ON THE RELATIVE CONTRIBUTION OF LEGACY SEDIMENTS TO SOME MID-ATLANTIC VALLEY FILL DEPOSITS

Sediments deposited in mid-Atlantic stream valleys following European settlement, often termed “legacy sediments”, are widely recognized and play an important role in watershed management and stream restoration strategies, but their extent has been poorly quantified. We combine detailed studies of three field sites with an extensive meta-analysis of published stratigraphic data. We adopt the Chesapeake Bay Program’s Science and Technology Advisory Committee’s definition of legacy sediments as those deposited between 1750 and 1900. Older deposits are termed “pre-Settlement” deposits, while those deposited after 1950 are termed “modern”. The extent of these geochronologic units is defined by dating eroding riverbanks at the South River and Difficult Run, Virginia, and at the White Clay Creek, Pennsylvania. Dates are obtained using OSL, ¹⁴C, dendrochronology, and the fallout radionuclides ¹³⁷Cs and ²¹⁰Pb. At these 5 sites, pre-Settlement deposits average 36 % of the preserved thickness, legacy sediments average 33%, and modern deposits average 17%. Pre-Settlement sedimentation rates are very low, as expected, but modern sedimentation rates exceed those of legacy sediments at 4 of the 5 sites. To generalize these results, we created synthetic stratigraphic sequences based on 85 published stratigraphic descriptions with age control from radiocarbon dating, fallout radionuclide analyses, Cr horizons of known age, and buried soils. Though highly variable, pre-Settlement deposits comprise a median percentage of 40% of the total thickness, while legacy and modern deposits comprise 36% and 11%, respectively. The median pre-Settlement sedimentation rate is 0.06 cm/yr, while median rates for legacy and modern sediments are 0.32 cm/yr and 0.26 cm/yr. The differences between legacy sedimentation rates and modern sedimentation rates are not statistically significant. Our results indicate that, on average, legacy deposits comprise less than half of the sediments in mid-Atlantic stream valleys, and modern fluvial sedimentation rates are similar to those that prevailed following European settlement.