Lake Missoula Flooding: One or Many?

1. Lake Missoula Flooding:One or Many? Richard B. Waitt Jr. Periodic Jökulhlaups from Pleistocene Glacial Lake Missoula – New Evidence from Varved Sediment in Northern Idaho and Washington (1984) John Shaw, et al. The Channeled Scabland: Back to Bretz? (1999) Mike Lamons

2. Brief Background 1920’s Harlen Bretz - Recognizes evidence of Spokane Flood, coins the term Channeled Scabland - Identifies ancient Glacial Lake Missoula as the source of the Spokane Flood Late 1970’s/Early 1980’s Richard Waitt - Suggests that a series of 40 or more floods were responsible for the carving of the Channeled Scabland

3. Evidence from Waitt Pend Oreille and Priest River Valleys - Flood gravel deposits in Pend Oreille River Valley suggest another route for Missoula Flood waters - Glacial Priest Lake - Clay/Silt varves periodically interrupted by beds of sand - Sand beds capped with massive silty clay - Evidence points to Priest Lake as a quiet lacustrine environment, sharply interrupted every few decades by backflooding current

4. Evidence from Waitt Latah Creek Valley - Sand and gravel beds (16 in all) - Thick (1-4m) flood deposits each topped with a thin (0-20cm) layer of mud, dip upvalley - Beds contain particles 8 to 12 grain-size units coarser than the intermediate mud layers - Boulders - Scattered throughout the sand/gravel beds, there are boulders as large as 90cm in diameter

5. Evidence from Waitt Latah Creek Valley (cont.) - Boulders - LCV carved out of basalt, but beds contain a number of nonbasaltic crystalline-rock types - Uncemented state of sand/gravel beds and lack of soil horizon indicates Late Wisconsin age - Evidence suggests that sand/gravel beds were carried in floodwaters deep enough to sweep violently up the Latah Creek Valley

6. Evidence from Waitt Latah Creek Valley (cont.) - Atop a disconformity in the LCV sit 12 thinner gravel beds with similar upvalley-dipping structures, perhaps formed after the glacial lake had drained - In all, evidence for 28 floods in the LCV

7. Evidence from Waitt Spokane Drainage Basin - Flood deposits form giant current dunes and bar-like landforms, all containing huge boulders - Evidence for an immense high-velocity discharge of water - Some sites contain sand/gravel beds separated by clay/silt beds (centimeters thick) containing as many as 35 varves - Suggests that decades of normal sedimentation occur between each catastrophic flood

8. Evidence from Shaw, et al Ninemile Creek Section - Thick silt beds interpreted as the result of rapid sedimentation, causing a variety of structures - Jökulhlaups from beneath Rocky Mountain trench glacier identified as best potential source of sediment - Thick clay records identified as turbidity currents rather than GLM emptying and refilling Sage Trig Section - Suggested that basaltic clasts should be seen with flooding from Glacial Lake Missoula, but are absent

9. Evidence from Shaw, et al Starbuck Section - Massive silt beds thought to be the result of bioturbation are explained as a result of mere suspension settling, burrow casts deemed modern - Changes in grain size are merely representative of rapid flow deceleration - Absence of desiccation cracks, rilling, eolian deposits, and paleosols indicates and unlikelihood of subaerial exposure between depositional events

10. Evidence from Shaw, et al Hypothesis - Shaw, et al suggest that evidence from Sage Trig section indicates flow from the north, which is unlikely to have come from the drainage of GLM - Possible alternate source of water could have been drainage from British Columbia through deeply incised tunnel channels - Drilling operation discovered coarse gravels on bedrock eroded below sea level, potentially formed by powerful subglacial drainage

11. Evidence from Shaw, et al Hypothesis (cont.) - Shaw, et al reason that the Scabland floods may have been partially formed by an enormous subglacial reservoir that extended over much of British Columbia - Volume of water estimated to be contained within this reservoir is 10^5 km3, greatly exceeding the 2000 km3 estimated fr0m Glacial Lake Missoula - Given discharge estimates from the Wallula Gap, the reservoir was capable of sustaining discharge for a period of 100 days

12. Conclusion While it seems the current consensus regarding the Channeled Scabland origin lies with multiple catastrophic floods from Glacial Lake Missoula, there are many who see inconsistencies as evidence for other flood sources. An alternative flood source could support hypotheses of a single Glacial Lake Missoula draining, rather than the periodic Jökulhlaups described by Waitt.