Summary of studies supporting cumulative effects analysis of upper Yellowstone River channel modifications

Abstract

During the last several decades, portions of the upper Yellowstone River have been modified for flood control and erosion prevention. The U.S. Army Corps of Engineers is responsible for administration of a permit program for evaluating construction activities affecting rivers, streams, and wetlands. The Corps regulates activities under the authority of Section 10 of the Rivers and Harbors Act and Section 404 of the Clean Water Act. Since assumption of jurisdiction in the mid-1970’s, the Corps has processed a total of 156 permit actions for the upper Yellowstone River. Over two-thirds of the permit actions occurred during or after two consecutive large floods during 1996 and 1997. In response to concern regarding the potential environmental and ecological consequences of channel modification, the Corps, in conjunction with State and local government agencies, initiated a series of scientific studies to better understand the effects of channel modification in the upper Yellowstone River (Figure 1). These included preparation of wetland and riparian inventory maps (Bon, 2001); hydraulic modeling and flood-plain delineation; watershed land-cover assessment (Pick and Potter, 2003); historic bottomland use analysis (Brelsford and others, 2003); analysis of channel modification effects on fish habitat (Bowen and others, 2003); comparison of juvenile salmonid use of modified and unmodified habitats (Zale and Rider, 2003); analysis of riparian vegetation and flood-plain turnover (Merigliano and Polzin, 2003); study of the relations between riparian habitat and bird communities (Hansen and others, 2003); analyses of geomorphology and historical channel changes (Dalby and Robinson, 2003); socioeconomic assessment (BBC Research and Consulting, 2002); and sediment transport investigations and modeling (Holnbeck, 2003). This report is a summary of results from the individual scientific studies as they bear on future programmatic cumulative effects analyses of channel modification of the upper Yellowstone River. We do not attempt a formal, cumulative impact assessment in the sense of evaluating alternatives or future scenarios. The first section presents major findings of the resource studies in terms of temporal comparisons, spatial comparisons, and causal relations. In this section, we present a series of conceptual models or flow diagrams of the major causal pathways of cumulative impacts from channel modification. These represent major pathways of potential impact based on knowledge from other rivers, concerns expressed about the upper Yellowstone, and results from the scientific studies. These diagrams serve to focus interpretation of study results as either supporting or not supporting the importance and magnitude of particular causal relations and to identify key linking variables appearing in multiple causal pathways. These key variables that connect channel modification actions to multiple, valued environmental attributes can serve as the foundation for both projecting and monitoring future responses of the system. A section on analytical realities outlines some of the limitations of projecting cumulative impacts from channel modification of the upper Yellowstone River on meaningful spatial and temporal scales and some of the difficulties of interpreting results from studies conducted shortly after two extreme floods and substantial increases in channel modification. A section on classification describes the two primary geomorphic classification systems of the upper Yellowstone River used in the various individual resource studies. Each of these systems has been valuable in supporting field sampling and expressing results concerning patterns of variation. Their integration or revision into a classification system to achieve some new purpose, such as a regulatory program or monitoring system, will depend on a crisp articulation of riverine management or regulatory objectives. A section on key variables identifies those that are central to the causal pathways connecting channel modification to impacts and provides a rationale for key variables as an alternative to other tools such as Proper Functioning Condition (Barrett and others, 1993), Index of Biotic Integrity (Karr, 1981), or the Synoptic Approach (Liebowitz and others, 1992). This section also explains relations among key variable to Hydrogeomorphic (HGM) assessment procedures (Hauer and Smith, 1998; Hauer and others, 2001) and outlines how these 2 variables might be monitored to track cumulative impacts. Examples of how selected key variables can be quantified using Geographic Information System data sets developed from the resource studies are presented for one reach. The largest portion of the document is an Appendix that summarizes each of the individual scientific studies in terms of scope and methods, findings, principal variables, and metrics used in the study or suggested by the study results, and important needs for further study.