Projected Climate and Geomorphic Changes in the Little Colorado River 

Watershed and Potential Links to Humpback Chub Habitat

With Emphases on Vegetation, Hydrology, and Sediment Transport

1 2

Gregory

David J. Dean and Joel Unema

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

2U.S. Geological Survey, Arizona Water Science Center, Flagstaff, AZ

Little Colorado River, 1927

Courtesy of NPS

Courtesy of 

Melanie Fischer, USFWS
First: 

A broad perspective of geomorphic change in Western 

rivers...
Little Colorado River

Gregory

1914

2016

Little Colorado River, AZ

Looking Downstream from 

Highway 89 bridge 

near Cameron, AZ 

Dean and Topping, 2019

H.C. Gregory
Gregory

Moenkopi Wash

1941

2015

Moenkopi Wash, AZ,

Looking upstream from 

Hwy 89 bridge 

Near Cameron, AZ

Unknown photographer

Dean
Gregory

Rio Grande/Bravo

2008

Rio Grande/Bravo, 

Big Bend National Park, TX,

Taken downstream from Castolon,TX

1945?

Dean and Schmidt, 2011
Gila River, near Calva, AZ

Gregory

1932

1964

Turner, 1974
Common Themes From the Above Slides

1) Narrower Channels

2) Increase in Vegetation

3) Arid/Semi-arid environments 

(drylands)

a) Variability in water supply

b) Growing human populations

c) Heavily managed water resources

1914

2016
But how does a river go 

from

Little Colorado River, 1914

2016

Dean and Topping, 2019

This

to

That
Why do channels change size and shape?

channels change form in response to changes in the balance between 

water and sediment supply

Lane, 1955, modified by Borland (1960)
Water and Sediment Balance...

Lane, 1955, modified by Borland (1960)
Water and Sediment Balance...Gets Messy When Streamflow/Sediment 

Transport are Modified by Other Processes (e.g. plants) – FEEDBACKS?

Little Colorado River, AZ

Dean and Topping, 2019, GSA Bulletin, Data Repository

Lane, 1955, modified by Borland (1960)
Geomorphic Effects of Vegetation: 

Historic Observations

Gila River, AZ 1932

Gila River, AZ 1964

Turner, 1974

• Vegetation aids in channel recovery after floods

• Vegetation establishment = abandonment of secondary channels

• Reaches with/without vegetation exhibit different channel 

patterns

Schumm and Lichty, 1963– Cimarron River, KS

Hadley, 1961 – Oraibi Wash, 

Little Colorado River Basin, AZ
Flume Experiments: 

Role of Vegetation on Channel Morphology

Tal and Paola, 2007

For other videos of their experiments, go to:

https://pubs.geoscienceworld.org/gsa/geology/article/35/4/347/129823/Dynamic-single-thread-channels-maintained-by-the

In this experiment, alfalfa sprouts were seeded, and allowed to grow. Flow Had simple hydrograph of low flow, and 

flood flow. 

Show Video
Flume Experiments: 

Role of Vegetation on Channel Morphology

Tal and Paola, 2007

Tal et al., 2004

Gran and Paola, 2001

Braudrick et al., 2009
Flume Experiments: 

Role of Vegetation on Channel Morphology

Nepf, 1999; Zong and Nepf, 2010; 

Chen et al., 2012; Manners et al., 2015; 

Diehl et al., 2017,

Flume experiments at plant scale:

• Effects on flow/Sed deposition

• Different types of plant seedlings

• Different plant/stem densities

Multi-stem Single-stem

Single-stem Single-stem

Multi-stem

Multi-stem

Velocity

Reduced sed transport 

with increasing stem density/area
How does this apply to the Little Colorado River?

Part 1: The Alluvial Valleys
Re-evaluating the Timing of Historical Hydrologic Changes

Analyzed LCR stream flow data to determine time periods of hydrologic change

Identified four alternating periods of high and low flow

1926-1943

high

1944-1964

low

1965-1995

high

1996-2016

low

Dean and Topping, 2019, GSA Bulletin
Temporal Changes in Annual Mean 

Flow and Peak Flow

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

Peak flows have continuously declined 

over the period of record. 

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Years with largest flow volume are years 

with large winter/spring runoff. 

Dean and Topping, 2019, GSA Bulletin
Geomorphic Analysis Period 1: Early 1900s – 1943

Large Total Stream Flow and Peak Flow

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow Magnitude (ft3/s)

• A flood of over 120,000 ft3/s occurred in September 1923.

Largest flood since 1870.

Big Floods/Wide/Braided

1940

J.A. Baumgartner

Dean and Topping, 2019, GSA Bulletin

Present

1920 1930s 1940 1950 1960 1970 1980 1990 2000 2010

1933/1934
Present

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Period 2: 1944 – 1964

Reductions in Total Flow and Peak Flow:

Onset of Channel Narrowing

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Dean and Topping, 2019, GSA Bulletin

• Reduced flood disturbance allows vegetation to begin

growing along channel margins.

Reduced Hydrology/

Channel narrowing Big Floods/Wide/Braided

1933/34
Period 2: 1944 – 1964

Reductions in Total Flow and Peak Flow:

Onset of Channel Narrowing

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Dean and Topping, 2019, GSA Bulletin

• Reduced flood disturbance allows vegetation to begin

growing along channel margins.

Present

1930 1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing

1920

1954
Period 3: 1965 – 1995

Increase in Total Flow, No Increase in Peak Flow

Floodplain Formation

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Below Grand Falls

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

Dean and Topping, 2019, GSA Bulletin

1954
1979

Period 3: 1965 – 1995

Increase in Total Flow, No Increase in Peak Flow

1

Reduced Hydrology/

Floodplain Formation

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

1500s

000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/BraidedChannel narrowing Increase in Volume,

No increase in Peak

Present

Below Grand Falls

Dean and Topping, 2019, GSA Bulletin
Period 3: 1965 – 1995

Increase in Total Flow, No Increase in Peak Flow

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

Floodplain Formation

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

Below Grand Falls

Dean and Topping, 2019, GSA Bulletin

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

1992
Period 3: 1965 – 1995

Reductions in Sediment Transport

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

Pasta=Sediment

The Strainer Effect

Water = Flood water

Strainer=Vegetation

Dean and Topping, 2019, GSA Bulletin

1992
Period 4: 1996-Present

Driest Period of Gaging Record

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

Driest Period of 

Gaging Record

Dean and Topping, 2019, GSA Bulletin

1992
Period 4: 1996-Present

Driest Period of Gaging Record

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

Driest Period of 

Gaging Record

Dean and Topping, 2019, GSA Bulletin

2013
Period 4: 1996-Present

Driest Period of Gaging Record

0

200

400

600

800

1,000

1920 1940 1960 1980 2000 2020

Annual Mean Flow (ft3/s)

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

Present

Driest Period of 

Gaging Record

Dean and Topping, 2019, GSA Bulletin

2013
Period 4: 1996-Present

Continued Narrowing/Vegetation Expansion/Bed Aggradation

No increase in Peak

Present

Gaging Record

Dean and Topping, 2019, GSA Bulletin

Reduced Hydrology/ Increase in Volume, Driest Period of 

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Big Floods/Wide/BraidedChannel narrowing

2013
Summary: Complete Geomorphic 

Transformation of the LCR

• Narrowing by > 80%

• Reductions in

sediment transport

• Bed aggradation in

some areas

• Dense riparian

forests

Dean and Topping, 2019, GSA Bulletin
Reasons for Hydrologic Change:

Declines in Peak Flow

Why have peak flows declined even though total flow has not?

• Hypothesis 1: Water management/dams = captured floodwater, reduced peak

flows.

• >100 reservoirs in basin, 3,700 stock ponds

• e.g. Diversion of water from East Clear Creek into the Salt River basin ≈ 4% of the

total flow measured near Cameron.

0

10,000

20,000

30,000

40,000

50,000

60,000

1920 1940 1960 1980 2000 2020

Annual Peak-Flow (ft3/s)

East Clear Creak - 1929

Dean and Topping, 2019, GSA Bulletin
Reasons for Hydrologic Change:

Declines in Peak Flow

Why have peak flows declined even though total flow has not?

1940

Hypothesis 2: Geomorphic change/Biogeomorphic

feedbacks have affected flood conveyance and 

cause flood attenuation. 

• Large floods inundate floodplains

• Floodplain vegetation slows flow velocities and

disrupts conveyance. Vegetation stabilizes

floodplains

2016
Case Study: The Little Colorado 

River, AZ - Ecogeomorphology

Why have peak flows declined even though total 

flow has not? 

Hypothesis 2: Geomorphic change/Biogeomorphic

feedbacks have affected flood conveyance and cause 

flood attenuation. 

Increase in Travel Time of Floods!!

Dean and Topping, 2019

1940

2016
0

500

1000

1500

2000

2500

3000

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

Discharge (ft3/s)

Change in Flood Shape

This is what flood 

hydrographs used 

to look like

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)

0

500

1,000

1,500

2,000

2,500

3,000

11/23/04 11/24/04 11/26/04 11/27/04

Discharge (ft3/s)

Grand Falls

Cameron

This is what they 

look like now

0

2,000

4,000

6,000

8,000

12/30/04 1/1/05 1/3/05 1/5/05

Discharge (ft3/s)

Rounded Peaks 

once floods go 

overbank

Dean and Topping, 2019; Block, LCR, 2014; Burkham 1976; Turner 1974; Gellis et al., 2017 
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)

Rain on Snow- East Clear Creek

LCR near Cameron

LCR near Winslow

preliminary data, do not cite

• Flood peak attenuation by ~85%

• No large reservoirs or diversion structures

• Attenuation likely solely caused by the biogeomorphic feedbacks.

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)
Reasons for Hydrologic Change:

Declines in Peak Flow

Why have peak flows declined even though total flow has not?

Hypothesis 3: Climate Change has resulted in storms that don’t produce big floods. 

• But adjacent basins, with similar climate, don’t show peak flow reductions

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)

Salt River near Roosevelt, AZ

0

50,000

100,000

150,000

1904 1924 1944 1964 1984 2004 2024

Annual Peak-Flow (ft3/s)

Gila River near Calva, AZ

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

1904 1924 1944 1964 1984 2004 2024

Annual Peak-Flow (ft3/s)

Dean and Topping, 2019, GSA Bulletin
Reasons for Hydrologic Change:

Declines in Peak Flow

Why have peak flows declined even though total flow has not?

Hypothesis 1: Water Management/Dams/Stock Ponds 

• Water management has some effect, but additional quantification needed

Hypothesis 2: Biogeomorphic feedbacks (channel narrowing/vegetation encroachment)

• Likely the strongest effect.

• Big floods can originate in Clear/Chevelon Creeks – rapid attenuation occurs

Hypothesis 3: Climate Change

• Peak flow has not declined in adjacent basins with similar climate

• Precipitation records don’t support this

Effects of Future Climate Change (i.e. Rising Temps, Decline in Precipitation)

• Affect soil moisture - could have a large impact on base flows.

• Further declines in flood flow (most importantly in the winter/spring).
One Last Note about Moenkopi Wash

• Supplies ~32% of the sand and 27% of the 

silt & clay transported past LCR Cameron 

gage

• >60% of the annual flood peaks in LCR were 

either entirely or partially from Moenkopi 

Wash over the last 20 years 

• But lots of geomorphic change in Moenkopi 

as well. Working to quantify its affects. 

1941

2015 Upstream,

Moenkopi at Moenkopi

Downstream,

Moenkopi near Cameron

Attenuation
Switching Gears:

Part 2: The LCR Gorge

Historical Floods of Geomorphic Change in the Lower Little Colorado River 

During the late 19th to early 21st Centuries

Joel Unema, David Topping, Keith Kohl, Michael Pillow, Joshua Caster

Scientific Investigations Report 2021-5049
Study Area
Significance

• Habitat for endangered humpback chub

• Chub spawning is sensitive to channel conditions

• Travertine dams can form barriers to chub movement

• Cultural Sites may be affected by travertine growth

NPS.gov

Illustration by Joe Tomelleri
Geomorphic Processes

Incision:

Reworking of dams and debris-flow 

deposits by floods

Aggradation:

Travertine dam growth

Side-canyon debris flows

All data are provisional and subject to change
Above Big Canyon 1911 2013

All data are provisional and subject to change
Above Big Canyon 1911 2013

All data are provisional and subject to change
Near the confluence

1950’s 2019

All data are provisional and subject to change
Baseflow 

long profile

change

1926 - 2013

1992 - 2013

-12

-10

-8

-6

-4

-2

0

2

4

6

8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Elevation change (m)

Upstream distance from confluence (km)

Change between 1926 and 2013

Change between 1992 and 2013

Change between 1971-1974 and 2013

(photograph)

Aggradation

Incision

Unema et al., 2021, USGS SIR 20215049
Conclusions

• Major incision and aggradation occurred between 1911 and 1950s

• Small change (only aggradation measured) since 1950s

• Decline in flood frequency and magnitude = Loss of disturbance

Implications

• Continued travertine dam growth and aggradation is likely without

erosion by large floods

• This could lead to more physical barriers to chub movement and loss of

habitat, and potentially impact cultural sites (Sipapu)