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

Human Impacts on Watershed Processes 1 - Mountain Watersheds

Paper Session 4101
Friday, 4/20/07, from 8:00 AM - 9:40 AM

Sponsorship(s):

Geomorphology Specialty Group
Mountain Geography Specialty Group
Water Resources Specialty Group

Organizer(s):

John Faustini - Oregon State University
Julia Jones - Oregon State University

Chair(s):

Julia Jones - Oregon State University

Abstract(s):

• 8:00 AM Introduction:
  John Faustini - Oregon State University
  
• 8:10 AM Author(s):
  *Anne Chin - Texas A&M University and National Science Foundation
  Peng Gao - Syracuse University
  

  Abstract Title: Modeling step-pool sequences in mountain watersheds.

  
  

  Mountain watersheds are increasingly under pressure from human land uses that include logging, mining, grazing, and urban development. Mountain channels are characterized by a preponderance of steps and pools, producing a striking, repetitive staircase-like longitudinal profile. Although the rhythmic character of the step-pool streambed has long attracted the attention of scientists, few studies have successfully quantified the nature and significance of this spatial rhythmicity. This paper develops a quantitative model to describe and predict the rhythmic step-pool morphology in a range of environments. Time-series analytic techniques are applied to a large database of step-pool streams to test the hypothesis that predictable, periodic step-pool streambeds develop in diverse environmental settings, and that a general model is capable of describing their characteristics. The model reveals the extent to which step-pools are expressions of meandering in the vertical dimension, similar to pools and riffles in downstream channels, and suggesting a fundamental adjustment to maintaining equilibrium channel geometries under high energy conditions. Such a model is potentially useful in the design, management, and restoration of steep channels. Because mountain streams are important habitats for many sensitive aquatic species, results of this study can also have implications for the ecological management of impacted mountain watersheds.

  Keywords: fluvial geomorphology, mountain streams, river restoration

  
  
• 8:30 AM Author(s):
  *Francis Kevin Rengers - Engineering and Hydrosystems, Inc.
  Jennifer M Patterson - Engineering and Hydrosystems, Inc.
  

  Abstract Title: A Method for Quantifying Geomorphic Change in Fluvial Systems.

  
  

  Hydrologic changes wrought by infrastructure development in a watershed, alter the sedimentation and erosional regimes of fluvial systems in the watershed from their pre-development functionality. Rohrer and Rosner (2005) have developed a method for quantifying flow frequencies and shear stress in order to show changes in erosive capacity at multiple recurrence intervals when hydrology has been altered. We have expanded on this method and suggest that it can be used effectively for quantifying fluvial geomorphic change based on recurrence interval. Beginning with hydrology, existing flows at many recurrence intervals can be plotted along with predicted flow alternatives. The difference between the area under each curve is calculated to determine the overall percent change between the existing flows and predicted flows. Subsequent parameters such as sediment transport capacity and stream power can be plotted versus recurrence interval for existing and proposed conditions, and percent change can be determined similarly to flow. Percent change results for many alternatives can then be displayed graphically so that parameters indicating either erosion or sedimentation can be evaluated. With this technique a single graph can be constructed with multiple erosion/sedimentation parameters showing changes between alternative hydrologic regimes over many recurrence intervals. Roesner, L.A. and C.A. Rohrer. 2005. Runoff Control Design for Hydrologic and Geomorphic Sustainability in Urbanizing Streams. Colorado Association of Floodplain and Stormwater Managers Conference. Steamboat Springs, Colorado. Rohrer, C.A. and L.A. Roesner. 2005. Matching the Critical Portion of the Flow Duration Curve to Minimize Changes in Modeled Excess Shear. 10th International Conference in Urban Drainage, Copenhagen/Denmark.

  Keywords: Fluvial Geomorphology, Hydrology, Land-use

  
  
• 8:50 AM Author(s):
  *John M. Faustini - Oregon State University
  Philip R. Kaufmann - U.S. Environmental Protection Agency
  

  Abstract Title: Land Use Impacts on Stream Bed Substrate Influenced by Geology in the John Day Basin, Oregon.

  
  

  Human land uses and land cover modifications (e.g., logging, agriculture, roads) can alter runoff and increase sediment supply to streams, potentially degrading aquatic habitat for benthic organisms and fish. This study used synoptic stream habitat survey data from a regional assessment (N=58) to quantify stream channel substrate characteristics and relate these to human disturbance within the riparian zone and upstream catchment at local to whole-catchment scales inferred from field observations and remote sensing data. We assessed potential excess fine sediments using several indices of bed stability and excess fine sediments based on the difference between observed particle size or fine sediment abundance and empirically predicted values for least-disturbed sites. Bedrock lithology was an important natural control on the relationship between streambed substrate and human disturbance. Bed stability was significantly egatively associated with a field-based index of local human riparian disturbance in catchments underlain by resistant rock types, but only weakly negatively associated with riparian disturbance in catchments containing a significant proportion of erodible rock types. In contrast, bed stability in resistant basins was significantly positively associated with catchment-scale road density, but strongly negatively associated with road density in basins underlain by erodible bedrock. Our results suggest that roads are an important form of (or surrogate for) human disturbance in this landscape, and that geology may play an important role in determining the response of stream channel substrate to human landscape and riparian disturbance. Disclaimer: This is an abstract and does not necessarily reflect EPA policy.

  Keywords: fine sediments, bed stability, stream habitat, anthropogenic disturbance, roads

  
  
• 9:10 AM Author(s):
  *Timothy D. Perry - Oregon State University
  *Aaron Stone Arthur - Oregon State University
  Julia A. Jones - Oregon State University
  

  Abstract Title: Long-term responses to forest thinning and clearcut treatments from the Coyote Creek paired watershed study, revived after a 25-year nap, Southwestern Oregon.

  
  

  The long-term consequences of forest thinning treatments on water yield, especially in seasonally drought-stressed climates, are essential for planning contemporary forest management in southern Oregon. After a 25-year snooze when streamflow and vegetation measurements lapsed, new records from the Coyote Creek watersheds in the South Umpqua Experimental Forest are providing insights into long-term vegetation and streamflow responses to forest treatments conducted in 1970. Historical vegetation and streamflow records were collected from 1963 to 1981 on four small watersheds, where clearcut and overstory thinning treatments occurred in 1970. In 2000, the U.S. Forest Service in cooperation with Oregon State University resumed streamflow monitoring, changes in forest composition and structure were related to landform effects (soil, slope, aspect, topographic position) and disturbance legacies (clearcut, overstory thin, and small patch clearcut harvest treatments, roads, proximity to harvest patch edges). Streamflow responses were related to the range of harvest treatments to determine the relationship between fractional canopy removal, subsequent vegetation regrowth, and water yield. Results from the clearcut basin showed an emerging water deficit in spring, summer and fall, under young plantation forests, but less pronounced long-term responses to thinning and partial harvest treatments. Interactions among forest harvest treatments, vegetation regeneration and water yield were more complex than could be described using simple relationships of leaf area removed vs. water yield changes. These results show awakening moribund experimental watersheds can yield important ecological and hydrologic findings relevant to contemporary forest management issues.

  Keywords: long-term ecological research, paired watershed analysis, streamflow response to logging, water storage, water use, Coyote Creek

  
  

Session Description: Human impacts are pervasive across the modern landscape. Land use, resource extraction, and other human activities alter vegetation cover and species distributions; alter surface topography and disrupt soil layers; move large quantities of rock, soil and other materials; re-route surface and subsurface water flows; and directly or indirectly introduce large quantities of chemicals across the landscape, among other impacts. These many impacts affect key watershed processes even in relatively remote areas, altering the routing and delivery of water, sediment, organic matter, and dissolved chemicals to rivers and streams and in turn altering channel and floodplain morphology, aquatic habitat quality, and riparian ecosystem structure and function. Because humans depend upon watersheds for water supply, recreation, and many ecosystem services, understanding and management of human impacts on watershed processes is profoundly important to human societies. This series of sessions explores human impacts to hydrogeomorphic, biogeochemical, and ecological systems and processes in watersheds in a range of environments. Primary focus areas include (1) mountain watersheds; (2) geochemistry, water quality, and nutrients; (3) watershed management, particularly with respect to streamflow and fluvial processes in urbanizing landscapes; and (4) ecological impacts and processes.