Application of the diminishing returns concept in the hydroecologic restoration of riverscapes.

Increasing our knowledge of unplanned anthropogenic synergies, which have affected ecosystems since prehistory, may facilitate ecological restoration. Predictive relationships between spatial pattern and ecosystem processes and functions in riverscapes have the potential to inform applied ecosystem restoration planning and design, where principles are needed for large-scale river reconnections. Although synergistic, additive, and antagonistic interactions affect ecosystems, the role of such interactions in restoration rarely has been evaluated. Using hydrodynamic modeling, we experimentally examine the aggregate effects of reestablishing hydrologic connections in a tidal freshwater tributary on the floodplain of the Columbia River, USA, which is currently undergoing dike breaching to restore juvenile salmon habitat. Sets of dike breaches yielded average wetted floodplain areas conforming to a two-parameter hyperbola ( r = 0.93). These findings demonstrate that the yield of inundated floodplain habitat area from dike breaching can conform to the well-established 'law of the diminishing increment,' developed in the study of agriculture and economics. Furthermore, the influence of spatial configuration on yield was strong, with midstream breaches yielding 63% and upstream breaches 2% of the wetted area produced by downstream breaches, although conditions of extreme high river flow were not studied. Opening the dike at 26% of the historically present channel outlets provided the maximum return on investment for the study riverscape. Verification of this relationship elsewhere in tidal areas of the Columbia River and on other large river floodplains would contribute to cost-benefit analyses in ecological restoration program planning and have implications for effects on biota. [ABSTRACT FROM AUTHOR]