Pennsylvania members of the Chesapeake Bay Commission sponsored a presentation this week on how legacy sediments—sediments left over from thousands of mill dams – are having a significant impact on sediment and nutrient loading in the Chesapeake Bay.

Sen. Waugh (R-York), who serves as chair of the Pennsylvania delegation to the Commission, along with members Rep. Zug (R-Lebanon) and Sen. Wenger (R-Lancaster) joined members of farm organizations, legislative staff and other guests to hear about the problem.

Franklin & Marshall College professors Dr. Robert Walter and Dr. Dorothy Merritts said more than 65,000 grist mills and small dams dotted streams all over the east coast, more than 600 in Lancaster County alone.

Sediments trapped behind these dams – from 4 to 12 feet deep accumulated over the years before dams were breached or otherwise disappeared. These sediments are now being eroded by the streams and carried to the Chesapeake Bay causing both sediment problems and carrying nutrients like phosphorus and nitrogen with them.

Wade Oberholtzer, P.E. from LandStudies, Inc. noted 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, according to actual measurements.

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

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.

Evaluating potential remedies for water pollution problems involving restoration, particularly in more developed areas, should include floodplain restoration, if these results hold.

Oberholtzer said thought should be given to developing a specific floodplain restoration best management practice.

Franklin and Marshall will be sponsoring a special two-day workshop on the legacy sediments issue on September 12 and 13.

For more information visit the Legacy Sediments webpage.

6/17/2005