The importance of floodplain width on hydraulic variability and aquatic-riparian habitat in semi-confined, regulated river systems

Hydraulic variability (e.g., reach-scale heterogeneity of depth, velocity, and shear stress) can generate diverse riverine habitat conditions that support species biodiversity and ecological resilience. To evaluate hydraulic variability over a range of discharges from seasonal low flow to overbank flood, we used a two-dimensional hydrodynamic model to simulate depth (D), velocity (V), shear stress (τ), and the hydromorphological index of diversity (HMID), a dimensionless index used to characterize hydraulic heterogeneity. Simulated hydraulics were also used to evaluate aquatic (bull trout rearing) and riparian (cottonwood and willow seedling recruitment) habitat quality. We compared each of these metrics against the dimensionless relative floodplain width (W*) of individual (1 km long) reaches over 22 km of a semi-confined gravel-bed river system.

Hydraulic variability, aquatic habitat, and riparian forest recruitment depended on discharge and W*. Hydraulic variability generally decreased with increasing discharge but increased with greater W*. Confined reaches (W* < 2.5) maintained low hydraulic variability regardless of discharge, whereas less confined reaches (W* > 2.5) exhibited greater variability during higher flows. Aquatic habitat quality also generally decreased as discharge increased because velocities became too fast in the channel for bull trout rearing habitat. The influence of W* was most important for aquatic habitat during higher discharges when greater off-channel habitat in the floodplain became hydraulically connected and increased habitat quality and area. Hydraulic variability during sub-bankfull discharges was not important for aquatic habitat, but became critical during overbank flow when hydraulic refuge was necessary during flood conditions. In contrast, overbank floods provided critical riparian recruitment processes that increased with greater W*. This ecohydraulic modeling approach can help watershed managers prioritize riverine restoration for increased hydraulic variability to improve/restore aquatic and riparian seedling recruitment habitat over large spatial scales.