Tuesday, March 17, 2009

Mill dams in the Midwest

Here's an interesting story from EARTH magazine:

Rewriting rivers: What it means for river restoration

Merritts began to look at the streams near Lancaster, Pa., in 2002, because she wanted her students to focus on geology close to the college. Surprisingly, the fine silts and clays she found in the layering along the streambanks did not look like material deposited by running water; the sediments were too thick and too laminated, plus they were underlain by a black, organic layer rich with seeds, pollen, roots and woody debris. The organics in turn rested on a gravelly bed. When Merritts showed them to her colleague, Robert Walter, he thought they looked more like pond sediments. The pair also wondered how the three-meter-wide, ankle-deep streams the students studied could deposit banks of sand and silt six meters deep.

Walter, who had grown up in the area, suggested that the sediments accumulated behind a dam. Despite finding stream after stream with similar sediments, Merritts says that she was initially skeptical that human-made dams could have such a widespread impact. To better understand what happened, she and Walter consulted historic maps, early census studies, county records and diaries. They also examined aerial photos and light detection and ranging (LiDAR) data (which removes ground cover to more easily reveal topography) and walked riverbanks in more than 20 watersheds, such as the Brandywine River, Seneca Creek and Watts Branch. All the data pointed to the same culprit: dams.

Those freakin' dams were everywhere, a fact that is easily overlooked today because most of them have been removed. But they so altered the riparian environment that engineers trying to restore rivers today may be trying to recreate the wrong kind of river.

1 comment:

Cranberry Necklace said...

Perhaps the dams were not all or only of human origin, but made by beavers (until a couple of centuries ago), or the giant beavers of the Pleistocene (a mere 10,000 years ago).