Holocene Sedimentary History of Chilliwack Valley, Northern Cascade Mountains
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Holocene Sedimentary Historyof Chilliwack Valley,
Northern Cascade Mountains
Jon Tunnicliffe & Michael ChurchUBC Geography
GSA, October, 2008
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Objectives
• to determine the post-glacial sediment budget
• to establish connectivity and changing rate of sediment yield with scale in the paraglacial sediment cascade
• to develop a mobile-bed sediment transport model that simulates the topographic and textural evolution of the valley mainstem
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Some consequential technical issues
• can we adequately reconstruct an end-Pleistocene initial condition?
• can we account for variable system forcing, or do we need to?
• can we model 13 000 years of sedimentary system dynamics?
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Chilliwack Valley
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Chilliwack Lake
Mt. Baker
Tamihi
Slesse
FoleyChipmunk
Liumchen
ChilliwackLake
Sumas Valley
Nooksack Valley
Lower valley
Upper valley
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Upper valley
low order drainage
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Church and Slaymaker, 1989. British Columbia; Decadal timescale
Sediment yield from low-order drainage
Chilliwack R
debris flow fluvial dominance dominance
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Lower valley
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Tamihi Moraine
LacustrineZone
Outwash Delta
Net Deposition
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Lower valley sediment budgetmillions m3
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Vedder Fan: endpoint deposit
Mapping from Levson et al. 1995
North
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Vedder Fan
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• We route sediments through the lower valley, from Foley Creek to Vedder Crossing (31 km distance)
the exercise includes initial evacuation of the Pleistocene valley fill
Holocene sediment routing
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Modelling criteria
Known conditions:• initial and final profiles
• sediment budget (mass constraint)
• sediment fining gradient
• subsurface sand content
• lithological composition
Known conditions:• initial and final profiles
• sediment budget (mass constraint)
• sediment fining gradient
• subsurface sand content
• lithological composition
Model parameters:(selectable)
• abrasion rate• water discharge• sediment feed rates• feed sediment calibre
Model parameters:(selectable)
• abrasion rate• water discharge• sediment feed rates• feed sediment calibre
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Δx
x
q
tsed
p
)1(
Wv
Wc
Valley Width = WvActive Channel Width = WcSinuosity = ΩAbrasion = Ã
Ax
q
Wv
Wc
tsed
p
~)1(
1D modelling frameworkfinite difference formulation
Parker (1990)
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Transport function:Wilcock and Crowe formulation
*rsm Sand fraction effect
Hiding function
Wilcock and Crowe (2003)
Percent Sand (surface)
rsm
ri
sm
i
D
D
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Bed texture and rates of abrasionβ = 0.015 β = 0.030
Per
cen
t co
arse
r
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Delta Front
Upper Sandur
Lacustrine Beds
Fluvial Gravels + Moraine
Model stratigraphy
Moraine
35 30 25 20 15 10 5 0
0
50
100
150
200
250
300
350
400
450
Distance (km)
Elev
ation
(m)
MajorTributaries
Knickpoint
Modern Profile
Tamihi Cr
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Model solution strategy
• we have a number of adjustable system parameters• therefore, we do not expect a unique solution (i.e., a
unique set of parameters that produces the present-day morphology and sedimentology)
• our lack of knowledge of Holocene variations in system forcing is an important reason for that
• therefore, we attempt to gain insight by finding a parametric solution space that reasonably satisfies the current system configuration
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Evolution of the bed
35 30 25 20 15 10 5 0
0
50
100
150
200
250
300
350
400
450
Distance (km)
Elev
ation
(m)
Reconstructed Profile
Modern Profile0 1 2 3 4
200
220
240
260
280
300
Ele
vatio
n (m
)
Model Time Step (x105)
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Model Runscontemporary long profile is the primary criterion for model fit
Reference Runs (Circled)
Qw = 225 m3/sQs = 1x Budgeted Quantitiesα = 0.02 km-1
+128 mm Load = +3%Sand Load (-4 mm) = - 5%
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Cumulative post-glacial sediment yieldVedder fan data
65 000 m3 bulk rate
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Perspective• Much of sediment delivered from steepland
headwater sources remains in storage
• Glacial sediments continue to influence patterns of sediment yield
• Many aspects of long-range fluvial evolution can be captured in a 1D framework
• The evolution of degrading river systems can be modelled but remains an important challenge
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Conclusions
• we can reconstruct an end-Pleistocene condition that is sufficiently accurate for studying the Holocene sedimentary history
• It appears that a detailed knowledge of variable system forcing is not necessary for first order reconstructions
• we can constrain the Holocene sedimentary history relatively narrowly, but we cannot (yet) arrive at a complete and precise reconstruction
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Acknowledgements• Ron Clowes and Phil Hammer, EOS UBC• Janet Demarcke, City of Chilliwack• Randy Enkin and Judith Baker, GSC-Pacific, Sidney• Natalie Helmstetter, BJ Kelly and Ted Hickin, SFU Geography • Rob Huggins, Geometrics, Sunnyvale CA• Vic Levson, BCGS, Victoria• Brian Menounos and Melanie Grubb UNBC• Bruce Thomson, BC MWLAP, Surrey• Murray Hicks and Jeremy Walsh, NIWA, Christchurch• NSERC Research Grant to M. Church• UBC Graduate Scholarship to Jon Tunnicliffe
In the Field:• Sydney Kjellander, Dason Commodore, Kathryn Black• André Zimmermann, Dave Campbell, Jason Rempel, Brett Eaton, Josh
Caulkins, Bonnie Smith, Kristiann Allen
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Chilliwack Lake
Depot Creek
Paleface Creek
Radium Creek
Centre Creek
LittleChilliwack
Creek Bear Creek
US Border
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Chilliwack Lake
Depot Cr fan
Paleface Cr fan
Holocene fill
Last glacial deposits
Preglacial valley
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Depot Creek Fan
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Chilliwack Lake sedimentation
(a) rate, based on 5 cores and (b) volume based on seismic stratigraphy
(a)
(b)