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

Geomorphology Specialty Group Graduate Student Paper Competition I

Paper Session 2139
Wednesday, 4/18/07, from 8:00 AM - 9:40 AM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

Michael Craghan - Middle Atlantic Center for Geog. & Envt. Studies

Chair(s):

Michael Craghan - Middle Atlantic Center for Geog. & Envt. Studies

Abstract(s):

• 8:00 AM Author(s):
  *Inci Guneralp - University of Illinois at Urbana-Champaign
  Bruce L. Rhoads - University of Illinois at Urbana-Champaign
  

  Abstract Title: Uncovering the Cumulative Effect of Upstream Curvature on Planform Migration of Meandering Rivers.

  
  

  Current theoretical mechanistic models aimed at predicting planform migration of meandering rivers relate the meander-migration rates to local and upstream planform curvature where the influence of upstream curvature on migration rate decays exponentially with distance upstream. However, this migration-curvature relationship is based on a theoretical assumption about how curvature should affect near-bank flow velocity in a bend, rather than on extensive empirical studies. Furthermore, although the theoretical models based on exponential-weighting of upstream curvature effect yield fairly realistic predictions of bend asymmetry and translation, they are incapable of reproducing complex forms of bend evolution, such as compound looping -quite common features along meandering rivers. This paper investigates empirically the spatial relationship between planform migration and curvature along a variety of meandering rivers, some including compound loops. For this purpose, the weighting distribution of the upstream curvature effect on local migration rates are analyzed using two time-series models: (1) distributed lag model and (2) transfer function model. Then, the results from both methods are compared to verify the robustness of the distribution. Accurate and detailed planform geometry and curvature data necessary for such extensive analyses are obtained using parametric cubic spline interpolation, and then, arc-length parameterization of the planform data. The results indicate that the weighting distribution of the upstream curvature effect on migration rate may be more complex than currently is assumed. Thus, the study improves our understanding of the spatial relationship between upstream curvature and planform migration of meandering rivers, specifically those with compound loops.

  Keywords: meandering, curvature, planform migration, time-series analysis, aerial photography, GIS

  
  
• 8:20 AM Author(s):
  *Michael L. Hughes - University of Oregon
  

  Abstract Title: Lateral movement and planform adjustment of the Umatilla River during and between two sequential flood events.

  
  

  Floods are viewed as disturbances to riverine ecosystems; however, linking ecological responses to flood-forced channel changes has been limited by knowledge of how river process-form relations differ during and between floods. Floods in 1965 and 1975 on the Umatilla River, northeastern Oregon, coupled with pre- and post-flood aerial photos, provide an opportunity to explore differences in channel change during and bewteen sequential floods and to investigate interplay between lateral movement and planform adjustment of the river. Based on a time series of georectified aerial photographs bracketing both flood and interflood periods from 1964 to 1977, I digitized in a GIS active bars, scoured and vegetated surfaces, and wet channels within the active channel, as well as the channel and floodplain centerlines, for nine reaches along a 100-km section of river. In all but one reach, the area of scoured surfaces increased during both flood periods (1964-1971 and 1974-1977) and decreased during the interflood period (1971-1974). Areas of bars and vegetated surfaces within the active channel changed inconsistently across reaches during both flood periods, but consistently increased and decreased, respectively, during the interflood period. Lateral channel movement was greater during flood periods than during the interflood period, but displayed no obvious relationship with change in any of the planform units or channel sinuosity. These results reflect distinct differences in reach-scale planform channel adjustments during flood versus interflood periods, but fail to implicate lateral channel movement as the primary driver. Sub-reach process-form linkages between channel movement and planform composition are therefore suggested.

  Keywords: floods, channel change, GIS

  
  
• 8:40 AM Author(s):
  *Carl J. Legleiter - Department of Geography, University of California Santa Barbara
  

  Abstract Title: Quantifying the spatial variability of river morphology and hydraulics in natural and restored gravel-bed rivers.

  
  

  Current research at the interface between geomorphology and ecology emphasizes linkages among geomorphic complexity, habitat heterogeneity, and the diversity and productivity of aquatic ecosystems. River restoration seeks to reestablish these connections by rehabilitating fluvial processes that create and maintain habitat and thus sustain viable populations of critical species. Achieving this goal will require a spatially explicit approach to quantifying variability in channel morphology and hydraulics. Geostatistics provides an appropriate, stochastic framework for this characterization, summarizing variance at different scales based on the dissimilarity between pairs of observations separated by a range of lag distances. Although Euclidean distance is not an appropriate metric for meandering channels, transformation to a channel-centered coordinate system allows variations along and across the channel to be described in an intuitive frame of reference. This approach to quantifying reach-scale spatial variability is demonstrated using field data from pristine and restored gravel-bed rivers in Yellowstone National Park and California's Central Valley, respectively. For example, variogram models fit to depth and velocity measurements collected at different discharges summarize the stage-dependent spatial structure of the flow field, and comparison of variogram models from different reaches illustrates the relationship between disturbance history and reach-scale spatial variability. For the recently restored river, geostatistical tools are used to characterize morphologic response to a period of sustained high flows: point bar development, bank erosion, and modification of the constructed pool-riffle geometry. This type of spatially explicit description efficiently and quantitatively summarizes the evolution of geomorphic complexity from the simple initial conditions of restored channels.

  Keywords: fluvial geomorphology; river restoration; spatial variability; geostatistics; gravel-bed river

  
  
• 9:00 AM Author(s):
  *Dale K. Splinter - Calvin College
  

  Abstract Title: Spatial patterns in the fluvial system: comparisons between three eastern Oklahoma ecoregions.

  
  

  Ecoregions are homogeneous regions delineated by similar associations of geology, climate, soils, land use and potential natural vegetation (Omernik, 1987). Ecoregions provide a spatial framework to establish whether geographic phenomena differ between ecoregions. A study was initiated to establish whether watershed morphology and the characteristics of stream channels could be differentiated by ecoregions in eastern Oklahoma: the Ozark Highlands, Boston Mountains, and Ouachita Mountains were selected for study. One hundred and forty-nine watersheds and stream reaches were randomly selected for investigation. Four lines of inquiry were used to evaluate whether differences exist between ecoregions. These include morphometric analyses of watersheds, upstream-todownstream trends in channel morphology, dissimilarity and cluster analysis of watershed and reach variables, and channel reach classification. Statistical results reported at a = 0.05. Morphometric analyses report statistical differences between some of the watersheds and ecoregions. Regression coefficients were used to test for significant differences in the upstream-to-downstream trends in channel morphology. Significant differences exist in particle-size, bankfull width, and width-depth ratio. Cluster analysis and dissimilarity were conducted using a sum of squares method. The most dissimilar watersheds and stream reaches exist in the Ouachita Mountains. A chi-squared test reported no differences between Rosgen stream classifications by region and stream order. However, spatial patterns of reach types could be inferred in the upstream-to-downstream direction. The results of this study show that ecoregions in eastern Oklahoma provide a spatial framework that explains differences in watershed and reach morphology, clusters and dissimilarity, and channel reach patterns.

  Keywords: ecoregions, geomorphology, streams, Oklahoma

  
  

Session Description: 1st of 3 sessions for the Geomorphology S.G. student paper competition.