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 II

Paper Session 2239
Wednesday, 4/18/07, from 10:00 AM - 11:40 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):

• 10:00 AM Author(s):
  *Mark W. Bowen - University of Kansas
  

  Abstract Title: Human Constructs, Channel Morphology, and the 100-Year to 500-Year Flood along the Colorado Front Range.

  
  

  High-magnitude floods, while rare, have potential to inflict catastrophic change; as more landscape becomes altered by humans this potential increases. Impacts of bridge construction on the natural environment were analyzed for one high-magnitude flood along two mountain streams in the southern Colorado Front Range. Penrose, CO received over six inches of rain in two hours, which, depending upon estimates, resulted in a 100-year to 500-year flood along 8-Mile and Brush Hollow Creeks. Bridges for U.S. Highway 50 constrict flow through lower reaches of both streams, which caused Highway 50 to flood. Also, high energy flow washed out several roads downstream. For both streams, crosssectional surveys and Wolman pebble counts were conducted across the floodway,and longitudinal profiles were surveyed upstream and downstream of Highway 50. 8-Mile Creek displays a unique pattern in channel morphology upstream and downstream of the bridge due to the constriction of flow backing up water upstream and increasing erosion downstream. Flow was constricted to such a degree that Highway 50, situated 6 meters above the stream bed, was overtopped.Brush Hollow Creek channel morphology was not impacted by Highway 50 even though flow overtopped the bridge. A small check-dam installed approximately 2 km downstream of the bridge initially restricted flow but was subsequently breached, controlling upstream channel morphology. This dam, when breached, caused considerable, rapid downcutting as evidenced by several knick-points alongthe stream. Thus, human constructs in these watersheds have significantly alterednatural channel response to high-magnitude, low-frequency flood events.

  Keywords: fluvial geomorphology, Colorado Front Range, Flood-100-year

  
  
• 10:20 AM Author(s):
  *Ranbir Kang - Oklahoma State University
  Richard A Marston - Kansas State University
  Daniel E Storm - Oklahoma State University
  

  Abstract Title: Geomorphic Effects of Urbanization in an Ex-Urban Watershed.

  
  

  This project evaluates the impact of imperviousness (roads, buildings, parking lots and sidewalks) on the channel morphology of Stillwater Creek, Oklahoma. Stillwater Creek is an ex-urban stream with mixed land-use (pasture, woodland and crops) lying adjacent to an expanding urban area. Urban growth in Stillwater has paralleled the expansion of Oklahoma State University. This research differentiates upstream-to-downstream changes (channel cross-sections, channel bed materials, channel unit types and riparian vegetation) from changes due to urban run-off and sediment load. Fieldwork was conducted to measure all these elements in 30 reaches of the creek. Aerial photographs taken at different times were used to measure growth of impervious areas. Although the creek receives runoff and sediment from construction and impervious areas, statistical analysis of data revealed no significant impacts of urban growth on channel morphology. The study demonstrated that three factors counter urban impacts on channel morphology: 1) a thick cover of riparian vegetation, 2) highly entrenched channel characteristics and, 3) the nature of bed materials.

  Keywords: urbanization, channel morphology, riparian vegetation

  
  
• 10:40 AM Author(s):
  *Mark D. Lange - University of Southern California
  Bernard O. Bauer - University of British Columbia Okanagan
  

  Abstract Title: Patterns of Flow at a Tidal River Divergence, Sacramento River, California.

  
  

  Flow through diverging tidal river channels have only recently received attention in the literature. A divergence, unlike a confluence, represents a 'decision point' where the flow of water and sediment discharge is distributed among two downstream channels, each with its own tidal signature and hydraulic geometry. While many studies have examined the macro geometry and dynamics of distributary channel networks, this study focuses on the changing pattern of flow within a single divergence system over a tidal cycle to enhance our understanding of the 'decision point' aspect of tidal river junctions. Threedimensional flow was mapped at the junction of the Sacramento River with Georgiana Slough near Walnut Grove, California, over a variety of hydrologic conditions using a boat-mounted acoustic Doppler profiler and Global Positioning System in conjunction with a temporary tide gage network. Measurements were taken during low flow conditions, when tidal processes were dominant, and during moderate flow conditions when fluvial processes became more significant. Secondary circulation is most strongly developed when the difference in discharge between the downstream branches is greatest, which produces a cross-stream gradient in surface elevation that drives the secondary flow. The dynamics of these secondary flows are related to out-of-phase changes in the water slopes in each downstream branch of the divergence. This phase difference is due to their connection to separate channel networks that attenuate tidal wave propagation at different rates. These results have implications for the distribution and delivery of suspended sediment between downstream branches of a tidal river divergence.

  Keywords: fluvial, coastal, delta, distributary, geomorphology, Sacramento River, California

  
  
• 11:00 AM Author(s):
  *Dennis Staley - University of Memphis
  

  Abstract Title: Linking Process and Form on Alpine Talus Cone Systems: Front Range, Colorado USA.

  
  

  This research examines the relationship between geomorphic process, surface form, and particle morphology on alpine talus cone systems in the Colorado Front Range. High resolution digital terrain data derived from terrestrial laser scanning and airborne laser swath mapping sources provided an unprecedented resolution for the analysis of talus cone system form. Point specific measurements of particle morphology and geomorphic process were made on several talus cones. Rockfall, debris flow, and slushflow are the primary processes that have formed and sculpted the talus cones. Surface forms associated with these processes were analyzed using a three-tiered spatial hierarchy consisting of analysis at the particle scale, feature scale and system scale. At all scales of analysis, talus cone form was related to debris transport mechanisms. Forms associated with rockfall reflect controls related to particle sizes of transported debris and in-situ material characteristics. Debris flow and slushflow related forms represent material shear strength and yield strength. The ability to decipher these properties using measurements of form from high-resolution digital elevation data allows researchers to decipher geomorphic processes at very fine-spatial scales across large areas from remotely sensed terrain data. The use of a conceptual hierarchy of relevant spatial scales elucidated the complex relationship between spatial scale, process, and form evident in talus systems.

  Keywords: Geomorphology, Mountains, Talus, LiDAR

  
  

Session Description: 2nd of 3 sessions for the Geomorphology S.G. student paper competition.