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

Fluvial Geomorphology II: Sediment and Channel Dynamics

Paper Session 3501
Thursday, 4/19/07, from 3:00 PM - 4:40 PM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

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

Chair(s):

Francis J. Magilligan - Dartmouth College

Abstract(s):

• 3:00 PM Author(s):
  *Francis J. Magilligan - Dartmouth College
  Carl Renshaw - Dartmouth College
  Kristen M. Svendsen - Dartmouth College
  Nislow H. Nislow - USDA: Forest Service
  James M. Kaste - Dartmouth College
  

  Abstract Title: The Effects of Flow Regulation by Dams on Channel Bed Sedimentation and Benthic Community Structure: Longitudinal Variation and the Role of Tributary Inputs.

  
  

  Flow regulation induces hydrologic, geomorphic and ecologic changes in impounded river systems, and tributaries may mitigate the effects of flow regulation by altering the flux of water and sediment into the flow-regulated mainstem. To capture the effects of channel bed changes and of tributary influences we use long-term discharge and cross-sectional data to assess the geomorphic and hydrologic impacts of impoundment on a flow-regulated river. In addition, we evaluate the use of the short-lived cosmogenic radioisotope 7Be (half-life 53.4 days) to link the geomorphic impacts to benthic macroinvertebrate community structure. We find that 7Be activity of transitional bed load sediment is highly seasonal and reflects both sediment source and residence time. Benthic populations also exhibit a strong seasonal variability. The mainstem radioisotope signal and benthic community composition change abruptly at tributary junctions. In the spring, neither the 7Be activity of the sediment, nor the benthic population exhibits a clear relationship with sample site location. However, during the summer the ratio of Ephemeroptera/ Trichoptera, representing benthic community structure, decreases below tributary junctions. In addition, increases in filter -feeding/shredder Trichoptera abundance strongly correlate with recently 7Be tagged sediment at the confluences. The results show that tributaries are impacting the flow-regulated mainstem. Moreover, we propose that the tributaries are exacerbating the effects of impoundment due to the reduced competence and capacity of the mainstem.

  Keywords: Dams, Isotopes, Macroinvertebrates, Tributary Junctions

  
  
• 3:20 PM Author(s):
  *Mary Ann Madej - USGS Western Ecological Research Center
  Diane Sutherland - USFS Redwood Sciences Laboratory
  Bonnie Smith - Anderson Consulting
  Thomas E Lisle - USFS Redwood Sciences Laboratory
  

  Abstract Title: Channel response to sediment pulses in steep, gravel-bed rivers: A Flume Study.

  
  

  Fluvial geomorphologic field studies provide useful information on rates and processes of channel formation and evolution; however, they are typically confounded by a myriad of variables. In contrast, flume studies can control variables to provide a stricter experimental design, but the scale of water and sediment must be adapted to laboratory conditions. In this study we used a flume experiment to track changes in sediment transport and channel storage following large sediment inputs. In a 12-m long, 0.75 m wide flume, water discharge and flume slope were held constant, but the sediment feed rate varied to initiate cycles of aggradation and degradation cycle. We used a laser microtopographic scanner to measure bed topography and elevation. Channel response to increased sediment loads followed a similar trajectory of change that had been documented in field studies of Redwood Creek, California. Initially the channel responded to increases in sediment load by textural changes in the channel bed (a fining of the bed surface). As the aggradational cycle continued, bedform size and form roughness decreased. Mean depth decreased slightly during moderate sediment feeds, but decreased much more rapidly during high sediment feeds. Locally, secondary channels formed, although the channel pattern never became fully braided. As the sediment pulse was processed by the channel, the channel reverted to a single thread system, bedforms became more pronounced, channel roughness increased, and channel armoring became stronger. Overall transport rates increased during aggradational phases and decreased during degradational phases.

  Keywords: sediment transport, storage, supply, flume, aggradation

  
  
• 3:40 PM Author(s):
  *James Colin Rogers - University of Georgia Department of Geography
  David S Leigh - University of Georgia Department of Geography
  

  Abstract Title: Estimating sediment yield from tributary stream banks in the Southern Blue Ridge Mountains.

  
  

  Hillslopes of the Blue Ridge Mountains underwent significant erosion due to logging during the late 19th and early 20th centuries, which caused rapid overbank sedimentation in stream valleys. Subsequently, circa 1910-1940, the tributaries incised and left the historical overbank sediment as a low terrace, which provides a significant source of sediment to overall basin sediment yield. We developed linear regression models that predict minimum estimates of bank erosion and yield as functions of the correlations between Shreve stream order, floodplain width, and historical terrace height. The models are based on field measurements from 40 to 50 catchments smaller than 50 km^2 and a drainage network delineated from digital elevation models using ArcView. Age estimates for incision and floodplain establishment come from stratigraphic, photographic, dendrochronologic, and geochronologic (luminescence and radiocarbon) measurements. Terrace height minus floodplain height is the basis for calculating net yield from bank erosion. Preliminary results indicate that banks erode at 1.0 to 1.5 cm/yr, which yields about 10-20 t/km^2/yr. This estimate is equivalent to published direct observations of sediment yield (sediment rating curve and reservoir sedimentation approaches) from fully forested catchments in the region where bank erosion is the primary source of sediment. Thus, our approach is validated as a suitable alternative to other methods of estimating bank erosion, such as process-based models and measurements from erosion-pins. Future research will investigate whether various types and intensities of land uses influence variation in bank erosion rates.

  Keywords: sediment budget, erosion, Appalachian, fluvial, geomorphology

  
  
• 4:00 PM Author(s):
  *David Michael Livingston - Indiana State University
  Basil Gomez - Indiana State University
  

  Abstract Title: Controls on Longitudinal Profiles: Waipaoa River Basin, New Zealand.

  
  

  Rivers adjust their gradient to discharge and the character of the rock or sediment that forms the channel boundary. Accordingly, as J.T. Hack demonstrated, rivers of similar size flowing across comparable substrates tend to have similar profiles. The neighboring 222 km2 Mangatu and 239 km2 Upper Waipaoa catchments of the Waipaoa River basin, New Zealand, offer an ideal setting to examine the interaction of these and other variables on river profiles. These catchments are under laid by similar lithologies, have been subjected to a similar climatic regime and have experienced a similar rate of uplift during the past ~15 kyr. There is little difference in total-relief, drainage density and distribution of slope angles, or in the median size of sediment present along the main stream channels. Despite the similar conditions, the profiles of the Mangatu and Upper Waipaoa rivers are quite different. The Upper Waipaoa has a gentler gradient and the upper reaches of the stream are ~80-m lower than adjacent reaches of the Mangatu River. These differences are attributed to how discharge increases in a downstream direction along the two rivers which serves to emphasize that the distribution of energy in the stream-channel system is dependent on the structure of the drainage network, and that an orderly empirical relationship between drainage basin area and main channel length may not always apply.

  Keywords: fluvial geomorphology, hydrology, gis, landscape evolution

  
  

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.