Glacial Lake Missoula:Outburst Flooding

Nicholas MisnerGEOL-309: Geomorphology

The Story of Bretz

● 1886: T.C. Chamberlain notes lacrustine formations above the town of Missoula

● 1909: Pardee maps Glacial Lake Missoula● 1923: J. Harlen Bretz presents at GSA about

research into massive floods in Washington-state○ It didn’t go very well○ “Catastrophism” = Heresy

● 1942: Pardee publishes another Glacial Outburst Flooding paper

○ GSA continues to resist the idea

● 1962: Quaternary Research Congress takes a field trip

○ They finally see what Bretz saw, and promptly apologize

Source: http://idahoptv.org/

Washington State Geography: Modern vs. Quaternary

The Cordilleran Ice Sheet advanced out of western Canada, blocking the Columbia River and forming the lake.

● River backs up along tributaries

● Surrounded by mountains● Stats:

○ Up to 600m deep○ 2170km3 volume

(Lakes Erie + Ontario))Source: Tony Waltham, 2010

Glacial Lake Missoula

Overlooking the town of Missoula

● 1886: T.C. Chamberlain recognize multiple terrace scoring on the hillside

Ninemile Creek (47°01’N, 114°23’W)

● Varves are seasonal bed formations seen in lakebed sediment formations

● (Hanson, et al.) Varved sediments have a gradual upwards-trending thickness decrease

○ This indicates progressive increases in lake depth

Source: Tony Waltham, 2010

Source: Hanson, et al., 2014

The Glacial Outburst Flood

After the ice dam failed, the entirety of Glacial Lake Missoula was free to flow out the Eddy Narrows at the northwestern outlet of the Lake.

● Stats:○ Transported Boulders predict Velocity: 80km/hr, and Soil Stripping predict cross-section.○ Together calculate discharge: 38km3/hr (9.12mi3/hr), 600 times modern Amazon discharge

● Camas Prairie○ Massive standing waves created ripples of gravel- and cobble-size sediment 30m high and

100m long

Source: Tony Waltham, 2010

The Glacial Outburst Flood

Latah Creek, south of Spokane

● Varved sediment is interrupted by gravel horizons, each marking a new flood

Ninemile Creek was located in Missoula’s basin

● The Königsberger ratio (Q), paleomagnetism: natural remanent magnetization against induced magnetization

● If Q > 1, sediment was deposited in low-energy environment○ Samples here all have Q > 1, except those taken during lake-filling

Source: Tony Waltham, 2010

Glacial Lake Columbia

Floodwaters escape through the Eddy Narrows, and into Glacial Lake Columbia, formed from ice damming by the Okanogan Lobe.

● Flooded the Grand and Moses Coulees, carving them out

● Manila Creek, northwestern arm of Columbia

○ Königsberger ratio (Q) < 1, indicating very rapid sedimentation

○ Consistent decreases in S-ratio of this sediment attributable by hematite deposition from Missoula

Channeled Scablands

Floodwaters from Missoula also overflowed south across the plains.

● Stratigraphy: late Tertiary flood basalts, overlaid by Pleistocene loess

○ Floodwaters stripped away all but high-elevation loess “islands”

○ This formed fertile, loessic soils between channels of useless basalt

Source: McDonald, et al., 2012

Channeled Scablands

● Three roadcuts here (McDonald, et al. Fig. 4) display various unconformities that indicate floods during & before OIS-2

● Use Tephrochronology to bracket the ages of loess layers 1 and 2 (L1, L2, informally)

Source: McDonald, et al., 2012

Bottlenecked Temporary Lakes Lewis and Condon

● Lake Lewis formed due to hydraulic damming at the Wallula Gap

○ Slackwater sediment & ice-rafted debris: 250m deep

○ Empties at 40mi3/day

● Columbia Gorge bottleneck, forming Lake Condon

○ Also 250m deep with pulses up to 100m deep○ Sediment load of floodwaters (by this point,

loess-heavy) contribute to local loessic soil○ Ice-rafted debris, 800km downstream, fell here

as angular-profiled boulders. Source: Tony Waltham, 2010

Pacific OutflowFinally, the floodwaters escaped through the wider Columbia valley and out to sea.

● Marine core tops off the coast of Washington (Lopes, Mix)

○ Abundance of freshwater diatoms (a type of freshwater algae)

○ Low paleosalinity levels, up to 6 Practical Salinity Units (psu) below ~35psu background levels

○ Paleosalinity drop documented 600km south and 400km offshore

○ Source: Sudden freshwater flows

Source: Lopes, Mix, 2009

Concluding Statements

● The advancing Cordilleran Ice Sheet blocked the upper Columbia River and formed Glacial Lake Missoula

○ Terrace scoring above Missoula show the formation of the lake

● When the ice dam there failed, the water flowed out and into Glacial Lake Columbia

○ Paleomagnetic studies show rapid deposition connected to the flooding○ Gravel horizons interspersed in varved sediments there show repeated flooding

● The Channeled Scablands were formed by the massive floodwater flows○ Roadcuts here feature unconformities that were dated to Lake Missoula flooding

● Paleosalinity levels off the Washington state coast indicate a sudden influx of freshwater, the time corresponding to predicted Outburst Flooding

Thank You!