HR: 09:35h
AN: H51F-06    [Abstracts]
TI: High Suspended Sediment Yields of the Conestoga River Watershed to the Susquehanna River and Chesapeake Bay are the Result of Ubiquitous Post-Settlement Mill Dams
AU: Merritts, D
EM: dorothy.merritts@fandm.edu
AF: Dept of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003 United States
AU: * Walter, R
EM: robert.walter@fandm.edu
AF: Dept of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003 United States
AU: Lippincott, C
EM: caitlin.lippincott@fandm.edu
AF: Dept of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003 United States
AU: Siddiqui, S
EM: sauleh.siddiqui@fandm.edu
AF: Dept of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003 United States
AB: The Conestoga River watershed of Lancaster County, Pennsylvania constitutes 1.7% of the total area of the Susquehanna River basin and drains mostly Piedmont Province with very low relief and hillslope gradients, yet it contributes the largest annual suspended sediment yield of any tributary in the basin. Nearly one-third of the Susquehanna's sediment is deposited in the northern Chesapeake Bay, causing significant impairment to water quality. Poor farming practices (60% agricultural land) and late 20th Century suburban sprawl have been blamed for the anomalously high sediment yields observed in the Conestoga watershed. Our study indicates, however, that the main cause of these high sediment yields is sediment trapped behind ~500 post-settlement mill dams. On average, there was one mill dam every 2.5 km of stream length. We calculate that a minimum of 27 x 10$^{6}$ m$^{3}$ of post-settlement legacy sediment was stored in Conestoga watershed valleys. This estimate is based on field mapping and coring and on analysis of historical records, air photos, and maps. We compiled a GIS database for all known mill dams and constructed stream profiles showing the location and height of each dam. Average dam height was 2.4 m (range 1-9 m) and average stream gradient is 0.001, so the average reservoir extended ~2.4 km upstream. Field measurements show that the average stream valley bottom is 100-m wide and coring reveals a broad, planar bedrock valley floor beneath post-settlement alluvium. The legacy sediments overly a thin veneer of organic-rich sediments (peats and leaf mats) and tree stumps (some cut), which yield late 17th century and older $^{14}$C ages. These organic layers overly ca. 20 cm of pebbly-sands, which together represent the pre-settlement to early-settlement valley floor. From these findings, we calculate that the average mill reservoir could trap 0.3 x 10$^{6}$ m$^{3}$ of sediment. Field surveys and air photographs taken in the 1930s and 40s reveal that all reservoirs were filled to capacity with sediment. Numerous factors contributed to the storage of such large volumes of sand, silt, and clay along Conestoga stream corridors: 1) parent material of thick saprolites developed on Paleozoic silty limestone; 2) widespread soil erosion with land clearing for agriculture from ~1700 to 1800 AD; 3) intense deforestation and industrial charcoaling from ~1850-1910; and 4) the advent of the mechanized plow in the late 1800s that initiated widespread gullying and associated downslope deposition of sediment. Pennsylvania leads the nation in the removal of low-head dams. Once these dams are removed, however, the streams quickly incise through the stored mill pond sediments to the level of their former valley floors, reaching gravels that are eroded more easily and undercutting steep banks of finer-grained, slightly cohesive legacy sediments. Bank erosion occurs along at least 80% of the 1036 km of stream channels in the watershed. We estimate that 10% of the sediment stored along valley floors since 1710 has been removed in the past several decades by channel incision and widening that closely resemble arroyo-cutting (lateral bank erosion rates of $>$0.5 m/yr measured at multiple sites). The large volume of legacy sediment has become a major source of suspended sediment load to the Susquehanna River and Chesapeake Bay in the past 35 yrs, and will remain so unless substantial remediation efforts are made.
DE: 1719 Hydrology
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting