This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Tuesday, November 9, 2010

Where did the groundwater in the Pacific Northwest come from?

Dry Falls, a 350 foot high, 3 mile wide group of cliffs
that formed during the floods that scoured the
Channeled Scablands.  Photo from the U.S. Geological Survey,
available here. Dry Falls is ten times the size of Niagara
Falls, and was formed over a short period of time during the
catastrophic Missoula Floods.
Toward the end of the last ice ages, approximately 20,000 years ago, ice plugged several of the rivers that normally drained water from the area we now know as Montana.  Behind the ice dams, a gigantic lake, Lake Missoula formed.  Repeatedly, the ice dams broke and catastrophic flood, with discharges on the order of a million cubic meters per second, raced down old and new channels, flooding parts of Idaho, Washington, and Oregon under hundreds of feet of water in just a few days.

In a new paper, Brown et al. propose that these floods, in spite of their brevity, provided the groundwater that is now present within the aquifers of the Columbia River Basalts that cover the region (Brown, K.B., et al., Isotopically-depleted late Pleistocene groundwater in Columbia River Basalt aquifers: Evidence for recharge of glacial Lake Missoula floodwaters, Geophysical Research Letters, vol. 37, L21402, 5 pp, 2010.).  This region is semi-arid today, and general climate models suggest that it was colder and drier during the Last Glacial Maximum.  The groundwaters have anomalously low del O18 and delD values, and radiocarbon ages between 15.7 and 19.6 thousand years before present, and the authors suggest that they were recharged from multiple pulses of the Missoula flood events.  Several groundwaters have even older radiocarbon ages and may have come from earlier undocumented Missoula flood events.  The mechanism by which aquifers could be charged with such huge volumes of water in times of just a few days, even with repeated episodes, are unknown, but the large depth (366 m) and volume (>1200 cubic kilometers) of the ponds formed during the floods may have helped drive the water into the aquifers.  These waters are important for agricultural and domestic water resources in central Washington, and--like the Great Lakes--may not be replenished under current geological conditions.


Mark said...

There's something about the massive scale of these events that make them even more interesting. I look forward to reading any subsequent studies on the potential mechanisms behind the proposed rapid groundwater recharge.

Anonymous said...

I have only read the abstract to the paper, so I don't know their full argument. But, I do have a question about the source of the groundwater. Since the terminus of the ice sheet was across the northern extent of the flood basalt province, wouldn't a more obvious source be melt water from the outflow there? I find it hard to envision a mechanism to recharge the aquifers in a brief flood surge.

Susan W. Kieffer said...

This is a great question! Rather than trying to answer it myself, I'm calling the attention of this question to the authors of the paper and asking them both to make sure that I wrote up the summary correctly, and to answer your question. I'm sure that they can do a better job than I can, and the dialogue may be interesting and fruitful. Thank you for your interest!