Evaluating ecosystem impact requires characterizing appropriate baseline conditions and teasing apart the impact of multiple stressors from environmental variability. Accurately defining baseline is hindered by the spatio-temporal disconnect between past, present and future environments including changes in climate, catchment parameters and multiple human impacts. Single large impacts are relatively clear; however, small, incremental, cumulative impacts are difficult to quantify due to their relative size to the natural variability and can be devastating to ecosystems. Focusing on large–scale ecosystem processes rather than desired endpoints might be a better method to handle system variability and unpredictability in the evaluation of impact and management of dynamic landscapes.

We used a mechanistic model of river channel meander migration, coupled with hydrological and ecological data, to evaluate the history of flow regulation and floodplain development on riparian forests. We used a 130-year record of channel planform and hydrology data to quantify multiple baseline conditions and model future impacts. 

Results/Conclusions

We found that flow regulation with the proposed off-stream storage diversion project reduced riparian forests by 6-9% from current conditions (US policy requirement), 22% from post-dam operations, 38% from pre-dam conditions and 79% if near-channel constraints are considered. By modelling an ecosystem process (river channel migration) we were able to isolate the effects of flow regulation on the establishment of riparian vegetation, taking into account multiple human alterations with high inter-annual system variability. In addition, although historic empirical data show no change in channel migration rates and sinuosity, our results challenge this conclusion and suggest that concurrent changes to ecosystem (forest conversion to agriculture) concealed the true impact of flow regulation.

Process-based methods can be used to assess cumulative impacts against appropriately defined baselines to improve evaluation procedures and management goals where empirical studies sometimes fall short. We found that defining baseline on current condition (as many impacts assessments do) does not protect the environment from degradation. Rather, this type of short-sighted, yet well intended, regulation disguises true protection by allowing the natural baseline by which projects are judged to slide closer to a human dominated landscape.