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

Fluvial Geomorphology I: River Channel Adjustment

Paper Session 3401
Thursday, 4/19/07, from 1:00 PM - 2:40 PM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

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

Chair(s):

Michael Slattery - Texas Christian University

Abstract(s):

• 1:00 PM Author(s):
  *James A Hyatt - Eastern Connecticut State University
  

  Abstract Title: Fluvial Erosion & Form Characterization in Bedrock Channels, West Greenland.

  
  

  Subglacial and proglacial discharge to1200 m3 s-1 eroded complex forms into a gneissic bedrock sill near the mouth of Watson River, West Greenland. These forms reflect interactions between bedrock, topography and flow structure. We quantify forms in order to test form classification schemes developed for bedrock-controlled glaciofluvial and fluvial settings, and to identify relationships with flow. Measurements of forms are derived from sub-cm resolution digital maps for sites on the upflow (site 1 > 220 m2), crest (site 2 > 350 m2), and downflow (site 3 > 500 m2) sides of the sill. These maps were constructed by edge mapping individual near-vertical images collected with a 4-m high camera pole. Mosaics were rectified using 2 m grid total station survey data. Sharp and smooth form boundaries were traced, scanned, and imported into medical imaging software for form analysis. The lengths, widths, perimeters, areas, orientations, centroids, and several derived variables were measured for each site (n1,2,3 ? 1600, 2800, 4100). Raster base maps were converted to vector format, registered, and analyzed in ArcGIS to map groups of forms. Initial analyses for site 3 identify spatial groupings of similar forms related to flow structure. Form asymmetry is generally greater and orientations are more tightly grouped parallel to flow at lower elevations where water depths are greater and erosive discharge events more frequent. Ongoing multivariate analyzes suggest that the number of form variables may be reduced without a loss of explanatory power but that statistical clusters may not coincide with spatial groupings.

  Keywords: fluvial; geomorphometry; erosion; Arctic; geomorphology

  
  
• 1:20 PM Author(s):
  *Robert T. Pavlowsky - Missouri State University
  

  Abstract Title: Geomorphic resistance and the lack of channel change in a disturbed Ozark river.

  
  

  Channel form is controlled by the balance between force and resistance in the fluvial system. Nevertheless, most geomorphic studies focus on understanding the influence of hydraulic forces on the channel bed and banks and spend little effort on evaluating the resistance factors involved. This study examines the channel forms and fluvial processes within the South Dry Sac River located on the northern edge of Springfield, Missouri. The objective is to better understand the geomorphic resistance factors linked to apparent channel stability and little channel change over the past century along a 1 mile long channel segment despite chronic disturbance. Indeed, the river has been subjected to a long history of local- and watershed-scale disturbance including land clearing and settlement in the 1840s, mill and reservoir dam construction in the late 1800s, and urbanization since 1980. A preliminary evaluation shows that narrow valleys and shallow bedrock limit channel adjustments in most places. Further, riparian vegetation and the composite banks reduce the rates of channel enlargement and bank failure. The adjustment periods for both geomorphic lag and recovery seem to be relatively long in this system. The main driver for instability appears to be excess gravel sedimentation in mega-bars and plane-bed channels that forces bank erosion where they clog the channel. The source of the gravel probably started during historical settlement but recent urbanization has also increased gravel loads from headwater tributaries due to bed scour and bank erosion.

  Keywords: fluvial geomorphology, channel change, bedrock rivers, Ozarks

  
  
• 1:40 PM Author(s):
  *Mark Alan Fonstad - Texas State University
  

  Abstract Title: The Three-Dimensional River Environment: Tests of Classical Hydraulic Geometry Predictions using Remote Sensing Approaches along the Brazos River, USA.

  
  

  Large rivers in semi-arid and mild mid-latitude areas often are wide, shallow, and produce physical habitats of great complexity. Policy and scientific decisions regarding management of these rivers require measurement techniques that can handle the high geodiversity and that are simple, straightforward, and can use existing geographical data. The Brazos River is a 500 km long clearwater river that has its headwaters in the eastern New Mexico and flows across Texas into the Gulf of Mexico. In clearwater rivers such as the Brazos, some light often reaches the riverbed and returns to the surface, providing optical information about different components of the physical habitat structure. The HAB-2 transform allows the estimation of river depth at each image pixel, and separation of the depth effect from the remaining image information. This remaining spectral information can be correlated with suspended sediment concentrations, bottom albedo, and water surface geometry. The widespread availability of CIR digital orthophotoquads across much of the United States allows the use of HAB approaches to extract three dimensional data for large area riverscapes. In so doing, they allow the testing of classical models of fluvial forms and large-area processes. The classical theory of the downstream hydraulic geometry, for example, postulates power relationships between a rivers upstream drainage area and its width, depth, and velocity. Calibrated high-resolution imagery shows systematic deviations from the classic theory as well as a level of randomness at most scales that support's Phillips's theory of multiple modes of adjustment in fluvial systems.

  Keywords: geomorphology, remote sensing, sediment transport, Brazos River

  
  
• 2:00 PM Author(s):
  *Frank Hopf - Texas A&M University
  Bernard O Bauer - University of British Columbia Okanagan
  Mark Lorang - University of Montana
  Douglas J. Sherman - Texas A&M University
  

  Abstract Title: The history, present and projected future of the mid-channel islands in the Sacramento River Delta.

  
  

  The Sacramento River Delta of California has undergone significant development and changes during the Holocene. The rate of change increased greatly with the human population explosion that started with the nearby discovery of gold in 1849. Large sections of the delta were converted to agricultural use by the augmenting the height of the natural levees. As a result, the periodic inundations by flood waters and flood sediments were restrained. The islands behind the augmented levees subsided to elevations below normal low water levees forcing landowners to slowly convert the levees into dams. Also during the last century, the channels have been modified and new ones created to facilitate moving about 20% of the total fresh water south to the southern California water supply system rather than west to San Francisco Bay and the ocean. This channel and levee building effort left behind, or in some case may have created, numerous small mid-channel islands. These small islands remain responsive to the modified fluvial and tidal forces processes while the bulk of the delta has been largely isolated or protected from those processes. They also represent some of the last riverine aquatic habitat in the delta and are therefore extremely valuable. This study looks at the historic record largely through maps and remote imagery and studies the active fluvial processes by analyzing up to eight years of quarterly erosion measurements to illuminate the past, understand the present, and project the potential future of these critical remains of the natural delta.

  Keywords: fluvial, levees, river deltas

  
  

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.