Categories
rivers the flood

Of Catastrophic Floods and Canyons

Looks like in Texas, we just got to see a catastrophic flood in action and have some fresh land forms to look at. This is really, really cool. Catastrophic floods aren’t events that can be produced on demand1, so being able to watch one happen and be on site as soon as the waters recede to check out the alteration to the surface features is a very neat opportunity.

I’m less thrilled about some of the writing in the article, however. For example, this:

Our traditional view of deep river canyons, such as the Grand Canyon, is that they are carved slowly, as the regular flow and occasionally moderate rushing of rivers erodes rock over periods of millions of years.

Such is not always the case, however. “We know that some big canyons have been cut by large catastrophic flood events during Earth’s history,” Lamb says.

While I do understand what the author is trying to say, they’re frankly doing a poor job of it that plays right into the sticky little hands of the Young Earth Creationists; the author makes it sound as if this example of catastrophic flooding somehow sheds doubt on to what we know about the Grand Canyon. It doesn’t.

Catastrophic flooding and canyons is a fairly recent area of research that was pioneered by J Harlen Bretz – who was very much a geologist, not a young-Earth nut. He mostly looked at portions of the western US that were shaped by the glacial outburst floods from Lake Missoula during the last period of glaciation. One example of the resulting land forms is the Channeled Scablands. And the big canyons that Lamb is talking about? The coulees in that same area, such as Grand Coulee.

Morphologically, the coulees look VERY different from river canyons such as the Grand Canyon. As one example, viewed in an aerial photo, the Grand Canyon has very obvious (and incredibly entrenched) meanders – a river feature. Take a look at the Scablands and the features become much more linear – here you’re looking for the features that seem to slant NE to SW; Banks Lake, for example, was made by filling in the Upper Grand Coulee with water. And there are many other features that could be compared and found different between these canyons. If you’re curious, here’s an aerial view of the new “canyon.”

Which is why this really bothers me:

Unfortunately, these catastrophic megafloods — which also may have chiseled out spectacular canyons on Mars—generally leave few telltale signs to distinguish them from slower events.

…so do those boulders pictured in the article itself just not count? Because the idea that catastrophic flooding (such as glacial outburst floods) were the cause of certain types of features is relatively new, we’re still researching what features should be associated with what phenomena and trying to understand how they form. That is really not the same thing as there being “few” features, or that somehow slower erosional events are difficult to distinguish from flood events. Part of the elegance of Bretz’s argument for the Scablands was that the Lake Missoula outburst floods explained features that really couldn’t be explained by the normal action of rivers.

I have no doubt the YECs are already picking their way through the paper, taking the bits of data that support their position. That’s to be expected. But the way this article has been written makes me cringe. Instead of simply focusing on the super coolness of catching a flood like this in action, or giving background of other catastrophic flood morphology in the US, we’re getting these bizarre little pokings at the idea that this somehow affects our understanding of the Grand Canyon’s formation. Again, it really doesn’t. To me it just sounds like whoever wrote the article was searching for some sort of controversy or dramatic angle, one that was ultimately unnecessary when the solid facts are nifty enough to stand on their own. Writing FAIL.

1 – Okay, technically you could. But probably only if it involved an elaborate plot to blow up the dam upriver of Metropolis because you’re just so tired of your world domination plots being foiled. And you’d likely have to be cackling all the while too, and that’s just tiring.

Categories
rivers

A big dam problem

At its most basic, a river is just a conduit for the transportation of sediment via water. Rivers pick up sediment and put it back down further downstream. There’s a fascinating balance between the amount of sediment available for transport and how much the water is capable of carrying. More sediment than the river can hold? The extra sediment gets piled up in to a bar. Not much sediment in the river? The water eats the bar down to nothing and carries the sediment downstream.

Rivers constantly change and rearrange their beds on their own, depending on flow and available sediment. Bars appear to move downstream over time, with new ones forming behind them. Where people come in to the process is with dams. Dams change river flow, taking out the possibility of annual floods (when you’ll get a whole lot of sediment washing downstream) and generally trapping sediment in the lakes that the dam forms. Since the rivers downstream of the lakes suddenly have a lot less sediment to carry, this often leads to them eating their own bars and then not having the sediment input near the dam to build new ones.

Which is what is happening in the Grand Canyon1. We actually talked about these studies a lot in the geomorphology class that I took since it was a very good example of how sediment load affects river morphology. The Colorado River is starved for sediment below the Glen Canyon Dam, so it’s been eroding its own sandbars away.

One of the proposed solutions to stop the sandbars from disappearing entirely has been changing the flow of water from the Glen Canyon Dam so that there are periodic floods, which will allow sediment to be swept out in to the river. This isn’t exactly a simple proposal, though.

Ted Melis, deputy chief of the USGS’ Grand Canyon Monitoring and Research Center, said Tuesday that the key to maintaining the sandbars is not simply manipulating the flows from the dam on the Arizona-Utah line. The frequency and timing of the flows would have to exceed the erosion that occurs between them, he said.

Exactly. It all boils down to how much sediment comes in to the system as opposed to how much sediment leaves. If they want to preserve what currently exists, they’ve got to do enough flooding and time it appropriately so that the system remains balanced. If they want to actually rebuild some of the eroded bars, they need to up the sediment input even more. It makes sense that the greatest benefit would come from controlled floods that coincide with the flooding of tributary rivers; that’s when the Colorado would naturally be getting its influx of sediment.

I suppose that this can be seen as a depressing lesson that even “green” power (the Glen Canyon Dam generates electricity) isn’t without a potential for high environmental cost. It sounds like a solution where seasonal flooding would make up for the presence of the dam may be workable, but would lower its power generated somewhat. To me, that still sounds like a real have your cake and eat it too kind of solution, though – the Colorado River gets its original character back, and the Glen Canyon Dam still gets to be a working dam.

Since the 1960s, Glen Canyon Dam has blocked 90 percent of sediment from the Colorado from flowing downstream, turning the once muddy and warm river into a cool, clear environment that helped speed the spread of extinction of fish species and pushed others near the edge.

90 percent? Ouch.

1 – You can tell the man in the picture is a geologist because he has a giant beard. True fact.

Categories
geomorph rivers

Modeling Meanders

Alfalfa Sprouts Key To Discovering How Meandering Rivers Form

Some very cool stuff from the world of Geomorphology. Now that we’re realizing that channelizing rivers sometimes isn’t the best idea (well, as far as the flood plains and nearby shores are concerned, it’s never a good idea) and trying to get them back to their natural state, we’ve never managed to copy nature. We can put a man on the moon, but we can’t make a meandering river, to paraphrase. So this is some very cool modeling on how the process works, which means some day we might be able to get the meanders right.

*Quick terminology: Meandering rivers are those wandering, looping rivers we’re so familiar with. Such as The Amazon or the Mississippi or the Nile. You’re probably not familiar with braided rivers unless you live near the mountains or other sources of extremely coarse sediment, but here are a couple examples: Waimakariri River, drainage near the Yukon River. Basically, braided rivers have a lot of in-channel sediment deposits that the river cuts through in a multitude of small channels.

I definitely want to see if I can get my hot little hands on a copy of their results. It sounds extremely interesting. (Though I’m sure all the really technical stuff will make my head spin.) Also, the researcher does bring up some good questions about Mars and Titan. We can be pretty sure that neither place has or ever had the verdant banks that would help build meanders. So the real question is, how would meanders form in an environment without vegetation? What would provide the bank stability that lets the point bars grow? Maybe that’ll be the next experiment, after they’re done with their alfalfa jungle.

By the way? Best use for Alfalfa sprouts outside of a turkey sandwich. Truly.