Section 4:  Conclusion

One of the primary objectives of this project is to develop a method for conducting habitat inventories with two dimensional hydraulic models. Modeling provides a means to quantify spatially and temporally variable aquatic habitats and a means to visualize hydraulic conditions at discharges which are difficult to measure in the field. This hydraulic framework provides input for both future species specific and community focused biological studies. For example, contour plots of channel area can be overlain with habitat suitably curves to determine discharge conditions which maximize habitat for a particular species or life-stage.

Our modeling also provides insight into the potential biological effects of future climatic shifts. Modeling suggests that race and riffle habitats at Ratcliff Ford are the most sensitive to hydrologic fluctuations.  Habitat duration calculations show that most of the net habitat loss between wet and dry climatic regimes is accomodated by a loss of race and riffle habitat while pool and glide areas are more moderately affected (Figure 3.5).  Edgewaters are the only habitat type that has a greater area during dry years.  Analysis of hydrologic records also shows that in addition to lower magnitude flows, dry years also tend to have less variability in the range of discharges experienced during the year (Figure 2.2C; Figure 2.4B).   This suggests that while habitat areas may be more limited during dry years, habitats are subject to fewer discharge variations and are therefore more stable.   Future biological studies need to investigate the relative importance of differences in hydraulic stability versus differences in habitat area.

The effects of climatically driven variations in discharge are mediated by channel and valley geometric characteristics that determine the distributions of depth and velocity. The sensitivity of habitats to climatic scenarios at Ratcliff Ford is determined in part by the particular geomorphic conditions in this reach. Observations along the Jacks Fork show that channel morphology varies longitudinally in response to changes in drainage basin area but also in response to valley characteristics. The Ratcliff Ford reach lies within a canyon-like segment of the Jacks Fork, where confinement of flow leads to higher stream power and many bluff-side pools (McKenney, 1997). Deep and narrow pools formed from these confined flows provide habitats that are likely to be relatively insensitive to periods of drought and may therefore provide critical low-flow refugia. Conversely, confined conditions in the Ratcliff Ford reach promote extensive areas of high-velocity race habitats during high discharges, which may limit high-flow refugia. Similar modeling efforts in other reaches of Ozarks stream may provide insight into habitat sensitivity in other valley segment types and into longitudinal controls on habitat responses to climatic variation.