Big Spring Run bottom land
Big Spring Run is a small tributary of the Susquehanna River. This stream and thousands like it contribute considerable sediment to the Susquehanna and ultimately to Chesapeake Bay. Federal regulation calls for substantial reductions in sediment flowing into the Chesapeake, and a long-standing question is how to do that. For a long time, it was thought that the high sediment loads were caused by poor farming practices. Contour plowing and other erosion reduction methods have become widespread, but still there is a lot of sediment entering the Chesapeake. The trip leaders have been intensively studying this watershed, and have concluded that sediment is not mostly derived from tilled land or even hill slopes in general, but rather from erosion of sediment in the bottom lands of small streams. The stream valleys, illustrated at this field trip stop, typically have flat floors that look like flood plains. The streams, considering their size, are deeply incised into the flat valley floors and the sediments are not made of normal stream deposits such as point bar crossbeds. The flat valley floors are almost never flooded, and then only briefly, and so they can’t be flood plains. Indeed, the streams here generally flow directly on limestone bedrock, indicating that the flat valley floors were never near the equilibrium stream level.
The proposed scenario starts with pre-settlement streams, which flowed on relatively wide, flat, marshy bottoms of forested valleys. The streams themselves flowed on lag gravel over bedrock on the valley floors, through a wet meadow assemblage that is now represented by a relatively thin organic-rich horizon ~50 cm above the bedrock. Samples from this horizon at the field trip location were 14C dated and range from 3220-690 years BP, and similar locations elsewhere in the area yield ages of 14,000-300 years BP (all Holocene, pre- or earliest-settlement). After settlement, in the period of approximately 1680-1880, thousands of mill dams were built in the area, to the extent that most small streams were chains of mill ponds. The mill ponds filled with sediment, a process that was accelerated by high erosion rates related to early logging and farming practices. The local Conestoga Formation limestone supplied silt for post-settlement mill pond deposits from the thick residual weathering horizon on top of it, and some silt possibly from Pleistocene periglacial loess. The mill pond sediments contain a starkly different pollen and spore assemblage, dominated by moss and fungus, than the organic-rich pre-settlement horizon that is dominated by ferns, marsh trees and shrubs, and upland trees. By the early to middle 1900’s almost all of the mill dams were removed or collapsed, and the mill pond sediments were rapidly incised to form lacustrine sediment terraces as the streams eroded down to their original gradient. As the streams slowly meander, they erode the mill pond sediments at rates that greatly exceed sediment input from the surrounding higher ground. Solutions to the high sediment loads entering the Chesapeake Bay will apparently have to concentrate on these “legacy” mill pond deposits rather than on the surrounding farms.
Big Spring Run, showing rolling topography in this carbonate bedrock region. This photo shows the cut bank of the stream, with an organic-rich layer only ~50 cm above limestone bedrock on which the stream flows. This organic-rich layer is thought to be the original valley bottom wet meadow soil. In pre-settlement time the stream was not channelized, but rather flowed as a broad sheet through the meadow. This organic horizon is in the same location throughout this small valley, and is characteristic of all the streams in this area. This location is therefore representative of a much broader region.
Closer view of the cut bank showing the dark pre-settlement organic horizon. The organic material contains, among other things, pollen and seeds of trees and other plants that do not exist in the valley at the present time. This particular stream valley is heavily monitored, and the many small stakes and some vertical pipes are for monitoring. The taller, large diameter white tubes are not for monitoring, but were to protect the bark of planted trees that have since died.
The stream meanders through its incised gully, creating cut banks and small point bars. The flat land outside of the stream channel looks like a flood plain, but it is really a terrace of incised mill pond sediment. Here the organic layer is again visible, as are stones on the bottom of the stream. The stones are a lag gravel derived from quartz veins in the underlying limestone.
Eroded flank of a point bar, showing that the pre-settlement organic layer is missing here, and some gravel is present in the fine-grained sediment. This is backfill of an old sewer line. The point of the photo is to show the large quartz vein boulder in the middle of the image. Because the stream typically carries stones only up to 3 cm or so even in flood, which does not usually rise to the flat land surrounding the stream, large boulders have probably traveled only very short distances if at all.
This shows a small point bar on the inside of a meander bend. The organic layer is absent on the eroded cliff on the left because of an old sewer line excavation, as mentioned above.
Slumping of the silty sediment into the stream from an undercut bank.
Another view of undercut blocks of sediment slumping into the stream, supplying new fine-grained sediment load.
Recent point bar deposits on the inside of the meander loop.
The cut bank of this meander loop exposes the pre-settlement organic-rich horizon to the left of the two people. Under the person on the right the organic layer is missing (it projects under water, but it really isn’t there), and some gravel is mixed in as fill for the sewer line, mentioned above. This view of the extensive flat surface of the lacustrine terrace behind the people shows why it was long mistaken as a flood plain.
I have to admit that I was fascinated by this stop. Since I was a kid I have seen places like this throughout New England, and wondered what changed to cause so many streams to be deeply incised into their “flood plains”. This answers the question by showing that the flats are not flood plains, but terraces of lacustrine sediments that were deposited behind mill dams, which used to abound in New England, Pennsylvania, and elsewhere. Although some places may be different, I bet most streams I wondered about owed their peculiar geomorphology to prior land use, and especially dams. This was definitely my favorite stop, until we got to Stop 3.
