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Dreams of Streams

By josh donlan on March 7, 2008.

You know what a natural stream looks like, right? The Yukon in northern Canada or the Onega in Russia come to mind. If you are like me, you are pondering images of a sinuous stream with meandering channels after meandering channels. Ever since scientists started studying fluvial geomorphology - the study of rivers - those meandering channels have become the backbone that defines a natural stream.

meander.pngLast month, two scientists from Franklin and Marshall College rocked the [river] boat with a paper in Science Magazine. They present a slew of evidence that suggests our view of a natural stream is a historical artifact and a consequence of our ancestors and their mills. "Gravel-bedded streams are thought to have a characteristic meandering form bordered by a self-formed, fine-grained foodplain." says Robert Walters and Dorothy Merritts, followed by a - not so fast. Combining historical sleuthing and lasers to study stream soils, the two scientists revisited the east coast streams where early studies formed the basis of what we think of as wild and natural streams. What's behind those meandering streams? Milldams from the 17th century. "The modern, incised, meandering stream is an artifact of the rise and fall of mid-Atlantic streams in response to human manipulation of stream valleys for water power" claim Walters and Merritts.

Turns out there were 16,000 milldams in Pennsylvania alone in the 1800s. Among other things, those damns substantially changed the way streams flowed. In in a nutshell, here are the roots of our "natural" east-coast streams: 1) people built thousands of small dams to power mills and other things, 2) those damns eventually filled up with sediment from the river upstream, and 3) those damns eventually broke which resulted in lots of water cutting deep streams with high banks, and the stream began meandering. So what?--you might be wondering. Well, the "natural" meandering river paradigm guides a multi-billion dollar stream restoration industry. This study is a classic example of how different environmental contexts and histories leave their mark on nature. It pays to know your ecological history. For rivers and for their restoration, it might behoove us to go beyond our normal baseline of 1492.

Dreams of streams or nightmares of milldams?

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Very interesting, Josh. I had caught some of that when it was first announced a while back but your perspective help to make it relevant in particular as it applies to our efforts to re-establish natural system now. Thanks.
What do you think stream restoration projects should be shooting for? Are there any examples of stream systems such as would have been naturally evolving on the east coast of the US that are still in existence?

PS...and I meant ask in additions; if we go back to the 1492 baseline have we gone back far enough? What about Native American activities in cultivating the landscape? Any thoughts on Charles C. Mann's book "1491"?

In many ways (in my view), settling on a baseline is a balance between knowing the ecological history of the environment your are dealing with and a value judgement by society (e.g., what kind of ecosystem/biodiversity does society want to co-exist with?). In some cases that might be 1492 in others it might be 10,000 years ago - these are issues that surprisingly have not been discussed or debated much.

I sorely need to read Mann's 1491. I had a copy last year, but it burned in a fire before I can tackle it. It has been on my list for a while now. You've motivated me to pick up a copy soon. Thanks.

By Josh Donlan (not verified) on 08 Mar 2008 #permalink

Hang on one cotton-pickin' minute! What about meandering rivers in the Amazon jungle? Those display a lot of the same fluvial morphology, and I'm pretty sure mill-dams have never touched that floodplain.

When I look at the streams that I am familiar with in Western Canada, it seems to me that the meandering is more a consequence of how steep the terrain is, and how much water is flowing. Three years ago, we had an abundance of spring rains that had a dramatic effect on a stream that I spend a lot of time around. Fish Creek generally meanders through a shallow valley, but when the torrent of water came through, the stream cut through the "s" shaped banks and went straight downhill, changing the flow pattern once again. If you stand at the top of the valley and observe, you can see evidence of earlier meandering stream beds that have been abandoned by the current flow of Fish Creek.

This is an important paper and shows the importance of recognizing baselines in highly dynamic systems, such as rivers. In terms of settling on a baseline, there are science-based data to consider in addition to value judgments. One of the most important points to consider about baselines and the conservation and restoration efforts to achieve them is how that baseline affects ecosystem structure and function. In the case of rivers and historic mill dams in the northeast, it may be that we have changed the shape of rivers in a way that has negatively affected nutrient retention, for example. There is great concern about nutrient inputs to downstream areas, particularly estuaries, and the role of rivers in retaining nutrients input from the uplands. Thus, a baseline that maximizes nutrient retention may not be consist with a value judgment baseline (based on human historical events), or a baseline that is best for biodiversity or migratory fishes. As Josh mentions, in addition to what baselines humans want, I think it is also important to discuss the associated trade offs with what baselines are needed to maintain ecosystem goods and services. Maybe there is a paper on this already, or maybe not? -Brad

"The modern, incised, meandering stream is an artifact of the rise and fall of mid-Atlantic streams in response to human manipulation of stream valleys for water power"
thanks

Hang on one cotton-pickin' minute! What about meandering rivers in the Amazon jungle? Those display a lot of the same fluvial morphology, and I'm pretty sure mill-dams have never touched that floodplain.