Most people blame current agricultural practices, sewerage treatment facilities, and development – strip malls, residential subdivisions, and paved roads and parking lots – for polluted waterways and unstable streams, but a greater portion of the problem, especially in the Chesapeake Bay region, goes back to the agricultural period of the 19th and early 20th centuries, when large-scale forest clearing and poor farming practices dumped millions of tons of soil into our local streams, valleys, and floodplains. Hundreds of mills and dams along Pennsylvania waterways caused water to slow down behind them and deposit additional tons of sediments. These sediments, deposited throughout our stream and river valleys within the past two centuries, are what we call “Legacy Sediments.”
Legacy sediments alter the geomorphology and hydrology of the valley bottom, producing an array of problems for the streams themselves and the communities through which they flow. Such problems include increased sediment yields, high nutrient contents, bank erosion, debris jams, habitat instability, and flash floods, all of which are common in the small streams of watersheds such as the Susquehanna, Schuylkill, and Delaware Basins. Many of these problems surfaced after the onset of urbanization.
By the mid 20th century, conservation farming practices slowed or stopped sedimentation in many streams in these watersheds. Urbanization in places like Lancaster County, where our Conestoga Watershed studies are being conducted, began in the 1950s, reaching a peak in the 1970s and 1980s, prior to the implementation of stormwater management policies. Stormwater runoff increased dramatically with urbanization according to models developed by Lancaster County Office of Engineering. Stream channels that had been aggrading (building up) for centuries began degrading (cutting down) – commensurate with increased runoff and removal of low-head, early American mill dams – and rapidly incising through thick stacks of legacy sediments, exposing peats, sands, and gravels of the submerged pre-settlement valley floors.
Once this pre-settlement floor is reached, the gravels erode easily and begin undercutting the banks of the slightly more cohesive, finer grained legacy sediments. Bank collapse and erosion now occur along at least 80 percent of the 1036 km of stream channels in the Conestoga watershed. We estimate that 10 percent of the sediment stored along valley floors since 1710 has been removed by channel incision and widening that closely resembles arroyo-cutting in the arid southwest (lateral bank erosion rates of >0.5 m/yr measured at multiple sites). The large volume of sediment trapped in the valley bottoms for several centuries has become a major source of suspended sediment load in local streams and in their downstream receiving water bodies during the past 35 years, and will remain so unless substantial remediation efforts are made. This same phenomenon of channel incision, channel bank erosion, and bank collapse is occurring throughout the Piedmont region of Pennsylvania and Maryland.
The deleterious impacts of legacy sediments on stream systems and their receiving waters are numerous and seriously affect groundwater recharge, flooding, water quality, aquatic environments, and native vegetation. Pre-historic floodplain areas that are naturally intended to store water are now filled with legacy sediments. Streambeds that are perched above their historical gravel levels interrupt the natural interplay between stream flow and groundwater recharge. Clays and sediments built up between the gravels and current, historically formed bank tops (often misnamed “floodplains”) prevent flows in the channel or on the surfaces of the legacy sediments from entering into the aquifer. Flow is directed, instead, into the channel and its downstream receiving waters.
The sediments now filling former groundwater recharge areas contribute to many of our current flooding problems. Individuals and entire communities grapple with frequent nuisance flooding, and often worse, because 1) less water is able to enter the aquifer as groundwater recharge, and instead is added to stream flow, and 2) legacy sediments have now filled the former floodplains, which used to serve as a storage area for water. As a result, many millions of acre-feet of storage space for groundwater have been filled and lost in watersheds.
Gravels that once served as channel beds still convey groundwater. Because modern streams are perched above the gravels upon which they once flowed, the streams no longer receive the flow of cold groundwater they once did, but rely mostly on warm runoff. The groundwater still flows along the gravels below the existing streambed. A stream that is detached from its historic gravels and base flow has impaired aquatic resources.
Old floodplains hold pre-settlement, 17th century seed-beds, which can re-germinate under the proper conditions. Today’s stream and floodplain degradation and erosion remove the historical seedbed and replace suitable, usually native, floodplain and riparian buffer vegetation with opportunistic, often invasive and unwanted species. This same erosive process removes or destroys historical and archeological evidence that also resides in the historic floodplain.
Floodplains and stream banks that typically should be 15 to 24 inches (0.3 to 0.6 m) high are, because of legacy sediments, three to 20 feet (1 to 6 m) higher than the historical floodplain. The result is bank erosion during all storm events and long-term effects on fish and other aquatic life due to increased turbidity that persists from beginning to end of precipitation events.
The legacy sediments stored along streams and the abnormally high stream banks contain massive amounts of phosphorus, which is released during channel erosion. Additionally, artificially high banks separate plant root zones from the nitrogen in groundwater. Thus, instead of nitrogen being taken up by plants, groundwater flowing through the sediments transports the nitrates, along with phosphates, into streams.
