RESUMO
Reshaping the scale of planning for hydropower development, from reaches to basin-scales, has been recommended as a more effective way to ameliorate the environmental impacts of hydropower. One approach is identifying mutually exclusive areas where development is precluded for conservation purposes and areas of low conservation value that present fewer barriers to development. This strategy, however, is less adoptable in developed countries where hydropower is already widespread and large-scale construction of new dams is unlikely. To broaden the adoption of basin-scale planning, alternative approaches and planning tools are needed for identifying mutually beneficial opportunities for simultaneous increases in hydropower capacity while improving environmental conditions. In this study, we present the Basin Scale Opportunity Assessment as a methodology to improve environmental conditions through either direct (on-site) or indirect (off-site) mitigation. We assess whether direct or indirect mitigation activities lead to optimal results in terms of added hydropower, environmental improvement, and monetary cost at a basin scale. We present two case studies for the Connecticut River and Roanoke River Basins, USA. Significant opportunities for expanding hydropower generating capacity are numerous in both basins. Results suggest that total hydropower capacity could be increased 4 to 7 % in the Roanoke and Connecticut Basins, respectively, without new dam construction and with net improvements in environmental conditions. We found that environmentally and economically optimal win-win strategies for increasing hydropower capacity and improving environmental conditions included improving environmental conditions in rivers downstream of existing dams. Off-site mitigation opportunities, such as dam removal and wetland mitigation, were identified as optimum solutions for achieving net environmental improvements only when they were associated with new hydropower construction. Our results demonstrate that opportunities to increase hydropower capacity and improve environmental conditions are expanded by viewing cumulative benefits at basin scales; however, increasing regulatory flexibility may be required to realize these opportunities.
RESUMO
Rivers and streams represent <0.6% of the Earth's land surface but play a disproportionately large role in global biogeochemical cycles and provide locally relevant ecosystem services. However, knowledge of how rivers influence material budgets and ecosystem services has major gaps due to the lack of explicit consideration of tidally-influenced reaches. Focusing on the conterminous US, we provide a foundation for understanding the role of tidal streams. We find that 66% of tidal stream length is contributed from low order streams (< 4th order), and that terrestrial ecosystem production in low-lying coastal zones is 30% greater than in adjacent terrestrial ecosystems. This prevalence of small streams indicates that small coastal watersheds dominate tidally influenced spatial domains. Furthermore, we find that relative sea-level rise (RSLR) will have a disproportionate impact on low order tidal streams and their terrestrial interfaces - 1 m RSLR will decrease the tidal stream land-water interface by 17% and the total surface area of US tidal streams by 31%. Upstream reaches of tidal zones will be extended in response to RSLR, but gains will be more than offset by coastal losses because topographic gradients become steeper moving inland, and accretion rates may not keep pace with RSLR. These results highlight previously unrecognized dominance, high productivity, and disproportionate future loss of low-order coastal ecosystems. This indicates a critical need to focus research on small tidal stream systems under contemporary and future conditions.