Legacy Sediments Could Have Big Impact on Bay Cleanup

Researchers from Franklin and Marshall College have determined that "legacy" sediments that built up behind old mill dams continue to haunt the region’s waterways and may pose a major threat to the Chesapeake.

The study estimates that more than half of the 180,000 tons of sediment flushed out of the Conestoga and into the Susquehanna River each year originates from legacy sediment, not “new” sources being washed off farms or development sites. It further estimates that legacy sediment each year carries with it about 135,000 pounds of phosphorus. The report concludes that “stream bank erosion is an important source of sediment and nutrients to tributaries of the Chesapeake Bay, and is at least as significant as runoff from upland sources in some watersheds.”

Past estimates of the amount of nutrients reaching streams—and ultimately the Chesapeake—have assumed that sediment and nutrients were coming from sources on the land, such as farms and development sites. The nutrient reduction “tributary strategies” prepared by states to meet Bay Program nutrient and sediment reduction goals called for a variety of actions to curb runoff from the land, such as reducing fertilizer use, changing farm tilling practices, planting streamside buffers and other actions. But those actions alone may not meet the goals if a large amount of the phosphorus and sediment reaching the Chesapeake is already in stream channels. 

State and federal officials will be examining these issues, as well as the effectiveness of new cleanup strategies aimed at dealing with sediment and phosphorus already in the stream. Such practices could include identifying erosion “hot spots,” then removing built-up sediments to restore the original wide flood plains. The work may also have ramifications for dams removed to improve fish migration—a major Bay region objective. Past studies have shown that initially, dam removals have little impact on sediment movement. The new study suggests that could change over time if the stream cuts through accumulated sediment and reaches long-buried layers of gravel. That may mean future dam removals could require increased efforts to either stabilize banks or remove stored sediment. http://www.bayjournal.com/newsite/article.cfm?article=3022