Inundation probability and geomorphic alteration explain current nesting distribution of a sandbar-dependent bird species in rivers of the Great Plains, U.S.A.

Emergent sandbars provide nesting, foraging, and stopover habitat for migratory waterbirds and local riverine species worldwide. Historically, rivers traversing the Great Plains of the United States and draining into the mainstem of the Mississippi River had abundant sandbar habitat, which emerged after recession of annual peak flows. Human alteration of the natural hydrogeomorphic character of these rivers reduced the abundance and quality of emergent sandbar habitats. In the northern Great Plains, substantial Federal and State resources have been used to recover sandbar habitats to bolster populations of listed bird species (interior least tern, and piping plover). Recent scientific analyses have undercut the motivation for recovery efforts, arguing that pre-development hydrology of northern plains rivers was too flashy to support metapopulations of birds because the probability of nest inundation was high. We examined this problem using Bayesian statistical analysis of streamgage records to generate estimates of pre- and post-development sandbar inundation probabilities (θ) across the Great Plains region. The results indicate that values of pre-development θ tended to be inversely scaled to stream size (order), but this scaling was disrupted after large-scale dam and navigation projects were completed. Prior to development, minimum nesting-season θ was lowest on the largest rivers in the region, the lower Missouri (θ 0.0 to 0.03) and lower Mississippi (θ 0.03 to 0.10), whereas tributaries had higher (>0.10) minimum θ values. After development, θ values increased substantially on the lower Missouri River (θ 0.15 to 0.60) because of flow re-timing by dams, but were only marginally higher on the lower Mississippi River (θ 0.05 to 0.16), where flow re-timing is attenuated. When accounting for channel alteration, post-development θ values map closely to the modern spatial distribution of nesting locations for interior least terns. The results highlight the important role of large rivers and natural hydrographs in supporting populations of sandbar-dependent species on the landscape, and the heightened importance of tributary habitat when the mainstem river is highly altered from its natural state.