2012-07-25

Large Pleistocene Floods along the Columbia River

ResearchBlogging.orgI spent the last weeks travelling through Idaho, Montana, Washington, and Oregon, tracking evidence for the floods that shaped the landscape of the US Pacific Northwest. For most of it I was privileged to be guided by Jim O'Connor (USGS, Portland), world-class specialist in these events (sample references below). The pictures of this geological trip show erosional and depositional features produced by outburst floods during the Pleistocene, including the Missoula ice-dam collapse and the Bonneville flood. 
Snake River gorge cutting the
Flood Basalt layers, through which
the Bonneville flood discharged
Detail of the boulder field. Note preferent tilt 
of boulders towards the left, in agreement with
flow direction. More pics here and here








Rounded boulders left by the high waters of the
Bonneville flood among a Neogene basalt ridge
spillway. Flood flow in the direction of the picture
More pics here.




One thing that makes this scenario key to understand the importance of large flooding events in shaping landscape is the ubiquitous Columbia River Basalt formation (one of the largest flood basalt regions) and the Yellostone-Snake River volcanic zone. Both covered many of the areas we visited with a nearly flat layer of very fluid basalt, during the Neogene, long before the Pleistocene floods (see e.g. Fouch, Geology, 2011). The simple relief and lithological setting left by the flood basalts enhances the erosional features related to the Pleistocene water floods and the presence of vertical tectonic motions. 



Shorelines of the Catlow Pleistocene Lake (map)
We visited other smaller Pleistocene pluvial basins in SE Oregon (e.g, the Alvord and the Catlow basins). As in the case of Lake Bonneville (more pictures of Lake Bonneville here), many of these basins overtopped during the end of the last glaciation, spilling along the lowest point of their drainage divide and triggering large flooding episodes evidenced by large rounded-boulder bars along the outlet (see this previous post). The same happened during the post-Messinian refill of the Mediterranean, and it dangerously occurs nowadays when lakes form by landslides damming mountain rivers, as they overflow and the spilling water  erodes into the rock dam, producing hazardous rapidly-increasing rates of water discharge.
The aim is now to find the parameters and the process-based, quantitative relationships between them that determine the development of intense outburst floods in turn of a slow drainage of the lake. For this we count on a wealth of data from tens of such overtopping basins that produced such floods (see ref. by O'Connor & Beebee, 2009). In parallel, Joe Walder and Dick Iverson (USGS at Vancouver, Washington) are carrying out beautiful dam breaching experiments that add to a large body of dam failure literature. Together, this provides the behavior of overtopping basins in a very wide range of space- and time-scales. 
2012 geofieldtrip: Outburst floods along the Columbia River. (all pictures georeferenced, map here)

Liz Safran, Jim O'connor and others are organizing a "Friends of the Pleistocene field trip" at Owyhee Canyon, Aug 23-26 where similar features will be discussed, in case you want to join.


O'Connor & Beebee (2009). Floods from natural rock-material dams Megaflooding on Earth and Mars, ed. Devon M. Burr, Paul A. Carling and Victor R. Baker. DOI: 10.1017/CBO9780511635632.008
J. E. O'Connor (1993). Hydrology, Hydraulics and Geomorphology of the Bonneville Flood GSA Special Paper, GSA, Boulder, CO, , 274 DOI: 10.1002/(SICI)1099-1417(199609/10)11:5