Legacy sediments in the Conestaga Watershed, Lancaster County

Sharing information among watershed groups is an important function we provide through the Digest. We invite your feedback on this article to the authors to see if others are identifying the same problems.

Watershed assessments funded by the Growing Greener Program and other programs have begun to document stream channel erosion as a much more serious source of non-point source pollution in watersheds than previously thought.

Studies in Adams, Armstrong, Chester, Lancaster, Susquehanna, and York counties and in Maryland have documented sediment loads from stream channel erosion that are orders of magnitude more than those documented in published studies that measured lateral bank erosion in stream channels around the world, including Pa.

In addition, studies show that 50 to 90 percent of the sediment load generated in a watershed is not coming from overland flow as previously thought, but from the stream channel banks themselves.

Some examples of stream channel sediment load versus what published studies by Prosser et al (2000), Rutherford (2001), and Green et al. (1999) would predict for “problem areas” –

Measured vs. predicted “problem area” erosion rates from stream banks in various areas of Pennsylvania and Maryland.

* Calculations based on studies by Prosser et al., 2000; Rutherford, 2001; and Green et al., 1999 that suggest stream bank erosion rates of between 1 x 10^-2 and 1 x 10^-1 may be typical of “problem areas” within highly erodible stream channels.

These sediments also carry with them nutrients—nitrogen and phosphorus – that contribute to nutrient loading in bodies of water downstream.

Soil sampling revealed stream channel erosion in a 193-foot portion of the Santo Domingo Creek Watershed in Lancaster County, resulting in an estimated 104 pounds of phosphorus and 289 pounds of nitrogen per year going to the Chesapeake Bay – more significant than any other source, yet one that has not been documented thoroughly until now.

The origin of the problem in many areas apparently stems in part from the history of land use, in particular, the location of grist mills by the hundreds that doted the landscape in the 18^th and 19th Century.

Franklin & Marshall College in Lancaster has documented many of the impacts of these old mills can have. Sediment from land clearing and poor agricultural practices accumulated behind the mills dams from just a few feet thick to more than 20 feet thick on top of the original floodplains.

As dams were removed or fell into disrepair, the streams began cutting down through the sediments and carrying those sediments downstream.

The policy impact of this more thorough understanding of the role stream channel erosion plays today in sediment and nutrient loading might be two-fold:

1. Programs such as Chesapeake Bay nutrient reduction strategies, modeling work done for water quality credit trading programs, the proposed Stormwater Management Manual, the Generalized Watershed Loading Function Model and TMDL planning that assign relative weights to pollution sources may need to be updated.

At worst, underestimating stream channel erosion in some areas may result in improper burdens being placed on agriculture or other sources of non-point and point pollution to cleanup their contributions to a stream’s pollution problem.

2. Evaluating potential remedies for water pollution problems involving restoration, particularly in more developed areas, should include floodplain restoration, if these results hold. Thought should be given to developing a specific floodplain restoration best management practice.

Franklin & Marshall College and LandStudies, Inc. have established a special Legacy Sediments webpage to provide additional background on their findings so far and to solicit information from other watershed groups and consultants on this topic to see if findings are similar in other areas.

“This is a good opportunity to learn from each other how significant the stream channel erosion issue is and its policy implications in a number of areas,” said Mark Gutshall, LandStudies, Inc.. “We look forward to sharing technical information with our colleagues all over Pennsylvania.”

To offer feedback visit the special Legacy Sediments webpage or email: land@landstudies.com .

6/12/2005