MARINE, EARTH, & ATMOSPHERIC SCIENCESArticle DOI:10.1002/esp.2150

Hillslope Response to Knickpoint Migration in the Southern Appalachians: Implications for the Evolution of Post-Orogenic Landscapes

GALLEN, Sean, WEGMANN, Karl, FRANKEL, Kurt, HUGHES, Stephen, LEWIS, Robert, LYONS, Nathan, PARIS, Paul, ROSS, Kristen, BAUER, Jennifer, and WITT, Anne 1 1 1111 12 3 3

North Carolina State University, Dept. of Marine, Earth, and Atm. Sciences, Raleigh, NC 27695 USA Georgia Institute of Technology, Dept. of Earth and Atm. Sciences, Atlanta, GA 30332 USA

North Carolina Geological Survey, NC 28778 USA

1

2

3The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass wasting in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for such activity in a post-orogenic landscape remain enigmatic. The non-glaciated Cullasaja River basin of southwestern North Carolina, with uniform lithology, frequent debris flows, and the availability of high-resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post-orogenic landscape through the lens of hillslope-channel coupling. We limit our investigation to channels with upstream drainage areas > 2.7 km2, a conservative estimate of the transition from fluvial to debris-flow dominated channel processes. We utilize values of normalized hypsometry, hypsometric integral, and mean slope vs. elevation for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Our results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. We capture the transient effect of knickpoint-driven channel incision on basin hillslopes by measuring the relief, mean slope steepness, and mass wasting frequency of tributary basins and comparing these results to the distance from major knickpoints along the Cullasaja River. We present a conceptual model of area-elevation and slope distributions that may be representative of post-orogenic landscape evolution in analogous geologic settings. Importantly, our model explains how knickpoint migration and channel-hillslope coupling is an important factor in tectonically-inactive orogens for the maintenance of significant relief, steep slopes, and weathering-limited hillslopes.

ABSTRACT:

80° W90° W

Miss

issip

piRi

ver

Little TennesseeRiver

CullasajaRiver

0 250 500km

Ohio

Ten nessee R.

Glacial Limit

River

40°

N30

° N

±

BRE

A

35°

N

Little Tennessee River

83° W84° W

0 20 40

km ±

CullasajaRiver basin

Blue Ridge Escarpmen

t

BNORTH CAROLINA

SOUTH CAROLINA

GEORGIA

TENNESSEE

(A) Map of the eastern United States showing major rivers, the Blue Ridge Escarpment (BRE), and the southern limit of continental glaciation (Thelin and Pike, 1991). (B) Hillshade image of southwestern North Carolina and adjacent states from SRTM digital topography.

Study Area:

Damage from debris �ow

Cullasaja Falls Quarry FallsDry Falls(A) Cullasaja Falls. (B) Dry Falls. (C) Quarry Falls. (D) Photograph of the upper portion of the September 15, 2004 Peeks Creek debris flow track, triggered by excessive precipitation from the remnants of Hur-ricanes Francis and Ivan. (F) & (G) Photograph of the debris flow deposit (Wooten et al., 2008). This debris flow destroyed 15 homes and took five lives and is a reminder of the mass wasting hazardpotential in the southern Appalachians (Wooten et al., 2008).

Waterfalls and Debris Flows:

B CA

DPeeks Creek Debris Flow

F

G

Cullasaja River Basin:

GFGF

GF

GFGFGF

GF

GFGF

GFGFGFGF

GFGF GF GFGFGF

GF GFGFGF

GFGFGF GF GF GFGFGFGF GF

GFGFGFGFGF GFGF GFGF GFGF GF

GFGF GFGFGF GF GF GF GFGFGF GFGF GF GFGF GFGFGFGFGFGFGFGF

GFGF GFGFGF GFGF

GFGFGFGFGF

GFGFGFGFGF GFGFGF

GF GFGFGF GF GF

GFGF GFGF GF

GFGFGF

GFGF

GFGFGFGF GFGF

GFGFGF GFGFGFGF

GFGF

GFGF

GFGF

GFGF GF

GFGFGF GFGF

GF

Highlands

1

2

3 4

Geology:P

D Quartz diorite togranodiorite

O Granite to quartz monzonite

Biotite gneissPB

AmphibolitePA

Meta-ultramafic rock

P Muscovite-biotite gneiss

M

P

D

O PB

PM

D

D

D D

D

PB

PA1

2

3 4

Highlands

A B

83°20'W

35°1

5'N

83°20'W

35°0

5'N

83°15'W83°20'W

35°1

5'N

83°20'W

35°0

5'N

83°15'W

Elevation

1562 m 609 m

0 2.5 5 10km

Peeks Creek debris flow

Slope failure locations

1. Cullasaja Falls2. Dry Falls3. Quarry Falls4. Kalakaleskies Falls

Knickpoint

Fluvial network is defined by channels draining areas greater than 2.7 km2. (A) Shaded relief map with locations of mapped slope failure locations from Wooten et al. (2006). (B) Geologic map of the Cullasaja River basin (North Carolina Geologic Survey, 1985).

(A) Airborne lidar digital eleva-tion model (6 m) of the Culla-saja River basin. Knickpoints were identified from longitudi-nal profiles as convex channel reaches dropping a minimum of 15 m in 0.5 km (gradient ≥ 0.3). (B) & (C) Channel longitu-dinal profiles of the Cullasaja River (blue line) and tributaries (pink lines).

Knickpoint Identification and Drainage Basin Delineation:

0 2.5 5 10km

W1

W6wW8

W7W6e

W5W4

W3

W2

E1Cullasaja

mouth

CullasajaheadE6

E-5

E-4

E3

E2

E1s

E1nb

E1na

Highlands

knickpoint

A

83°20'W35°15'N

83°15'W35°15'N

600

1000

1400

Ele

vatio

n (m

) E2E3

E4E5E6

E1na

E1nb

E1sCullasaja

East side tributary longitudinal profiles Tributary Longitudinal Profiles

1234Knickpoint

100 m smoothing window 5 m contour

E1

BCullasaja FallsDry Falls

Kalakaleskies Falls

123 Quarry Falls 4

West side tributary longitudinal profiles Tributary Longitudinal Profiles

Knickpoint

051015202530354045600

1000

1400

Distance from mouth (km)

Ele

vatio

n (m

)

W2

W4W5

W6w

W6eW7

W8

W3

Cullasaja

100 m smoothing window 5 m contour 1

234

Cullasaja FallsDry Falls

Kalakaleskies Falls

123 Quarry Falls 4

C

W1

83°20'W35°05'N

±

Swath profiles of maximum, mean, and minimum values of elevation, relief, and slope taken along the approximate trend of the Cullasaja River valley, each ~30 km long by 2 km wide. (A) Topographic swath extracted from the DEM. (B) Relief swath derived from a 500 m focal range analysis. (C) Slope swath extracted from a 500 m focal mean analysis of a slope map. The numbers 1, 2, and 3 correspond to the location of Cullasaja Falls, Dry Falls, and Kalakaleskies Falls, respectively.

Elevation (m)1562

595

0 5 10km

0 5 10km

0 5 10km

knickpointRelief (m)

602

15

knickpointSlope (°)

35

1

knickpoint

A B C

A A’ B B’ C C’

A

A’

B

B’

C

C’

BRE

83°20'W35°05'N

83°15'W35°10'N

BREBRE

SlopeReliefTopography

0 5 10 15 20 25 30600

1000

1400

distance (km)elev

atio

n (m

)

1 2 3

BRE

Max

Min

Mean

100

300

500

relie

f (m

)

0 5 10 15 20 25 30distance (km)

1 23

BRE BRE

0

102030

slop

e (°

)5 10 15 20 25 30

distance (km)

1 23

83°20'W35°05'N

83°15'W35°10'N

83°20'W35°05'N

83°15'W35°10'N

1

23

1

23

1

23

Swath Profiles:

0 10 20 30 40

10

20

30

Mea

n sl

ope

(°)

W-1

W-8

B

CF

DF

KF

CF

DF

KF

0 10 20 30 40

100

200

300

Mea

n re

lief (

m)

W-1

W-8

C

CF

DF

KF

0 10 20 30 40

0.2

0.6

1.0

1.4

Land

slid

e fre

quen

cy p

er k

m2

W-1

W-8

D

Tributary distance from mouth of Cullasaja (km)

F

GFGF

GF

GFGFGF

GF

GFGF

GFGFGFGF

GFGF GF GFGFGF

GF GFGFGF

GFGFGF GF GF GFGFGFGF GF

GFGFGFGFGF GFGF GFGF GFGF GF

GFGF GFGFGF GF GF GF GFGFGF GFGF GF GFGF GFGFGFGFGFGFGFGF

GFGF GFGFGF GFGF

GFGFGFGFGF

GFGFGFGFGF GFGFGF

GF GFGFGF GF GF

GFGF GFGF GF

GFGFGF

GFGF

GFGFGFGF GFGF

GFGFGF GFGFGFGF

GFGF

GFGF

GFGF

GFGF GF

GFGFGF GFGF

GF

0 5 10km

W-1

W-3W-2

W-4W-5

W-8W-6

E-1

W-7

E-6

E-2

E-4E-5

E-3

1

2

3 483°20'W

35°05'N

A Slope failure locations

1. Cullasaja Falls2. Dry Falls3. Quarry Falls4. Kalakaleskies Falls

Knickpoint

Drainage Basin Metrics: Slope

Relief

Landslide frequency

(A) Map highlighting the main tributary basins and mapped landslides (Wooten et al., 2008). (B) and (C) Values of mean slope, relief, and standard (1-std) errors. (D) The frequency of mapped landslides for each tributary basin. Waterfall locations are denoted by vertical dashed lines.

knickpoint

F E-1

E-6E-5

E-4

E-3E-2

W-8W-7

W-6

W-5W-4

W-3

W-2

W-1

Normalized hypsometry and mean slope v. elevation

Hypsometric curves

Nor

mal

ized

fre

quen

cy

Slo

pe (°

)

Nor

mal

ized

el

evat

ion

Cumulative area

W-1

0

0.1

0.2

0.3

+1 std

-1 std

mean

600 900 1200 1500

Tributary Basin Hypsometry & Mean Slope v. Elevation: E-1

600 900 1200 1500

E-2

Elevation (m)600 900 1200 1500

0

20

40W-7

600 900 1200 1500600 900 1200 1500

W-4

W-7

HI = 0.23

0.2 0.6 1.00

0.2

0.6

1.0

W-1

HI = 0.340.2 0.6 1.00

E-1

HI = 0.40

0.2 0.6 1.00

E-2

HI = 0.43

0.2 0.6 1.00

W-4

HI = 0.49

0.2 0.6 1.00

Spatial distribution of normalized hypsom-etry, mean slope, and individual hypsomet-ric curves for repre-sentative tributary basins along the Cul-lasaja River. Vertical black lines identify the elevation of knickpoints. Each plot shows tributary basins in different stages of landscape development.

knickpoint

0 0.2 0.4 0.6 0.8 1.00

0.2

0.4

0.6

0.8

1.0Hypsometric curve

Normalized cumulative areaN

orm

aliz

ed e

leva

tion

HI = 0.43

C

800 1200 16000

0.02

0.04

0.06

Normalized hypsometry

Elevation (m)

Norm

alize

d Fr

eque

ncy

10

20

30

Mea

n Sl

ope

(°)

B +1 std

-1 std

Mean Slope

0 5 10km

knickpoint

A

83°20'W35°05'N

Cullasaja River Basin Hypsometry & Mean Slope vs. Elevation:(A) Map highlights steeper slopes located be-tween 750 and 975 m in elevation, corresponding to the shaded elevation band in B. (B) Plot of hypsometry and mean slope v. elevation; black bars correspond to the elevations of Cullasaja Falls (~810 m) and Dry Falls (~965 m). (C) Hyp-sometric curve

Conceptual Model:

D Reach Basin

mea

n sl

ope

Nor

mal

ized

freq

uenc

y

Elevationlow high

B

t2

Reach Basin

mea

n sl

ope

Nor

mal

ized

freq

uenc

y

Elevationlow high

Stream flow direction

A Reach scale

Hypsometry

Mean slope Basin Scale

mea

n sl

ope

Nor

mal

ized

freq

uenc

y

Elevationlow high

E

Stream flow direction

ReachBasin

Elevation

mea

n sl

ope

Nor

mal

ized

freq

uenc

y

low highHypsometry

Mean slope

C ReachBasin

mea

n sl

ope

Nor

mal

ized

freq

uenc

y

Elevationlow high

Block diagrams show the migration of a knick-point through a channel and the transient hill-slope response at the reach scale. (A) undis-turbed landscape. (B) to (D) knickpoint migrates upstream forcing transient hillslope response. (E) Landscape returns to equilibrium with local baselevel.

Conclusions: A spatial relationship exists between knickpoint location and hillslope relief, steepness, and landslide frequency within the Cullasaja River basin.

Evidence shows that there is a transient hillslope maintenance of slope and relief by migrating knickpoints. Put another way, there is a linkage between waterfalls on North Carolina streams and the frequency of landslides.

Hypsometry and mean slope vs. elevation analysis is useful in distinguishing between waxing and waning landscapes which cannot be done by the traditional approach using hypsometric integral alone.

The conceptual model presented is applicable to other geologi-cally analogous post-orogenic landscapes.

Future work:Determine rates of knickpoint retreat by conducting a field based study to map and obtain age-constraints on river ter-races genetically related to knickpoints.

Quantitatively test the hypothesis that there is a link between fluvial process (i.e. knickpoint migration) and rates of hillslope denudation using terrestrial cosmogenic nuclide dating.

References:

North Carolina Geological Survey. 1985. Geologic map of North Carolina, scale 1:500,000: [accessed at http://www.nconemap.com; 02/01/2010].

Thelin, G. P., and Pike, R. J., 1991, Landforms of the conterminous United States- A digital shaded-relief portrayal. USGS, Report: i-2206: United States Geological Survey, scale 1:3,500,000.

Wooten, R. M., Gillon, K. A., Witt, A. C., Latham, R. S., Douglas, T. J., Bauer, J. B., Fuemmeler, S. J., and Lee, L. G., 2006, Slope movements and slope movement deposits map of Macon County, North Carolina: North Carolina Geological Survey, scale 1:50,000.

Wooten, R. M., Gillon, K. A., Witt, A. C., Latham, R. S., Douglas, T. J., Bauer, J. B., Fuemmeler, S. J., and Lee, L. G., 2008, Geologic, geomorphic, and meteorological aspects of debris flows triggered by Hurricanes Frances and Ivan during September 2004 in the Southern Appalachian Mountains of Macon County, North Carolina (southeastern USA): Landslides, v. 5, no. 1, p. 31-44.