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Abstract
Restoration has become a major tool of river management, used to re-establish the physical, biological and chemical processes of riverine ecosystems. Despite the significant effort that has gone into restoration, there is only sparse evidence that we can restore basic functions to river systems and river restoration projects are rarely evaluated after completion (Kondolf and Micheli 1995). The focus of many river restorations is physical manipulation, such as flow modification and removing aging structures to create a more natural flow regime. While there has been considerable interest in the geomorphologic and hydrologic results of physical manipulation restorations (Graf 2001) and the impacts these actions have on fish (Roni et al. 2002, Lenhart 2003), there have been relatively few studies done to determine their effect on other ecosystem attributes or functions.
In this body of work, I evaluated the responses of streams to three different restoration projects where hydrologic manipulations were completed to promote rehabilitation. The restoration techniques discussed here are major hydrologic manipulations including dam removal in and levee breaks. Vegetation returned quickly to a pair of dewatered reservoirs, however the resulting plant communities were dominated by invasive species and planting had little impact on the resulting assemblages. Dam removal in a small, second order stream had localized impacts on channel geometry and benthic sediment composition, but had an overall modest impact on nutrient uptake. The re-flooding of a historic large-river floodplain did not have the expected enhancing effect on soil denitrification rates, although it was found that wet sites did have measurably higher rates than drier sites.
These studies show that there are unpredictable components of river restoration and it is not always clear what important aspects of a river are altered, when a manipulation is done to promote a particular outcome. Refining management of any ecological system is based on an accumulation of information from individual studies. Future projects need to build on past surprises and use them to generate new hypotheses which can be tested with new restoration techniques.
