Geomorphic Change and Biogeomorphic

Feedbacks in the Little Colorado River, AZ

Project A – LTEMP Sediment Goal

David J. Dean, David J. Topping

E.C. LaRue, 1926, Q ~ 500 m3/s ~17,500 ft3/s
The Little Colorado River Basin
The Little Colorado River:

A Century of Change

• A formerly braided river – now single-

threaded

• The former channel devoid of riparian 

vegetation – now dense floodplain 

forests (non-native tamarisk)

Study objectives: 

1) Determine magnitude, timing, and rate 

of geomorphic change, and the 

reasons for that change.

2) How has this affected sediment delivery to 

the Colorado River in Grand Canyon? 

3) How do geomorphic changes in LCR affect 

floods in lower LCR?

H.E. Gregory

D.J. Dean

1914

2016
Historical Context: Land Use and Human 

Development within the LCR Basin (1)

• Irrigated agriculture at small scales was practiced by the Ancentral Puebloans possibly as 

early as 1000 B.C. (Damp et al., 2002)

• Spanish explorers in the mid to late 1500s described many groves of cottonwoods and 

willows. Cottonwood galleries compared to that of the Rio Grande in New Mexico (Colton, 

1937). 

• U.S. army officers leading expeditions through the LCR valley in the 1800s described a river 

that contained bayous and sloughs, large cottonwoods, and plentiful grasslands 

(Sitgreaves, 1854; Stacey and Beale, 1929).

1500s

1000 B.C.Mid - 1800s

Present
Historical Context: Land Use and Human 

Development within the LCR Basin (2)

• In the 1870s, Mormon settlers began building diversion dams and irrigation networks.

• Completion of the railroad in the 1880s brought ranchers and their large herds of sheep and cattle 

(150,000 head of cattle, 120,000 head of sheep).

• “Widespread denudation” (gullying/erosion) of the LCR ranges had occurred by the 1890s. Mostly 

in headwaters (Balling and Wells, 1990; Gellis, 1998). Primary cause believed to be above average 

rainfall (Graf 1986). 

• Riparian corridor was denuded for railroad construction/settlements/dam building.

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

Present
Historical Context: Land Use and 

Human Development within the 

LCR Basin (3)

• 23 dam failures between 1876 and 1900.

• Lyman Lake constructed in 1910; It failed in 1915 and 

was rebuilt in 1920 and 1949. Stores 2X the annual 

stream flow of the LCR upstream.

• A flood of over 3,400 m3/s (120,000 ft3/s) occurred 

in September 1923. Largest flood since 1870.

• Stream gaging began in 1926; substantial changes to 

LCR hydrology/geomorphology had already 

occurred.

1500s

1000 B.C.Mid-1800s

1870s

1880s

1890s

1900

1910

1923

1926

Present

1930s

1940 1950 1960 1970 1980 1990 2000 2010

Dam Construction Groundwater Pumping
Previous Work on the LCR:

Between the early 1900s and the 1980s:

• Three alternating periods of erosion and deposition

• 1900-1940s – frequent large floods, high annual discharge –

the channel was wide and sandy

• 1940s-1950s – precipitation and discharge decreased – the 

channel narrowed and floodplains developed – salt cedar 

(Tamarisk spp.) became widely established on these 

floodplains

• 1950/60s-1980s – precipitation and discharge increased –

the largest floods caused additional overbank deposition 

and vertical accretion. 

Conclusion: hydrologic and geomorphic changes were 

primarily driven by changes in climate.

Hereford, 1984
Hydrology – Winter/Spring

• Long-duration winter/spring runoff driven by large frontal storms.

• Floods can be large, but suspended-sediment concentrations are generally small

Preliminary data, do not cite.
Hydrology – Summer/Fall

• Short-duration, high-intensity, 

summer/fall floods driven by 

convective thunderstorms

• Floods can be large, and 

suspended-sediment 

concentrations can be huge. 

Sediment can be deposited 

within/on LCR channel and 

floodplain, or delivered to the 

Colorado River. 

Preliminary data, do not cite.

Preliminary data, do not cite.
Re-evaluating the Timing of Historical 

Hydrologic Changes

• We created a cumulative discharge 

curve of LCR stream flow data

• Detrended the data to determine 

periods of high and low stream 

flow. 

• Our findings are consistent with 

Hereford’s (1984)

• The most recent low-flow period 

has had the lowest flow over the 

entire period of record. 

Dean and Topping, 2019, GSA Bulletin

1926-1943

high

1965-1995

high

1944-1964

low

1996-2016

low

Dean and Topping, 2019, GSA Bulletin
Temporal Changes in Annual Flow Volume and Peak Flow

Largest flow volumes = years 

dominated by winter/spring flow

Peak flows have continuously declined 

Dean and Topping, 2019, GSA Bulletin

Dean and Topping, 2019, GSA Bulletin

Preliminary data, do not cite.
Geomorphic Analysis Period 1: Early 1900s – 1943

Large Total Stream Flow and Peak Flow

J.A. Baumgartner

1300

1305

1310

1315

0 10 20 30 40 50 60 70 80

12/11-17/1928, 0.1-0.7 m3/s

4/6/1929, 365 m3/s

7/29/1929, 663 m3

/s

Elevation Above Mean Sea Level (m)

Distance From Left End Point (m)

Measured historical

Extent of scour/fill

Dean and Topping, 2019, GSA Bulletin

Present 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

wide

narrow

wide

narrow

1933/1934

1940
Period 2: 1944 – 1964

Reductions in Total Flow and Peak Flow:

Onset of Channel Narrowing

Dean and Topping, 2019, GSA Bulletin

Present 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

wide

narrow

wide

narrow

Reduced Hydrology/

Channel narrowing

1933/341954
Period 3: 1965 – 1995

Increase in Total Flow, No Increase in Peak Flow

Dean and Topping, 2019, GSA Bulletin

Present 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 Flow,

No increase in Peak
Period 3: 1965 – 1995

Continued Channel Narrowing

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

1979 195492

Present

Floodplain Formation
Period 3: 1965 – 1995

Reductions in Sediment Transport

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

1979 1954

1990 2000 2010

Big Floods/Wide/Braided

Reduced Hydrology/

Channel narrowing Increase in Volume,

No increase in Peak

1992

Present
Period 4: 1996-Present

Driest Period of Gaging Record

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

1992

2013

Present

Driest Period of 

Gaging Record

Summary: Complete Geomorphic Transformation 

of the LCR

• Narrowing by > 80% 

in wide alluvial 

valleys

• Reductions in 

sediment transport

• Substantial bed 

aggradation in some 

areas

• Dense riparian 

forests now exist. 

Wide Alluvial Valleys Narrow Alluvial Valleys

Wide Alluvial Valleys Narrow Alluvial Valleys

Dean and Topping, 2019, GSA Bulletin
Channel Widening During 

Large, Long Duration Floods

Dean and Topping, 2019, GSA Bulletin

1992 1997

Widening

1972/73

1993

Dean and Topping, 2019, GSA Bulletin
Channel Widening During 

Large, Long Duration Floods

Large, long duration floods ≈ channel widening

Narrowing resumes shortly thereafter. 

These floods usually occur during the winter/spring.

Dean and Topping, 2019, GSA Bulletin

1992 1997

Widening

1972/73

1993
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, disrupted flood 

conveyance, 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 at Cameron.

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?

Hypothesis 2: Biogeomorphic processes (channel 

narrowing, floodplain development, vegetation 

establishment) has affected floodwave propagation, 

and causes flood attenuation. 

• Large floods inundate floodplains

• Floodplain vegetation slows flow velocities and 

disrupts conveyance. Vegetation stabilizes 

floodplains

1940

2016
Biogeomorphic feedbacks in the LCR

• Increases in Sinuosity = decreases in slope of ~ 30% (less streampower)

• Increases in veg = increased drag of channel banks and floodplains? 

• Sequestration of floodwater on the expanded floodplains?

Increased travel time = Greater flood attenuation, 

potentially a cause of peak flow decline?

Wide Alluvial Valleys Narrow Alluvial Valleys

Changes in Travel Time of Floods/Attenuation 

Between Grand Falls and Cameron

Dean and Topping, 2019, GSA Bulletin Dean and Topping, 2019, GSA Bulletin
Biogeomorphic feedbacks in the LCR: 1D Flood Routing

Added roughness, reductions in width, and 

lower channel slope disrupts the movement of 

floodwaves. 

Dean and Topping, 2019, GSA Bulletin

Same channel length/slope,

2013 channel width 2013 channel width, longer

channel, lower slope

2013 channel width, lower 

slope, higher floodplain 

roughness

1954

Grand Falls

Cameron

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

Flood Attenuation

• Flood peak attenuation by 

~85%

• No large reservoirs or 

diversion structures

• Attenuation likely solely 

caused by the bio-geomorphic 

feedbacks. 

Rain on Snow- East Clear Creek

0

50

100

150

200

250

300

350

2/15/2019 2/21/2019

Discharge (m

3/s)

Preliminary data, do not cite

LCR near Cameron

LCR near Winslow

Preliminary data, do not cite
Hydrologic Change - Adjacent Basins

Dean and Topping, 2019, GSA Bulletin
Conclusions

• Fluctuations in total flow - changes in climate (Hereford, 1984).

• Peak flow declines, mostly in winter/spring - primary cause of channel narrowing.

• Peak flow declines in second half of 20th century 

• 1) human water use and development 

• 2) geomorphic changes within the river. 

• Large, long duration floods - temporarily channel widening. Widening short lived.

• Biogemorphic feedbacks

• Veg = increased drag/floodplain stabilization

• Sediment accumulation  Increase in Sinuosity  Reductions in Slope  Reductions 

in Sediment Transport  Increased Deposition/Narrowing

• Unlikely to see increases in flood magnitude.

• Sediment Delivery to GC...Likely permanently reduced. Further evaluations 

being conducted to determine magnitude of reduction.