GEOMORPHIC EVOLUTION OF THE UPPER ARKANSAS RIVER, COLORADO: DRAINAGE REARRANGEMENT AND INCISION

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INCISION HISTORY OF THE EASTERN COLORADO ROCKIES
STATUS REPORT
Eric Leonard, Colorado College Peter Sak,
Dickinson College Scott Miller, Penn State/MIT
Western U.S. Geodynamics - CREST Workshop Santa
Fe, NM September 1, 2007
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Outline of Talk

• Summary of Arkansas River incision results
• Initial work on analysis of long profiles and
  knickpoints on Arkansas and South Platte rivers

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RIO GRANDE RIFT -- UPPER ARKANSAS VALLEY
UPPER CANYON
SALIDA

CANON CITY
OFF-AXIS GRABEN -- PLEASANT VALLEY
LOWER CANYONS
PIEDMONT
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Magnitude of Late Cenozoic Incision
gt100 - 120 m
Browns Canyon
700 - 800 m
gt250 - 300 m
1100 - 1200 m
Salida Exit
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Magnitude of late Cenozoic Arkansas River
incision (m) (Leonard 2002, McMillan et al. 2006,
this study)
700-800
100-300
1160
460
1200
770
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Timing of Inception of LateCenozoic Incision
Browns Canyon
Post 3.6 ma, pre ca 1.2 ma
?
Post 9 ma, possibly accelerated after 3.6 ma
Salida Exit
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Age Constraints on Incision
Dated Incision magnitudes (ages)
Tephra ages
Travertine U-series age
Pending carbonate cement U-series ages
lt40 m (640 ka)
38 m (640 ka)
50 m (223ka)
68 m (760ka)
113 m (640ka)
15 m
14 m
29 m
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Rate of Neogene Incision
11 cm/ka (2 my)
lt6 cm/ka
6 cm/ka 9 cm/ka
Browns Canyon
21 cm/ka
18 cm/ka
12 cm/ka (9 my)
Salida Exit
Sources Luiszer, 1997 Stroud, 1997 Jemison,
1999 Miggins, unpublished this study.
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Leonard et al., 2002
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McMillan et al., 2006
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South Platte 400-600 m
Arkansas 1100-1200 m
McMillan et al., 2006
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KNICKPOINT PROJECTION
Slope KsA-q Where Ks steepness index A
upstream drainage area q concavity
Derived from above-knickpoint slope/area plot
and an assumed reference concavity of 0.45.
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ALTITUDES OF KNICKPOINT CLUSTERS
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CLUSTERS OF KNICKPOINT PROJECTIONS (projected
incision in meters at range front)
Arkansas River tributaries below Royal Gorge have
few, if any knickpoints indicating less than 550m
of incision. Tributaries above Royal Gorge have
knickpoints that project to altitudes barely
above the modern river channel in addition to
high knickpoints.
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State of the Study

• Stratigraphic work on Arkansas River erosional
  history is (slowly) yielding results.
• Stratigraphic work of South Platte erosional
  history is waiting to be done!
• We need to talk with folks in this room about
  making sense of the knickpoint data.

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Dethier, 2001
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Arkansas 1100-1200 m
McMillan et al., 2006
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Thank You!