Special Sessions
Sponsored by the Geomorphology Specialty Group
Association of American Geographers 102nd Annual Meeting
Chicago, Illinois, March 7-11.

Fluvial Geomorphology: Works in Progress and Preliminary Results

Interactive Short Paper Session 2107
Wednesday, 3/8/06, from 8:00 AM - 9:40 AM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

Dr. Lisa Boulton - University of Alabama
Martin D. Lafrenz - University of Tennessee

Chair(s):

Dr. Lisa Boulton - University of Alabama

Introducer:

Dr. Lisa Boulton - University of Alabama

Abstract(s):

• 8:05 AM Author(s):
  *James D. Riley - University of Illinois at Urbana-Champaign
  Bruce L. Rhodes - University of Illinois at Urbana-Champaign
  

  Abstract Title: Flow dynamics and channel morphology at natural confluent-meander bends.

  
  

  The planform geometry of stream confluences is an important component of conceptual models of confluence dynamics. Planform geometry represents a controlling factor that influences flow structure, sediment transport and bed morphology at confluences. The planform of confluences typically is depicted as straight channels converging at symmetric or asymmetric junctions. However, natural channels, particularly those for meandering streams, curve and bend. This property has led to the hypothesis that natural stream and river confluences tend to occur on the concave (outer) bank of meander bends. The resulting confluence planform, referred to as a confluent-meander bend, was observed over a century ago but has received little scientific attention. This paper outlines the framework for a process-based field investigation of natural confluent-meander bends. The regularity of this confluence planform will be determined throughout watersheds in central Illinois and west-central Indiana, the likely location of study sites for the field investigation. Three sites at confluent-meander bends of varying size and with differing tributary entrance locations will be selected to compare confluence dynamics across different spatial scales and geometric characteristics. Variation in momentum ratio on patterns of time-averaged three-dimensional flow through confluent-meander bends and resultant changes in bed and channel morphology will be measured during a range of flow conditions. Historical aerial photographs will also be examined to determine if the confluent-meander bends of the study sites represent a quasi-stable channel configuration in meandering drainage networks.

  Keywords: fluvial geomorphology, confluences, meander bends

  
  
• 8:10 AM Author(s):
  *Mark D. Lange - University of Southern California
  Bernard O. Bauer - University of British Columbia
  

  Abstract Title: Suspended Sediment Transport At A Tidal River Divergence

  
  

  This research advances our understanding of the flow of suspended sediment through a bifurcating tidal river channel, a complex landform feature largely ignored in the geomorphic literature. In particular, the assumption that suspended sediment ‘goes with the flow’ is examined to test whether the ratio of flow discharge divided between downstream branches yields a similar ratio for sediment by proxy. This assumption is contrary to observations of non-uniform distributions of suspended sediment concentrations across channels driven by secondary flow patterns and by different source-sink areas within a channel. Three-dimensional flow and suspended sediment concentrations were mapped across a channel divergence in the Sacramento River near Georgiana Slough over a variety of hydrologic conditions using a boat-mounted acoustic Doppler profiler and optical backscatterance sensors in conjunction with a temporary tide gage network. The pattern of across-stream suspended sediment concentration varies over a tidal cycle due to shifts in the location of secondary flows. The dynamics of these secondary flows are related to tidal currents as well as out-of306 phase changes in the water slopes in each downstream branch of the bifurcation. This phase difference is due to their connection to separate channel networks that modulate the rate of tidal wave propagation. More problematic is the question of exchanges of sediment with the bed across a bifurcation due to the difficulty of interpreting acoustic Doppler profiler measurements near the bed.

  Keywords: suspended sediment, secondary flow, tidal river, fluvial,geomorphology, delta, California

  
  
• 8:15 AM Author(s):
  *Brendan Thomas Yuill - Arizona State University
  Mary H Nichols - USDA ARS
  Mark W Schmeeckle - Arizona State University
  
  

  Abstract Title: Using Bed Texture Maps to Understand the Morphodynamics of Ephemeral Channels

  
  

  Sediment transport in dryland ephemeral channels is often limited by transport capacity rather than by sediment supply. Field examination of sediment samples collected during a series of flow events in a low-ordered sub-watershed of the Walnut Gulch Experimental Watershed near Tombstone, Arizona shows substantial signs of hysteresis, which is generally an indicator of sediment supply limitations. The hysteresis is observed within flow events and among events temporally dispersed throughout the monsoon season, the period in which most flows occur. This study was initiated to compare the characteristics of sediment sampled during flow events with the spatial patterns of channel bed sediment resulting from the flow events. The evolution of channel bed texture was examined through comprehensive mapping using high resolution digital imaginary. It is assumed the bed material is the primary source of entrainable and transported material, with the heavily armored hillslopes only contributing a fraction of the finest material. Changes in bed texture after each flow event are compared to sediment measured during each flow event to test if the bed material dynamics offer an adequate explanation for the observed hysteresis. Analysis of the mapped channels between events show local areas of coarsening textures and scour indicating a winnowing of fines and discontinuities within the scour-and-fill process, matching some of the observed trends in the hysteresis. Future research will seek to understand the spatial and temporal variations in the flow hydraulics responsible for sediment redistribution along the channel bed during monsoon enerated flood flows.

  Keywords: sediment transport, channel morphology, drylands

  
  
• 8:20 AM Author(s):
  *M. A. Lisa Boulton - University of Alabama
  

  Abstract Title: Sediment Storage and Channel Widening in a Channelized Tributary

  
  

  Changes in sediment dynamics often occur as a result of channelization and are generally associated with vertical incision processes and failure of oversteepened banks. However, the role of reach-scale sediment dynamics in determining the location of other geomorphic processes, such as channel widening, remains unclear. This research examines the connections between reach-scale sediment processes and channel widening processes in a channelized tributary stream. Bank failure and storage of channel sediment in widening reaches was monitored for change over an extended time (months). Results suggest that bank failure is progressive and is caused by bank undercutting. Sediment is stored in berms and bars for prolonged periods, suggesting bar-bend processes may be occurring and involved in bank failure through flow deflection. Results of this research emphasize the importance of localized sediment sources in disturbed tributary streams, which limits the use of numerical models based on watershed-scale sediment sources. The results also suggest that sediment storage in channelized tributary streams may be involved in determining the location and duration of channel widening processes. This observation highlights the significance of long-term sediment storage outside of the channel, through recoupling with floodplains, as a limiting factor in channel widening processes. Finally, the interactions observed in this study between sediment storage and hannel widening processes shows that lateral migration processes are involved in post-channelization adjustment in addition to vertical adjustment processes.

  Keywords: sediment, storage, channelized

  
  
• 8:25 AM Author(s):
  *Jessica Block - Arizona State University
  J Ramon Arrowsmith - Arizona State University
  

  Abstract Title: Vertical and Lateral Stability of Bedrock and Alluvial channel systems: Central Arizona’s Salt River Paleo Geomorphology Constrained by Late Quaternary Geologic History

  
  

  As Basin and Range extensional tectonism waned in the late Pliocene,continental drainage shifted as the southern Colorado Plateau uplifted and the Gulf of California opened. Regional drainage integration connected the Plateau to the Gulf via the lower Colorado River and its tributaries. The Salt River drains central Arizona westward and joins the Gila River 20 km southwest of Phoenix. Abundant geologic and geomorphic evidence can be combined to constrain the timing and processes that have influenced channel migration vertically and horizontally. The Salt River has a well preserved fluvial terrace sequence from Phoenix eastward for 50 km and a buried paleochannel south of the modern channel system that establishes the history of channel migration. Subsurface information shows extensive Salt River channel gravels joining the Gila River roughly 25 kilometers(km) from where it joins the Gila today, implying a significant diversion of the main channel to its modern location occurred around South Mountain (16m higher local elevation difference). The diversion initiated flow of the modern Salt River through the Papago Narrows (10 km east of Phoenix). The channel moved 8 km laterally from its southern path, an unlikely localization given the local bedrock highs there). The mid Pleistocene terrace can be traced from the eastern mountains to the Papago Narrows, constraining a minimum age for the modern channel. The buried paleochannel may represent the downstream equivalent of the missing earlier Pleistocene terrace, indicating significant changes in Salt River channel gradient and horizontal position since he early to middle Pleistocene.

  Keywords: fluvial geomorphology, GIS, channel migration, desert geomorphology

  
  
• 8:30 AM Author(s):
  *Martin D. Lafrenz - Portland State University
  

  Abstract Title: The Influence of Different Sized Contributing Areas on Stream Channel Morphology

  
  

  It is often stated that hillslopes are more tightly coupled to stream channels in headwaters than occurs further downstream in larger rivers. This relationship is often modeled as a continuum, whereby in colluvial hollows the most important disturbance processes would be fire, wind, and landslides; debris flows in steep canyons would have the greatest impact on ephemeral and small streams, flooding would be the most important disturbance process in larger streams, and channel migration would drive channel morphology change in the largest alluvial channels. It follows that hillslope connectivity, from the divide to the stream channel, should decrease with increasing stream size, and the relative influence of riparian areas on stream channel morhophology should subsequently increase. In order to test this hypothesis, I assessed the relative importance of hillslope attributes on stream channel morphology for various sized streams and with varying sized contributing areas from the entire watershed down to the riparian conditions adjacent to the reach. Rather than finding consistent patterns with respect to stream size, I found that the intensity of hillslope disturbance and the lithology in the catchment dictated whether the entire watershed or only local reach conditions had the greatest influence on stream channel morphology.

  Keywords: fluvial geomorphology, hillslope processes, gis

  
  
• 8:35 AM Author(s):
  *Jeff Dahoda - University of Tennessee
  

  Abstract Title: Geographic Information Science Analysis of Factors Affecting Acidity in Crab Orchard Creek Watershed, Cumberland and Morgan Counties, Tennessee.

  
  

  In 2001, the Tennessee Department of Environment and Conservation published a report on the total maximum daily load for pH in Crab Orchard Creek, a watershed in the Emory River Subbasin on the Cumberland Plateau in Tennessee, with a history of surface mining and stream acidification. The report defines the level of stream acidification in terms of two components: pH of waters originating from non-point sources, and a net alkalinity load duration curve, but it does not provide a predictive relationship between stream acidification and land and mine characteristics. My ongoing research analyzes flow paths to the Tennessee Department of Environment and Conservation sampling sites relative to the spatial distribution of factors affecting stream acidity, including surface mine location and shape, land cover, soil type, geology, slope, stream network/channel, and watershed boundaries between headwater/ network systems and local/watershed process regions. Watershed models using Geographic Information Science are often based on the percentage of area occupied by each land use, thus disregarding the spatial relationships important to understanding how that land use affects water along its flow path. My research goal is to develop a step-wise logistic regression model to determine which landscape factor best predicts stream acidity.

  Keywords: Geographic Information Science, surface mining, acidity-stream, Tennessee

  
  
• 8:40 AM Author(s):
  *Mark Gossard - Missouri State University
  Robert T. Pavlowski - Missouri State University
  

  Abstract Title:Sediment budget approach to evaluate urban-related instability in an Ozark Stream.

  
  

  Sediment load as well as discharge variations can cause channel instability in gravel bed rivers. Accelerated gravel sediment inputs due to land use-related disturbances have been shown to cause channel and bank instability in Ozarks streams. Ward Branch is a heavily urbanized watershed in Springfield, Missouri for which restoration measures are planned. This study aims to use a sediment budget approach to understand the sources and transport patterns of bed load in a particularly unstable 300 meter long “disturbance” reach along Ward Branch where bank erosion and channel incision is releasing excess gravel to the stream. Urbanization has increased the 2-year flood peak by more than three times the pre-settlement condition. Channel form and erosion/deposition will be monitored over a 15 month period. The methods used in this study are (i) repeated crosssection and longitudinal surveys of the channel, (ii) scour chains and bank erosion pins, and (iii) pebble counts and sieving of both the bank and bed materials. Rainfall records and flood peaks are also being monitored. We hope to be able to use volumetric and grain size distribution comparisons between bed deposits and gravel inputs from bank erosion and bed scour to understand how and when excess gravel enters the stream. Results from this study will be used to plan bed and bank stabilization measures and understand channel storage and mobility dynamics of Ward Branch.

  Keywords: sediment budget, urban streams

  
  
• 8:45 AM Author(s):
  *Stacey Armstrong - Missouri State University
  

  Abstract Title:Detention basin influence on Ozark stream stability.

  
  

  The Little Sac Watershed on the northern edge of Springfield, Missouri has experienced rapid urbanization over the past few years. The watershed will continue to experience more urbanized growth in the future because part of the watershed lies in a location designated the Springfield urban growth reserve. The study will assess fluvial geomorphology of second and third order streams impacted by recent subdivision development within the last five years. Detention basins utilized for flood control often interrupt natural sediment transport and cause changes in the hydraulic energy of a stream. In many cases channel degradation occurs in streams below the detention basin outfall and increased sediment loads can further destabilize downstream reaches. The study will use a paired-reach approach to identify and quantify effects of detention basins on offsite stream morphology and stability in urbanizing Ozark watersheds. Reference conditions will be compared with detention basin-affected streams of similar drainage area and slope. Reach-scale variations will be evaluated over a distance of four sequential riffles. Three reaches will be examined in affected streams to understand the spatial distribution of fluvial response in the downstream direction: proximal (500 meters away). Total station channel surveys, GPSmapping of fluvial disturbance indicators, and Wolman-type pebble counts will measure channel variables. GIS analysis will help indicate spatial patterns of disturbance such as incision, bar formation, and eroding banks. Results will help evaluate the ability of current detention basin designs and policies to maintain a stable channel.

  Keywords: fluvial gemorphology, detention basins

  
  
• 8:50 AM Author(s):
  *Karen Williams - Montana State University
  Williams W Locke - Montana State University
  

  Abstract Title:Energy expenditure in a burned watershed: Fish Creek, Glacier National Park

  
  

  This presentation examines the effect of wildfire on stream channels through the lens of energy expenditure. Wildfire can produce short-term increases in precipitation runoff, sediment production, and burned woody debris. Portions of the Fish Creek watershed, in Glacier National Park, Montana, burned in 2003. To examine the possible effects of this burn on stream energy, bankfull width, depth, and channel slope were measured over 9000 ft. of stream length. The bottom third of the reach burned at high severity and drains a tributary that burned at high severity. The upper two-thirds of the reach burned at moderate severity, and the remaining upstream watershed was unburned. Unit stream power was calculated using field data and DEM-derived data. The spatial distribution of stream power in the moderate severity burn reach is more scattered than in the high severity burn reach, but values are lower. Increased scatter is reflected in the “stairstep” nature of the longitudinal profile resulting from the trapping of bedload behind burned and unburned large woody debris jams. The longitudinal profile of the high severity burn reach reveals a convex surface. This “ungraded” profile may indicate that the channel is responding to multiple spatial and temporal scales of disturbance, including increased delivery of sediment and wood. Local disturbance conditions can be seen in stream power distributions, but the integrated effect of recent and past disturbance is seen in the longitudinal profile. Preliminary conclusions indicate that the downstream reach has not experienced the reorganizing flood necessary to re-create a concave-downward profile.

  
  
• 8:55 AM Author(s):
  *Sabrina Rust - Oklahoma Cooperative Fish and Wildlife Research Unit and Oklahoma State University
  William L. Fisher - Oklahoma Cooperative Fish and Wildlife Research Unit and Oklahoma State University
  Richard A. Marston - Kansas State University
  

  Abstract Title:The role of fluvial geomorphic features in the distribution of freshwater mussels (Bivalvia: Unionidae) in the Kiamichi River, Oklahoma.

  
  

  Unionid mussel occurrence in streams can be predicted with some degree of certainty using macrohabitat variables such as stream size, stream gradient, hydrologic variability, chemical constituents, and other catchment-level variables such as surficial geology. Mussel occurrence can usually be predicted using microhabitat variables such as water depth, current speed, bottom roughness, extent of fine sediment patches, and other point specific variables. However, very little is known about which mesohabitat variables, such as channel unit type, channel dimensions, and sediment particle size, can be used to predict mussel occurrence in streams. The Kiamichi River has a diverse mussel fauna, including three federally endangered species, which are threatened by proposed water withdrawals. This study was undertaken in an effort to better understand which fluvial geomorphic features may be important to the Unionid mussel fauna of the Kiamichi River. The objectives of this study were to 1) characterize the current landscape and geomorphic features of the Kiamichi River, and 2) determine the relationships between freshwater mussel bed occurrence and these geomorphic features of the Kiamichi River. We measured mesohabitat variables at 114 transects along the Kiamichi River and used logistic regression analysis to determine which geomorphic variables best predict mussel occurrence in stream channel units. Preliminary results show a positive relationship between mussel presence and the median particle size and a negative relationship between mussel presence and the channel width-to-depth ratio.

  
  

Session Description: This series of three special paper sessions are co-sponsored by the Coastal and Marine Geography (CoMa) and Geomorphology specialty groups. The research presented in the paper sessions cover a broad spectrum of spatial and temporal scales within aeolian geomorphology.