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Title Pre-versus post-settlement alluvial sedimentation rates in the Upper Tennessee River Valley, Blue Ridge Mountains
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Population growth and associated expansion of agriculture and timber harvest commenced at about A.D. 1870 in the Upper Little Tennessee (ULT) River basin, a date which separates pre- and post-settlement alluvial strata. Rampant gullies and rapid bottomland sedimentation accompanied the late 1800s to early 1900s population growth. Rapid bottomland sedimentation continues today in mainstem river valleys, possibly related to erosion from a recent surge in population growth and second-home construction. Sedimentation rates from numerous sites, including both mainstem and tributary bottomlands are presented. Chronologies are determined with radiocarbon, cesium-137, stratigraphic and historic records. These data confirm an 1870 estimate for the boundary between pre- and post-settlement strata marked by the top of a prominent buried A horizon. Pre-settlement sedimentation rates typically register less than 1 mm/yr, with occasional episodes up to 5 mm/yr. There is no indication that Native Americans increased sedimentation rates during the last millennium prior to the 1800s due to erosive agricultural practices. Post-settlement sedimentation rates commonly are an order of magnitude faster than pre-settlement rates, with natural levees typically registering 9 to 18 mm/yr. Meander scars exhibit slower rates, but still register at least an order of magnitude increase for post-settlement strata. Many post-settlement natural levees along mainstems exhibit their fastest sedimentation rates since 1963, but levee progradation may exaggerate this trend. The ability to resolve decadal changes that may be responses to historic land use changes is hindered by a lack of appropriate dating techniques. Rapid post-settlement sedimentation rates in tributary valleys (<20-30 km2) appear to have ceased by the middle 1900s, as indicated by cesium-137 peaks that are within the surface A horizon. Cessation occurred because a new floodplain has formed within the past century in tributary valleys, rendering the formerly rapid post-settlement accretions on a low terrace. This low historical terrace in tributaries now functions as a sediment source due to lateral channel erosion, but usage of "legacy sediment" for such bank-derived sediment should be avoided, because half of it may be from pre-settlement strata.

Contributor D. S. Leigh

Leigh, D.S. 2012. Pre-versus post-settlement alluvial sedimentation rates in the Upper Tennessee River Valley, Blue Ridge Mountains. Pages 66 in: Southeastern Section of the Geological Society of America: Session No. 20 Hydrological Processes and Problems in the Southern Appalachians. Geological Society of America, Asheville, North Carolina.

Key Words alluvium, sediment, vertical accretion
File Date 2012
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