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When settlers came to the Susquehanna River Valley, one of the first things they did was build dams to harness the power of the river so they could grind grain into flour and animal feed and operate sawmills that fed the young nation's appetite for lumber.

Before long, the river and the creeks and streams that were part of its watershed were home to hundreds of mill dams.

Although the mills and dams are mostly gone - a few remain as historic landmarks - the impact they left on the watershed remains today.

Sediment accumulated in the ponds over decades - called "legacy sediment" - and it may be a serious impediment, as well as a long-term solution, to efforts to clean up the Chesapeake Bay.

According to Lycoming County environmental planner Megan Lehman, as the dams deteriorated or were torn down, the flow of the river and streams increased, cutting through the sediment deposited in the mill ponds and washing it downstream.

Once a stream reached bedrock, it eroded laterally, undercutting its banks and causing more sediment to be washed away, Lehman said.

The process not only releases sediment, but the nitrogen and phosphorus - major factors in the polluting of the bay - trapped in it, she said.

According to county Commissioner Jeff C. Wheeland, restoring stream beds to their original pre-mill dam state could result in the removal of thousands of tons of sediment, nitrogen and phosphorus that would otherwise be washed downstream.

The process also could be a major driver of a nutrient credit trading program the county is attempting to launch, Wheeland said.

Credits generated through the initiative, and other practices designed to reduce sediment and nutrients from entering the watershed, could be sold to sewage treatment plants as a way of becoming compliant with federal and state pollution load caps.

Although the idea of legacy sediment is rather new, a recent report to the state Department of Environmental Protection by Drs. Robert Walker and Dorothy Merritts suggests it is a major contributor to water quality problems in the region.

According to the report, legacy sediment may be responsible for 50 to 80 percent of the sediment load in some, and possibly many, of the watersheds of Pennsylvania and Maryland.

Mark Gutshall, president of Land Studies, the Lititz-based environmental consulting firm working with the county on developing a credit trading program, said legacy sediment isn't even on the radar screen when it comes to initiatives focusing on reducing nutrients and sediment in the watershed.

"There has been a lot of studies over the last 20 or 30 years with academia and government looking at causes of the problem of the bay," Gutshall said. "They've focused their efforts on a model based on three components: sewage treatment plants, agriculture and land development."

Gutshall compared the model to a "three-legged stool," adding that a fourth leg of the stool - legacy sediment - must be considered.

The region's watershed today bears little resemblance to the rivers and streams that greeted settlers when they first arrived, Gutshall said.

Streams meandered through shallow beds with a wide expanse of slightly elevated adjacent floodplain.

The floodplain served as a wetland that could accommodate stormwater runoff - allowing it to spread out and flow at a much slower pace - and allow flood waters to percolate into the ground and recharge the water table, Gutshall said.

It also contained native plants that prevented erosion by slowing the flow of water and holding the soil together.

As settlers established farms and towns along the rivers and streams, they significantly altered the historic makeup of the streams by building mill ponds that turned entire watersheds into sediment rich slackwater ponds.

At the same time, the widespread clearing of land through farming and lumbering resulted in massive amounts of soil being washed into the watershed, Gutshall said.

According to Gutshall, his firm has been involved in about 30 projects designed to deal with legacy sediment through floodplain restoration.

Lycoming County is the first to propose floodplain restoration as a way of generating nutrient credits, he said.

Gutshall said the county's initiative to develop a nutrient credit trading program could be a model for other areas of the state.

"I think (the county) is taking a huge leadership position for the state of Pennsylvania," Gutshall said. "They're dealing with this head on, trying to save money as well as do something for the environment.

"These guys (the commissioners) are stepping up. There are 67 counties in the state and you've got one saying, 'We're taking the lead on this.' "