Two-Dimensional Flow Modeling as a Tool for Meso-Habitat Delineation

The Instream Flow Incremental Methodology (IFIM) and the Physical Habitat Simulation System (PHABSIM) are commonly used in the instream flow water allocation process. This methodology is based on the premise that physical habitat plays a major role in structuring fish populations and that physical habitat can be described through modeling of rivers. PHABSIM is a series of programs designed to calculate depth and velocity distribution in the stream for unmeasured flows. PHABSIM uses a one dimensional flow model, such as HEC2 , to predict the water surface elevations and velocity along the survey reach. Problems with this approach include poor spatial resolution, a heavy dependence on habitat suitability indices, and equal weighting of preference criteria. These problems are especially problematic on warmwater rivers due to high species richness and poor knowledge of habitat requirements for most species. The use of two-dimensional models allows for spatially explicit representation of the river and analysis of the local fish population based on meso-habitat preference. The assumption that physical habitat plays a role in structuring fish populations can be tested and insight can be gained into which physical factors are most important in structuring those populations. RMA2 is a depth-averaged 2D finite element hydrodynamic model that supports wetting and drying. Reaches on the Colorado and Yampa Rivers were modeled using RMA2 and fish population data for subreaches were compared against meso-habitat delineations based on model output. Preliminary analysis suggests that fish populations are indeed influenced by physical habitat and that new insights may be gained as to the importance of different physical factors on specific species.