Geologic Evolution of the Channel Migration Zone

J. Riedel – National Park Service BT LSWT

OT CMZ defined by:

1) Areas within FEMA floodplain of Skagit River (100 and 500 year);

2) Areas within meander belt of the Skagit River identified with LiDAR;

3) USGS lahar (debris flow) inundation zone;

4) All areas mapped by Dragovitch et al. within Kennedy Cr. lahar deposition zone;

5) Excluded tributary FEMA floodplains; and

6) Excluded high terraces along river above Birdsview, which date to end of last ice age. river meander pattern

6,000 year lahar -planar glacial braided

Birdsview terraces Day Cr . Terrace The Skagit Basin formed where two mountain ranges overlap.

The Coast Mountains of Coast British Columbia are ~80 Mountains million years old.

The Cascade Mountains Skagit Valley are less than 35 million years old.

Cascade Mountains Skagit River Skagit River Pacific Crest Skagit River North Cascades Crest

Skagit Crest Pacific Crest Skagit River North Cascades Crest

Skagit Crest Fraser R. Mt. Baker

Skagit River

Glacier Pk. BT LSWT

OT Baker alpine valley glacier ca. 27,000 yr BP

- Area = 538km2

-Δ ELA =980m

-Δ T = ~7°C

?

Lake Concrete

? Lower Skagit Valley

Finney Cr. Cedar Grove

Ovenell Cedar Grove Section

24,725 yr. BP

30,112 yr BP 23,754

Ovenell Big Boy Section Ovenell - Big Boy Section

?

Cordilleran Ice Sheet

CIS advance outwash 19,515 ybp

Glacial Lake Concrete

MSH M Between 30,000 and

Baker ~19,000 years ago, the Skagit likely drained via Skagit River the Stillaguamish to

Cordilleran Ice Sheet

Redrawn from Waitt and Thorson (1981) Before the ice ages, perhaps 3 million years ago, the Skagit likely flowed in two directions.

Skagit River Since the first ice ages, the Skagit has become the focal point for ice sheet drainage, and carried water from the Fraser and Okanagan Rivers. ice sheet

Skagit River Lower Skagit Gorge looking west Fraser R.

Skagit River Fraser R. Cordilleran Ice Sheet

Between 19,000 and 15,000 years ago, the Skagit continued to drain via the Stillaguamish to the sea, but was eventually overwhelmed by ice. Mount Josephine from Skagit valley – photo by Scurlock Skagit valley looking east over Marblemount (photo by Scurlock) Advance of the Puget and Okanagan Lobes of the Cordilleran Ice Sheet into lower Skagit valley about 18,000 years ago.

Baker Valley Puget Lobe

Burpee Hills

Lyman Concrete

RSP Okanagan Lobe

Frontal retreat of the Cordilleran Ice Sheet 13,000-12,000 years ago; note mouth Baker Blocker and Lake Tyee-Grandy Creek is outlet.

N

ice margin ice margin

ice sheet Maximum extent of Glacial Lake Mt. Baker Baker about 12,000 years ago; surface elevation 1100 ft.

-After retreat of CIS opens southern outlet, lake stands at 825 ft.

-GLB persisted until at least 3,000 years ago due to bedrock at it’s outlet.

ice sheet Baker Valley

Skagit R.

(photo by Scurlock looking north) OT Marblemount

RSP

Rockport IC

Sauk R. Mt. Baker

Skagit River

Glacier Pk. LSWT Burlington Terrace

OT Kautz Creek 1947 debris flow Dee Moleenaar (10,451 ft.)

-Second to MSH in size and frequency of explosive eruptions since ice age; nine ash layers identified.

-Major eruptions ~13,000 and ~6,000 years ago.

-13,000 year old lahar reaches Puget Sound via Stillaguamish, forces Sauk to merge with Skagit.

-6,000 year ago lahar fills Lower Skagit into sea, and forms prominent terraces from Lyman to Burlington. Mt. Baker (10,775 ft.)

-Present cone built within last 30,000 years, major eruptions 13,000 and 9,500 years ago.

-Mud flow (lahar) to Deming 6800 years ago.

-1843 eruption kills fish.

-0.43 cubic miles of ice is more than all other Cascade volcanoes combined (except MR). After ~13,000 years ago, the Sauk and Suiattle Rivers joined the Skagit.

Skagit River 7,500

500 6,000 13,000

7,500 Ocean Sedro-Woolley

6,000 year old lahar terrace edge of sea 6,000 years ago

Burlington

Ocean

Mt. Vernon Geologic Evolution of the Skagit River Channel Migration Zone

J. Riedel – National Park Service The Skagit River Watershed Divides

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NCC

NCC

S C

P C