Linking Stream Geomorphology, Watershed Condition and Aquatic Ecosystem Health

Abstract

Twenty-five streams in northwestern Vermont spanning a range of geomorphic conditions were surveyed to determine the linkages between stream geomorphology, watershed condition and aquatic ecosystem health as part of an overall watershed classification project. Both field scale and watershed scale assessments were completed. At the field level, rapid geomorphic assessments (RGA) and rapid habitat assessments (RHA) were completed for all sites following Vermont Department of Environmental Conservation protocols. Detailed geomorphic and habitat assessments were also completed to provide quantitative data for each site. Watershed-scale parameters were evaluated using a geographic information system (GIS) and the Soil and Water Assessment Tool (SWAT) watershed modeling software. At the field-scale, data were assessed for relationships between the RGA and water quality, habitat quality and biodiversity. At the watershed-scale, we saw a broad trend relating RGA with average annual runoff, but the result was not significant. No trend was seen for annual average sediment loading. These results provide evidence that rapid geomorphic and habitat assessments done by trained individuals can provide useful information in managing and prioritizing watersheds and stream reaches for protection and restoration. Copyright ASCE 2005 Watershed 2005 Introduction Classification systems are used to establish order and provide a method for grouping objects based on similar characteristics (Kondolf 1995). McDonnell and Woods (2004) identify classification as the “first step in the process of scientific analysis and synthesis.” As the desire and commitment to manage, restore and protect watershed systems has increased, so has the need for watershed classification systems that can provide both a means of organizing data and a method to communicate. Lotspeich (1980) recommended the approach of viewing watersheds as ecosystems with streams the result of the ecosystems processes. Adopting this view requires an assessment of both the existing physical characteristics of the system and the processes occurring that affect how a particular system responds to change and stressors. Choosing which characteristics or processes to study is the first step and often determines how useful and in what contexts a classification system can be applied. Hierarchical classification systems have been used to simplify this task by establishing multiple layers of classification with each subsequent layer requiring more detail. For example, Frissell et al. (1986), described a hierarchical system identifying key variables at each level that determine the major characteristics of stream systems from the watershed level to microhabitats within the stream. A variety of classification systems focusing specifically at the stream level have also been developed (see reviews in: Kondolf 1995; Mosley 1987). Rosgen’s (1996) widely-used system uses physical stream measurements including: entrenchment ratio, width:depth ratio, dominant channel materials, slope, bed features, sinuosity, and meander width ratio to classify streams. Montgomery and Buffington (1997) adopted a process oriented approach focusing on differences in stream slope and roughness due to transport capacity and sediment supply for mountain drainage basins. In this study, we first evaluate characteristics at the stream reach level to gain an understanding of the relationships between reach-scale geomorphic characteristics and aquatic ecosystem health. Secondly, the geomorphic characteristics are related to watershed-scale characteristics. Finally, the foundation for a watershed classification system is developed that reflects these relationships and identifies the vulnerability of a watershed to degradation based on watershed-scale impacts such as land use change. Specifically, the research objectives of this study were to: 1) Determine the relationship between reach-scale geomorphic characteristics (and geomorphic condition) and both habitat quality and biotic diversity and health. 2) Determine watershed-scale characteristics that relate to the geomorphic characteristics affecting habitat and biotic health. 3) Use these measures combined with riparian and landuse change data to develop a hierarchical watershed and stream classification system. Copyright ASCE 2005 Watershed 2005 Methods Field-scale and watershed-scale data were collected for 25 stream reaches in northwestern Vermont during the summers of 2003 and 2004 (Figure 1). Reaches were chosen to span a range of geomorphic conditions (VTDEC 2002). Average reach length ranged from 200-450 m, with all reaches 10-20 bankfull widths in length. At each reach rapid and detailed geomorphic and habitat assessments were conducted. Biota (macroinvertebrates and fish) were also sampled at each reach. At the watershed-scale, the Soil and Water Assessment Tool (SWAT) (Neitsch et al. 2001; Srinivasan and Arnold 1994: SWAT http://www.brc.tamus.edu/swat/) was used for modeling of each stream reach and generating watershed characteristics.