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

Sediment Transport in Fluvial Systems I

Paper Session 2419
Tuesday, 3/8/06, from 1:00 PM - 2:40 PM

Sponsorship(s):

Geomorphology Specialty Group

Organizer(s):

Michael Slattery - Texas Christian University

Chair(s):

Michael Slattery - Texas Christian University

Abstract(s):

• 1:00 PM Author(s):
  *Peng Gao - Syracuse University
  Gregory B. Pasternack - University of California, Davis
  Bali M Khaled - University of California Desert Research & Extension Center, Holtville, Ca
  Wesley W Wallender - University of California, Davis
  

  Abstract Title: Suspended sediment transport in an intensively cultivated watershed in southeastern California

  
  

  The purpose of this study was to investigate the variation of sediment movement at multiple spatial and two temporal scales of an agricultural watershed in southeastern California where surface runoff is primarily supplied by irrigation. An extensive turbidity and stage monitoring program based on grab sampling using 500-mL plastic bottles and in situ monitoring using both turbidity sensors and pressure transducers was designed to capture the spatial and temporal heterogeneities of suspended sediment loads. The collected data of weekly and 5- minute turbidity and stage were subsequently converted to suspended sediment concentrations and discharges, respectively, using established calibration methods. The flux estimation assumption adopted in the mass balance approach was verified by cross section measurements and analysis of spatial distribution of suspended sediment loads. Furthermore, an empirical model describing the process of sediment transport at various spatial scales was developed. The model revealed that drain channels at larger spatial scales (i.e. downstream channels) tended to be dominated by weekly net deposition, while those at smaller spatial scales (i.e. upstream channels) were controlled by weekly net erosion. Analysis of sediment transport at the 5-minute scale showed that in small-scale drain channels, most of the weekly net deposition occurred intensively over about one day. For the remainder of the week, low intensity erosion prevailed. Generally, sediment transport in the study watershed was controlled by deposition, which supports the necessity of on-going drain-channel dredging by Imperial Irrigation District.

  Keywords: Suspended sediment transport, agricultural watershed, deposition and erosion

  
  
• 1:20 PM Author(s):
  *David W. May - University of Northern Iowa
  Matthew G Hill - Iowa State University
  David J. Rapson - Iowa State University
  

  Abstract Title: Early Holocene Alluvial Sedimentation in an Ephemeral Stream Channel in Western Nebraska.

  
  

  Alluvial stratigraphic investigations in Ash Hollow Draw in western Nebraska during the summers of 2004 and 2005 have revealed much about Early Holocene sedimentation along this ephemeral channel and the stratigraphic context of two Paleoindian sites. Cutbanks expose up to five Late Wisconsin and Early Holocene sedimentary units. Three units appear restricted to tributary confluences (alluvial fans), but two units are ubiquitous throughout the middle and lower reaches of the drainage basin. The older of these two Early-Holocene units in channel-axis and alluvial-fan settings is a massive, brown, silt loam containing ten percent, locally-derived, subangular, carbonate gravel. In slackwater settings along the margin of the valley this older, massive unit is dark brown to dark grayish brown silt loam. A weakly-developed soil is developed in it. The younger of the widespread Early-Holocene units is dark grayish brown, horizontallylaminated silt loam. This is the unit in which Paleoindian cultural material is found at the Clary Ranch and O.V. Clary Paleoindian sites in the basin. A chronology of Early Holocene sedimentation has been developed from 24 AMS radiocarbon ages on mostly wood charcoal. Twenty-two fractionation-corrected ages fall between 10,230 and 8,730 yr B.P. The ubiquitous, channel-axis brown silt with carbonate gravels dates between 10,230 and 9,510 yr B.P. Ages for the finer-grained, valley-margin portion of this unit range from 9970 to 9870 yr B.P. Thirteen ages for the widespread, horizontally-laminated silt loam unit from four localities in the basin range from 9,270 to 8,780 yr B.P.

  Keywords: Keywords: Holocene, sedimentation, stream, Paleoindian, Nebraska

  
  
• 1:40 PM Author(s):
  *Robert T. Pavlowski - Missouri State University
  

  Abstract Title: Geomorphic Assessment and Restoration Plans for the Ward Branch Restoration Project, Springfield, Missouri

  
  

  Urbanization often leads to flooding and instability in adjacent streams. While there are oft cited examples of projects in other regions, there are relatively few examples of successful urban stream restoration projects in the Ozarks. This presentation provides a progress report on the Ward Branch Restoration Project in Springfield, Missouri done in partnership with local governments and the university. The role of geomorphologic assessments is emphasized. Begun in Fall 2004, both the baseline geomorphic assessment and preliminary restoration plan have been completed. Historical aerial photographs, channel cross-section and longitudinal bed surveys, bed sediment mapping and size counts, bank stability indicators, and HEC-RAS hydraulic modeling were used to develop a scientific understanding of the causes of geomorphic instability over the 1 km long study reach. Increased urban stormwater discharge from the upper watershed is causing bank erosion and some bed incision along a 200 m long reach of Ward Branch. This channel widening process is releasing excessive amounts of gravel-sized sediment from floodplain storage. This remobilized bed material organizes to form bars and aggrade downstream channels. In turn, these downstream reaches are being destabilized by bank erosion and extreme meandering in response changes in flow velocity and direction forced by excess gravel deposition. Preliminary restoration plans are targeted to control flood peaks from the upstream urbanized watershed, utilize vegetation to increase bank roughness and decrease erosion, expand the floodwater storage capacity of the floodplain, and reduce bank erosion by the use of deflectors and weirs calibrated to local hydraulic conditions.

  Keywords:Ozarks, fluvial geomorphology, restoration

  
  
• 2:00 PM Author(s):
  *Jordan A. Clayton PhD - University of Colorado
  

  Abstract Title: Spatial and temporal variability in bed load transport in a gravel-bed river bend.

  
  

  Models of equilibrium transport for gravel-bed streams tend to be formulated for uniform flow fields, meaning that the depth, median grain size, and slope are similar in all areas of the channel. However, models of meandering streams are complicated because the geometric properties of river channels vary in space, whereas the flow varies in time, and because natural streams tend to have a heterogeneous morphology. The spatial and temporal variability in bed load transport was investigated for bends of the Colorado River in north-central Colorado. Approximately 190 coupled measurements of flow and sediment transport were taken at a range of locations within one of the bends during an extended period of above-average runoff in 2003. These data show that the finer particles of the bed load tended to be swept inward while coarser grains were routed outwards, leading to a near-balance in the overall mass transport between the inner and outer regions of the bend. This differential routing of the fine versus coarse fractions of the load decreased with increasing discharge beyond bankfull, indicated by a shift in the locus of coarse particle transport from the outside of the bend inward to the channel center. At the same time, the size of various grain size percentiles of the bed load increased with increasing shear stress. Together, these results suggest that in curved gravel-bed rivers the bed load transport regime varies laterally across the channel to promote the equilibrium transport of the full range of particle sizes present in the load.

  Keywords: sediment transport, gravel rivers, stream bend, variability,Colorado

  
  
• 2:20 PM Author(s):
  *Frank J. Magilligan - Dartmouth College
  Nira L. Salant - UBC, Department of Geography
  Carl E. Renshaw - Dartmouth College, Department of Earth Sciences
  Keith H. Nislow - USDA Forest Service
  Arjun Heimsath - Dartmouth College, Department of Earth Sciences
  

  Abstract Title: The use of short-lived fallout radionuclides to quantify transitional bed load transport in a regulated river

  
  

  Fine bed sediment can have a significant impact on aquatic ecosystems, thus accurate methodologies for quantifying its transport are essential. Here we investigate the use of the short-lived fallout radionuclide 7Be (t1/2 = 53.4 days) as a tracer of fine (~0.25 - 2 mm) mixed or “transitional” bed load transport. We measure 7Be activities in approximately monthly samples from point bar and stream bed sediments in an unregulated and regulated stream. In the regulated stream our sampling spanned an array of flow and management conditions during the annual transition from flood control in the winter and early spring to run-ofthe- river operation from late spring through fall. Sediment stored behind the dam during the winter quickly became depleted in 7Be activity. This resulted in a pulse of “dead” sediment released when the dam gates were opened in the spring which could be tracked as it moved downstream. Measured average sediment transport velocities (30 ? 80 m/d) exceed those typically reported for bulk bed load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in bed sand fraction. Results also show that the length scale of the downstream impact of dam management on sediment transport is short (~1km); beyond this distance the sediment trapped by the dam is replaced by new sediment from point bars, tributaries and other downstream sources.

  Keywords: dams, radionuclides, sediment, embeddedness

  
  

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.