Print PageThe science of instream flows evolved out of a growing concern in the decline of our riverine resources due to flow/habitat alterations and the recognition that quantitative tools were needed to aid resource managers when assessing environmental flow regimes. Much of the early work centered on the use of hydrologic based indices (e.g., Tennant Method, New England Aquatic Base Flow Policy), that were criticized for lacking apparent linkages to biology. This evolved through the development of assessment frameworks epitomized by the Instream Flow Incremental Methodology and its corner stone the Physical Habitat Simulation System. Early applications focused on recommending a single minimum instream flow requirement that often involved consideration of a single management species of interest. This subsequently gave way to assessments based on multiple species and life stages and recommendations that embraced the natural flow paradigm but remain primarily based on physical habitat modeling approaches. The current trends indicate increasing use of multidimensional hydrodynamic modeling to improve the spatial resolution of the physical habitat assessments that incorporate more behavioral aspects of fish ecology. Trends also show a better integration of water quality and sediment transport as overlays in the assessment flow regime recommendations. Although available for over a decade, mechanistic approaches such as application of bioenergetics, individual based models, and full life cycle models are beginning to emerge but are typically limited to a single or at most a few species.
Results/Conclusions
Whether based on physical habitat or more mechanistic approaches current assessments have not addressed the fundamental challenge facing environmental flow recommendations. Namely, that none of the existing methodologies can explicitly determine what specific flow regime or at what increment of flow alteration associated with magnitude, frequency, duration, rate of change, or timing can be equated with the state of the aquatic system at some point in the future. Even adherence to the natural flow paradigm does not explicitly or implicitly indicate how much of a change will provide what response or level of protection to the aquatic community except at the two extremes: all the natural flows versus no flow in the river. The primary conclusion is that to address our greatest challenge is not development of more quantitative tools or their basic application but rather to focus on understanding quantifiable thresholds of flow regime changes that equate to specific targets protective of our riverine resources. This may only be achieved through empirical assessments of existing altered systems.
