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Monday, 21 January 2013

On Spillways

These two photos are from the Ouimet Canyon spillway in Canada -- 2.5 km long and over 100m deep.  It has carried a phenomenal amount of meltwater from the glacial lakes that developed at the end of the last glaciation as the Laurentide ice sheet was retreating back to its core area.  But this feature is not the result of a single catastrophic event -- the channel has a very complex history, and is now accepted as a "composite" channel which has probably been used many times, over millions of years.....

This is the Red Rock Pass, which carried vast quantities of water from the overflowing of Lake Bonneville in the USA.  It's a  very impressive feature, and is justly famous -- but again it seems to have been used more than once.


Click on this one and give it careful scrutiny.  Beyond the ice edge, this is complex terrain, with many areas of glacial and fluvioglacial deposits among the bedrock outcrops.  The terrain is hilly, and there are many meltwater lakes.  But note that the water NEVER flows back into or under the ice.  On the other hand there are long stretches of ice front where the meltwater flows in well defined streams along the ice edge, going from one lake to another.

4 comments:

Dave Maynard said...

So are there lots of small spillways here each taking the surplus from one lake to another?

Is this what we may have in North Pembrokeshire, many small channels that don't seem to go anywhere in particular.

Has anyone tried to estimate where these pools might be from the channels we think we can see. I'm thinking pools smaller than Llyn Brynberian, but then they might not have the volume to cut some of the channels. Perhaps the Llantood channel might be on the smallish side rather than the Cippyn example

BRIAN JOHN said...

Dave, I have argued ever since I was a research student that the majority of meltwater channels in N Pembs are very old, composite in origin, and probably used many times by meltwater during the waxing and waning phases of various glaciations. Dai Bowen and others wanted them to be Devensian in age, but I could never accept that there was enough meltwater around in the Devensian to cut all those huge channels -- and that was supported by the fact that there was till and periglacial materials inside the channels which would have been flushed out, had they really been late Devensian spillways. Thankfully, most geomorphologists now seem to accept the great age of the channels.

But their models are still far too simple -- as I have argued before, the straight lines that they draw on maps as indicating Devensian ice edges are quite unrealistic. there are always multiple lobes and embayments in an ice edge, whether advancing or retreating, unless the ice is moving on a perfectly flat plane -- hardly ever found in nature.

And yes, I think you may be right. Maybe we should explore the idea of multiple meltwater pools coming and going, coalescing and draining and emptying, in a very complex fashion -- maybe with small short-lived spillways running in all sorts of unexpected directions.....

Constantinos Ragazas said...

Brian,

Beautiful photos of spillways! But did meltwater made these? Or merely used them!

Could glacial lakes drain without etching such dramatic characteristics in the landscape? My sense tells me “yes”. And “yes” implies “no” to evidence on the ground!

Kostas

BRIAN JOHN said...

Very deep glacial meltwater spillways can be cut, and have been cut, where vast quantities of meltwater (measured in cubic km) have to be evacuated catastrophically. But meltwater will always use pre-existing channels if possible -- natural processes always use the lines of least resistance. The biggest meltwater channels in the UK have probably been used, deepened or refreshed many times.

And yes, some spillways may be rather shallow, if water levels in a lake have been stable or falling -- through leakages via otherc routes. So short-lived channels, used by small lake overflows for just a few years or decades, may not be very prominent in the landscape.

And in the great scheme of things, Lake Brynberian (if it existed) was a small lake......