Special Sessions
Sponsored by the Geomorphology Specialty Group
Association of American Geographers 103rd Annual Meeting
San Francisco, California, April 17-21.

Fluvial Geomorphology III: Floodplain Processes

Paper Session 3601
Thursday, 4/19/07, from 5:00 PM - 6:40 PM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

Paul F. Hudson - University of Texas
Michael Slattery - Texas Christian University

Chair(s):

Paul F. Hudson - University of Texas

Abstract(s):

• 5:00 PM Author(s):
  *David S. Leigh - University of Georgia
  

  Abstract Title: Human Influence On Floodplain Sedimentation Along The Upper Little Tennessee River, Southern Blue Ridge Mountains, USA.

  
  

  This research examines human-induced geomorphic transformation of the Upper Little Tennessee River valley within a 363 km^2 catchment of the Southern Blue Ridge Mountains. Objectives focus on measuring late prehistoric and historic overbank floodplain sedimentation rates and deriving ungaged flood records from sedimentology. Stratigraphy, particle size, and geochronology (radiocarbon and cesium-137) data were measured at three mainstem floodplain sample sites from cores and outcrops. Particle size was measured in continuous down-column overbank sediment samples (1-3 cm increments). Results indicate that late prehistoric sedimentation rates were less than 1 mm/yr, whereas historical sedimentation rates are an order of magnitude higher (5 to 17 mm/yr). Furthermore, the most rapid historical sedimentation rates of 13-17 mm/yr occur after 1960. These latest high sedimentation rates correspond to a time period of gentrification in the region characterized by second home construction, road construction, and other erosive land uses related to population and infrastructure growth. Comparison of particle size data with post-1945 stream gage data indicates that the percent of particles larger than 0.25 mm is a good indicator of the frequency of large overbank floods, but that little information is provided about flood magnitude. Particle size data indicate that the highest frequency of large floods occurs after 1960, relatively few large floods occurred circa 1910- 1960, whereas many large floods occurred during the peak time of timber harvest in the region circa 1875-1910. Ongoing research concerns the importance of wet versus dry climatic periods on sedimentation versus changes in land use.

  Keywords: Appalachian, fluvial, geomorphology, sediment

  
  
• 5:20 PM Author(s):
  *Scott Lecce - East Carolina University
  Robert T. Pavlowsky - Missouri State University
  Gwenda J. Schlomer - Missouri State University
  

  Abstract Title: Mercury Contamination of Floodplain Sediments from Historic Gold Mining in Gold Hill, North Carolina.

  
  

  Although mercury is the most common contaminant of aquatic ecosystems worldwide, its sources, pathways and toxicity-controlling processes are complex and relatively poorly understood. One source of mercury contamination is gold and silver mining where use of the mercury amalgamation process has led to the release of unprecedented amounts of mercury to the environment. The first documented discovery of gold in the U.S. occurred in 1799 in the Piedmont of North Carolina, leading to the nation’s first gold rush between about 1830 and 1860. Although North Carolina led the nation in gold production until 1848, and produced more gold than any other state in the southern Piedmont gold belt, few studies of mercury contamination associated with this mining exist. The purpose of this study is to determine the magnitude and explain the distribution of mercury contamination in floodplain sediments more than 100 years after largescale gold mining ceased in the region. This paper presents preliminary data suggesting that floodplain sediments are highly contaminated downstream from the Gold Hill mining district, one of the most intensively mined areas in the North Carolina gold belt. Maximum background Hg concentrations in 25 samples collected from source area soils is about 0.1 ppm. Samples collected from exploratory floodplain cores show that about half are contaminated above 0.1 ppm and 25% had more than 5 times the maximum background concentration. Well-defined peaks in mercury concentrations in overbank deposits are used to assess historic rates of floodplain sedimentation.

  Keywords: mercury contamination, gold mining, floodplain sedimentation, North Carolina

  
  
• 5:40 PM Author(s):
  *Paul F. Hudson - University of Texas at Austin
  Alexandra G Myers - University of Texas at Austin
  

  Abstract Title: Geomorphic controls on floodplain lake variability on the Texas Coastal Plain.

  
  

  River management requires understanding the time-scales for river channels to adjust to disturbances. The San Marcos River (640 km2) emits from a large spring (4.5 m3/s) at the eastern edge of the Edwards Plateau in Central Texas, and has been significantly impacted by Cryptocoryne becketti, an exotic water trumpeter from Sri Lanka. State and federal river management agencies removed the Cryptocoryne becketti via extensive channel dredging. This paper provides an overview of the extent of geomorphic adjustment over a one year period, spanning the predisturbance, disturbance, and the initial stages of the postdisturbance regime. A variety of approaches were utilized to characterize the geomorphic response of the San Marcos River to channel dredging, including repeat channel surveys, acoustic Doppler bathymetry surveys, channel bed erosion pins, bank erosion pins, turbidity measurements, and estimates of bed load transport. Based on repeat channel cross-sectional surveys, 155 m3 of material was dredged from the river channel bed. There was an average increase of 1.9 m2 in channel cross-sectional channel area, but was as high as 6 m2 where Cryptocoryne becketti was densest. The maximum depth of dredging exceeded 0.75 m in some reaches, creating dredge holes and channel knickpoints After several months dredge holes showed minor (7.6 cm) upstream expansion, and associated downstream infilling (6.1 cm). Data from bank erosion pins reveals very low rates of bank erosion. The San Marcos River does not show significant signs of adjustment, which may be due to inherent channel stability, as well as persistent low flow conditions.

  Keywords: fluvial geomorphology, river channel erosion, channel dredging,

  
  
• 6:00 PM Author(s):
  *Patricia F. McDowell - University of Oregon
  Jim E. O'Connor - U. S. Geological Survey
  Pollyanna Lind - University of Oregon
  

  Abstract Title: Holocene and historical floodplain development in the Sprague River, south-central Oregon.

  
  

  To provide an understanding of river and floodplain processes for restoration decisions, we used historical aerial photographs and maps, LiDAR data, soil maps, and field stratigraphic investigations to map the floodplain and present and former channel positions for a 150km-long river section. The Sprague has alternating wide and narrow valley segments, with a high-sinuosity (1.5-1.9), low-gradient (<0.01) sand-bed channel in wider segments. The floodplain is relatively unincised, is well-watered with springs and irrigation, and supports extensive riparian wetlands. The Mt. Mazama eruption at 6700 yr BP injected much sand-size pumice into the watershed, as air-fall deposits and in a large sediment-laden flood down a tributary. The modern (late-Holocene) floodplain is rich in Mazama pumice with a finer top-stratum, while the pre-Mazama floodplain has loam to silty clay soils. Deposits of the 2006 peak discharge were dominated by Mazama pumice sands. In contrast to the smooth pre-Mazama floodplain, the modern floodplain shows much channel activity in the form of lateral migration (scroll bars), avulsion (anabranches), and shorter cutoffs (neck cut-offs, abandoned meanders). Many neck cutoffs date from the late 20th century. Filled meander scars date from the 20th century to 3500 yr BP or older. No incision is evident from 3500 yr BP to present. The Mazama eruption apparently had a significant impact on the river's style of floodplain development and channel migration.

  Keywords: fluvial geomorphology, LiDAR, channel change, Quaternary, historical

  
  
• 6:20 PM Author(s):
  *Michael C Slattery - Texas Christian University
  

  Abstract Title: Forensic fluvial geomorphology: lessons learned from being an expert witness in geomorphic litigation.

  
  

  This paper presents the results of a field study on potential sediment source areas, soil erosion, and sediment delivery in a 93 ha basin in Gregg County, Texas. The work was conducted as part of litigation pertaining to sedimentation of three water bodies located on properties owned by the plaintiffs. In the case, the plaintiffs contended that the three ponds had been significantly impacted by accelerated erosion from property owned by the defendant, who had constructed a storage facility that had (a) caused significant disturbance of the topsoil, and (b) increased runoff from the impermeable surfaces, thereby eroding and transporting sand into the ponds. While geologists and engineers frequently testify as expert witnesses in geomorphic-type forensic activity, here I argue that geomorphologists should become far more involved in such cases in which they clearly could contribute to an appropriate outcome.

  Keywords: fluvial geomorphology, sediment transport, litigation

  
  

Session Description: These three sessions in Fluvial Geomorphology are organized into sediment transport, river channels, and floodplains. The geographic focus is broad and includes a range of climatic and geologic settings, and the papers consider fluvial processes from the basin scale to channel-reach scale. The sessions are sponsored by the Geomorphology Specialty Group.