the 

Little Colorado River 

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Decline in 

Peak Flows

Dean and Topping, 2019, GSA Bulletin
• Narrowing by > 80% 

• Reductions in 

sediment transport

Dean and Topping, 2019, GSA Bulletin

Physical Changes within the Little 

Colorado River 

Pasta=Sediment

Water = Flood water

Strainer=

Vegetation

The Strainer Effect
Physical Changes within the Little 

Colorado River 

1940

2016

Increase in Travel Time of Floods

0

2,000

4,000

6,000

8,000

10,000

3/21/54 3/24/54 3/28/54 3/31/54

Discharge (ft3/s)

Grand Falls

Cameron

This is what flood 

hydrographs used 

to look like

0

500

1000

1500

2000

2500

3000

1/28/05 1/30/05 1/31/05 2/2/05

Discharge (ft3/s)

This is what they look 

like now

(i.e. delay in peak Q)

Dean and Topping, 2019, GSA Bulletin
February 2019 

Flood Attenuation

0

2,000

4,000

6,000

8,000

10,000

12,000

2/15/19 2/18/19 2/21/19 2/25/19

Discharge (ft

3/s)

LCR near Cameron

LCR near Winslow

• Flood peak attenuation by ~85%

• No large reservoirs or diversion structures

preliminary data, do not cite

Rain on Snow- East Clear Creek
February 2019 

Flood Attenuation

Rain on Snow- East Clear Creek

0

2,000

4,000

6,000

8,000

10,000

12,000

2/15/19 2/18/19 2/21/19 2/25/19

Discharge (ft

3/s)

LCR near Cameron

LCR near Winslow

• Attenuation likely solely caused by the biogeomorphic feedbacks. 

preliminary data, do not cite

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Decline in 

Peak Flows
Summary: Moenkopi Wash and LCR

• Channel narrowing and vegetation invasions 

• Dense vegetation slows floodwater – reductions in peak discharge (attenuation) Dean and Topping, 

2019, GSA Bulletin

• In lower LCR, progressive growth of travertine dams? Impact fish habitat/passage? 

(Unema et al., 2021)

• Dense vegetation traps sediment – reductions in sediment transport Dean and Topping, 2019, GSA Bulletin 

• Sediment loads from Moenkopi Wash have been reduced to LCR (preliminary data, do not cite)

• Sediment loads from LCR have been reduced to Colorado River (preliminary data, do not cite)

• Reduction in sediment loads unlikely to be reversed 

preliminary data, do not cite
3) Colorado River in Grand Canyon

How will possible low GCD 

releases affect vegetation 

and geomorphology in 

Colorado River? 

Desire to maintain Lake Powell 

elevations above minimum power pool 

(3490’)

preliminary data, do not cite
Vegetation will track large changes in river stage. 

Colorado River at Lees Ferry

• Decrease in disturbance (e.g. decline in flood peaks)

• Increase in baseflow 

Riparian Zone will encroach upon the river IF

Hazel Kaplinski

Hazel et al., 2022, USGS Professional Paper

1991 2009
Vegetation will track large changes in river stage. 

Increase in baseflows = vegetation expansion above 8,000 ft3/s stage

Lack of flood disturbance = greater vegetation at higher elevations 25,000-45,000 ft3/s 

8,000-25,000 ft3/2

25,000-45,000 ft3/s

45,000-210,000 ft3/s

Durning et al., 2021, Ecohydrology
2000 – Low Steady Summer Flow experiment

• Dramatic increase in vegetation during low flows

• HMF (habitat maintenance flows) reduced stem density

• Additional declines in stem density after resuming “normal” operations

• However, not all vegetation was removed...some residual survival

Habitat Maintenance Flow ~31,000 ft3/s

Ralston, 2011, USGS Open-File Report

?
2022 Flows

15,000 ft3/s 

7000 ft3/s 

Calendar Year 2022

preliminary data, do not cite
15,000 ft3/s 

8,000 ft3/s 

6 MAF

preliminary data, do not cite

Possible low flows to stay above power pool
6 MAF

Low flow encroachment

8,000 ft3/s 

preliminary data, do not cite

Possible low flows to stay above power pool
8,000 ft3/s 

6 MAF

Low flow encroachment

High flow recession (likely not complete)

15,000 ft3/s 

preliminary data, do not cite

Possible low flows to stay above power pool
8,000 ft3/s 

<4.9 MAF – LTEMP does not apply

Low flow encroachment

5,000 ft3/s 

preliminary data, do not cite

Possible low flows to stay above power pool
8,000 ft3/s 

<4.9 MAF – LTEMP does not apply

Low flow encroachment

5,000 ft3/s Veg induces sediment deposition (Butterfield 

et al., 2020)

Vegetation increases stability

Reduced dynamic adjustment of sand 

deposits

preliminary data, do not cite Once established, very difficult to remove

Possible low flows to stay above power pool
Many other examples:

Rio Grande - Dean and Schmidt, 2011; Dean et al., 2011

Yampa River - Manners et al., 2014

Little Colorado River – Dean and Topping, 2019

San Rafael River – Fortney, 2015

Green River (Dinosaur) – Grams and Schmidt, 2002; 2005

Rio Puerco – Friedman et al., 2005; Vincent et al., 2009

Channel Narrowing in other rivers (e.g. Green River)

• Vegetation encroachment 

during low-flow years

• stabilize deposits and trap 

additional sediment. 

• Channel narrowing

• No widening after subsequent 

Increases in discharge

Walker et al., 2020, GSA Bulletin

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

1880 1900 1920 1940 1960 1980 2000 2020

Annual Peak Discharge (ft3/s)

Year
Channel Narrowing in other rivers (e.g. Green River)

• Vegetation encroachment 

during low-flow years

• stabilize deposits and trap 

additional sediment. 

• Channel narrowing

• No widening after subsequent 

Increases in discharge

Even a few years of reduced flows may 

result in nearly irreversible changes to 

channel morphology

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

1880 1900 1920 1940 1960 1980 2000 2020

Annual Peak Discharge (ft3/s)

Year

Walker et al., 2020, GSA Bulletin
• Reduced flows – vegetation encroachment upon river

• A few years of reduced flows – vegetation maturation, very difficult to 

remove

• Mature vegetation will trap sediment

• Channel narrowing

• Loss of important aquatic habitats (e.g. backwaters)

Summary: Colorado River in Grand Canyon

preliminary data, do not cite
Questions References

• Butterfield, B.J., Grams, P.E., Durning, L.E., Hazel, J., 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.

• Dean, D.J., and Schmidt, J.C., 2011, The role of feedback mechanisms in historic channel changes of the lower Rio Grande in the Big Bend region: Geomorphology, v. 126, p. 333-349, 

http://www.sciencedirect.com/science/article/pii/S0169555X10001157.

• Dean, D.J., Scott, M.L., Shafroth, P.B., and Schmidt, J.C., 2011, Stratigraphic, sedimentologic, and dendrogeomorphic analyses of rapid floodplain formation along the Rio Grande in Big Bend National Park, 

Texas: Geological Society of America Bulletin, v. 123, no. 9-10, p. 1908-1925, http://gsabulletin.gsapubs.org/content/123/9-10/1908.abstract.

• Dean, D., and Topping, D., 2019, Geomorphic change and biogeomorphic feedbacks in a dryland river: The Little Colorado River, Arizona, USA: GSA Bulletin, v. 131, no. 11-12, p. 1920-1942.

• Durning, L. E., Sankey, J. B., Yackulic, C. B., Grams, P. E., Butterfield, B. J., & Sankey, T. T. (2021). Hydrologic and geomorphic effects on riparian plant species occurrence and encroachment: Remote sensing of 

360 km of the Colorado River in Grand Canyon. Ecohydrology, 14( 8), e2344. https://doi.org/10.1002/eco.2344

• Fortney, S.T., 2015, A Century of Geomorphic Change of the San Rafael River and Implications for River Rehabilitation: Logan, UT, Utah State University, MS Thesis, 227 pp. p., 

https://digitalcommons.usu.edu/etd/4363/.

• Friedman, J.M., Vincent, K.R., and Shafroth, P.B., 2005, Dating floodplain sediments using tree-ring response to burial: Earth Surface Processes and Landforms, v. 30, no. 9, p. 1077-1091, <Go to 

ISI>://000231784300001.

• Grams, P.E., and Schmidt, J.C., 2002, Streamfow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Unita Mountains, Colorado and Utah: 

Geomorphology, v. 44, no. 3-4, p. 337-360, <Go to ISI>://000175677600010.

• Grams, P.E., and Schmidt, J.C., 2005, Equilibrium or indeterminate? Where sediment budgets fail: Sediment mass balance and adjustment of channel form, Green River downstream from Flaming Gorge Dam, 

Utah and Colorado: Geomorphology, v. 71, no. 1-2, p. 156-181, <Go to ISI>://000233058200011.

• Hazel Jr, Joseph E., et al. Multi-decadal sandbar response to flow management downstream from a large dam—The Glen Canyon Dam on the Colorado River in Marble and Grand Canyons, Arizona. No. 1873. 

US Geological Survey, 2022.

• 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: Geomorphology, v. 

211, no. 0, p. 100-115, http://www.sciencedirect.com/science/article/pii/S0169555X14000026.

• Unema, J.A., Topping, D.J., Kohl, K., Pillow, M.J., and Caster, J.J., 2021, Historical floods and geomorphic change in the lower Little Colorado River during the late 19th to early 21st centuries, Scientific 

Investigations Report: Reston, VA, Report, 34 p., http://pubs.er.usgs.gov/publication/sir20215049.

• Vincent, K., Friedman, J., and Griffin, E., 2009, Erosional Consequence of Saltcedar Control: Environmental Management, v. 44, no. 2, p. 218-227, http://dx.doi.org/10.1007/s00267-009-9314-8.

• Ralston, B.E., 2011, Summary report of responses of key resources to the 2000 low steady summer flow experiment, along the Colorado River downstream from Glen Canyon Dam, Arizona: U.S. Geological 

Survey Open-File Report 2011–1220, 129 p., https://doi.org/10.3133/ofr20111220

• Walker, A.E., Moore, J.N., Grams, P.E., Dean, D.J., and Schmidt, J.C., 2020, Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah: Bulletin, v. 132, no. 11-12, 

p. 2333-2352.