HYDROGEOMORPHOLOGY AND RIVERINE HABITAT DYNAMICS

The template of physical riverine habitat is dominated by interaction of flow regime and channel-floodplain morphology. At high flow exceedance or high critical shear stress, understanding of habitat dynamics is essentially the hydrodynamic problem of distributing a time series of flow within the channel and floodplain. Ecological effects of different flow regimes under hydrodynamic assumptions can be evaluated with the concept of effective discharge to evaluate which flows on average maintain the most of a habitat type (Doyle and others, 2005). However, effective discharge methods may identify a discharge without regard for when it occurs in an organism’s life cycle; therefore, a complementary approach calculates daily habitat duration to evaluate variation in habitats over the year. The quality of either assessment depends on biological validity of habitat definitions and on robust discharge-habitat relations. The former requires rigorous biological understanding of what habitat means to an organism or a community of organisms. The latter requires hydraulic models at ecologically appropriate scales. Discharge-habitat curves for the Lower Missouri River illustrate typical relations from multidimensional hydraulic models: hydraulic habitats (combinations of depth and velocity, floodplain connections), derivative hydraulic variables (gradients between units), patch statistics (total edge, diversity of habitat types), and reach-scale integrative metrics (water residence time). Combined with modeled alternative flow regimes, the discharge-habitat curves are used to assess sensitivity of habitats to flow-management decisions using effective-discharge and daily habitat-duration approaches. At low flow exceedance or low critical shear stress, habitat issues can be considered morphodynamic: what conditions determine significant changes in substrate conditions and channel morphology? Analogous to the concept of geomorphic effectiveness used by Wolman and Gerson (1978), those events responsible for the most geomorphic work over the long term are not necessarily those that form or rejuvenate physical habitat. Sandbar building events on the Lower Missouri River, for example, have substantially higher discharge and longer durations than discharges that transport the most sand.