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Re: What did Pennsylvania Piedmont streams look like in the 1600?

Joined:
2006/11/2 8:50
Posts: 6198
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CRS, If you're still there. Could you talk more about this:
"I've even talked with employees of restoration companies that think the floodplains have always been elevated several (or many) feet above the water surface. The truth of the matter is that the natural floodplains I've described probably only accumulated ~1 foot of sediment over 11,000 years and yet for decades people have assumed that our impaired streams have built high floodplains naturally."

I understand that there were large accumulations of sediment behind dams. I've seen that in many places. But it's surprising to me that the original floodplain would have been only 1 foot thick sitting on top of bedrock.

When you are looking at the profile of the sediment, how do you determine the dividing line between what was the original floodplain sediment, and what was added because of disturbances?

And was this 1 foot thick floodplain consistent in different sites? Small streams and large, throughout the Piedmont?

Posted on: 2008/3/2 10:55


Re: What did Pennsylvania Piedmont streams look like in the 1600?

Joined:
2006/9/11 11:47
From Hollidaysburg (originally Lititz)
Posts: 320
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There is usually a distinct color change from a yellow/brown sediment to a dark gray sediment found at the bottom of the stream bank. This is common for almost every stream in the Piedmont that has been impaired by sedimentation. Usually the banks are around 6 feet high and the dark sediment layer is only a few inches to one foot thick. It is not always possible to see this buried floodplain because streams are often pushed to the side of the valley and are perched on the valley wall rather than lying at the low point of the valley. Therefore, the dark, hydric soil from the native floodplain is not present because the channel is not where the historic water flow occurred. We have used carbon dating, isotope dating, and historical records to determine the ages of stream bank profiles. A great example is a stream in Lancaster County called West Branch Little Conestoga Creek. There is a site just upstream of the mouth that has a 20 ft high, vertical bank. The dark, hydric soil, which comprises about 8-12 inches at this site, has been dated using radiocarbon dating to 1640 AD. A leaf layer that lies just on top of the dark soil layer has been dated to 1735 and is believed to be the first layer of leaves in the new mill pond. Historical records show that a 20 ft dam was built at the mouth around 1732. 4" from the top of the 20 ft high bank a Pb210 (lead) isotope date of 1850 was obtained. Therefore, we have shown that over 19 feet of sediment was deposited behind the dam in 120 years (a wedge of sediment stretching for miles upstream, uncountable tons of sediment). The dating is quite useful in verifying that seed samples are taken from the historic, buried floodplain; however, they are expensive and cost between $600-900 each. I hope this explanation helps.

Posted on: 2008/3/3 10:11
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Re: What did Pennsylvania Piedmont streams look like in the 1600?

Joined:
2006/11/2 8:50
Posts: 6198
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CRS,
Thanks for the additional details. I look forward to reading more of the details. It's very interesting stuff. If there will be any field trips regarding this stuff, please send me a PM.

I'm very surprised you are not seeing the following in any of your profiles:

A layer of well-developed forest floodplain soil, with tree roots, maybe another foot on top of the low profile you described. I've seen that in some other places in PA, and in Wisconsin.

I would have thought that the original condition would have been some low, wetland areas such as you described, and some areas slightly higher in elevation, with riparian/floodplain specialist trees growing there on forest soils.

That may not be the case on the Piedmont streams you studied, since each area has its own geology and landscape. But if further research shows that it was such a mosaic of higher and lower areas, including both floodplain forests and wetlands, remember you heard it here first!

The low profile you observed right behind the dam could have been caused by the excavation of much of the floodplain sediment when they built the dam, to increase storage area in the millpond.

I agree with the anabranching idea. The very important distinction between anabranching floodplain systems and braided channels is often missed, in certain stream classification systems, taught at certain short courses.

The observation that channels were often moved to the floodplain edges, against the hillslopes, is also very good. That was very widespread all over PA and elsewhere, but not many people are aware of it. And it's very important to know that when considering stream management options.

Posted on: 2008/3/3 15:23



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