Holocene Sedimentary Historyof Chilliwack Valley,

Northern Cascade Mountains

Jon Tunnicliffe & Michael ChurchUBC Geography

GSA, October, 2008

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

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?

Chilliwack Valley

Chilliwack Lake

Mt. Baker

Tamihi

Slesse

FoleyChipmunk

Liumchen

ChilliwackLake

Sumas Valley

Nooksack Valley

Lower valley

Upper valley

Upper valley

low order drainage

Church and Slaymaker, 1989. British Columbia; Decadal timescale

Sediment yield from low-order drainage

Chilliwack R

debris flow fluvial dominance dominance

Lower valley

Tamihi Moraine

LacustrineZone

Outwash Delta

Net Deposition

Lower valley sediment budgetmillions m3

Vedder Fan: endpoint deposit

Mapping from Levson et al. 1995

North

Vedder Fan

• 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

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

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)

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

Bed texture and rates of abrasionβ = 0.015 β = 0.030

Per

cen

t co

arse

r

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

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

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)

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%

Cumulative post-glacial sediment yieldVedder fan data

65 000 m3 bulk rate

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

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

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

Chilliwack Lake

Depot Creek

Paleface Creek

Radium Creek

Centre Creek

LittleChilliwack

Creek Bear Creek

US Border

Chilliwack Lake

Depot Cr fan

Paleface Cr fan

Holocene fill

Last glacial deposits

Preglacial valley

Depot Creek Fan

Chilliwack Lake sedimentation

(a) rate, based on 5 cores and (b) volume based on seismic stratigraphy

(a)

(b)