Strategies for Restoring River Ecosystems: Sources of Variability and Uncertainty in Natural and Managed Systems
1. Strategies for Restoring River Ecosystems: Sources of Variability and Uncertainty
R. C. Wissmar and P. A. Bisson
Restoring complex river ecosystems can be a hard task. Major difficulties arise from spatial and temporal variability in natural processes and uncertainty regarding present and future management practices affecting river corridors and their adjoining landscapes. Uncertainty occurs when factors influencing natural and human systems within river drainages are imperfectly understood and are, therefore, unpredictable. Variability contributes to uncertainty when knowledge about natural change is incomplete.
The purpose of this book is to bring together perspectives on sources of variability in physical and biological processes in river ecosystems—and origins of uncertainty when managing these systems—to help develop prudent strategies for renewing and conserving rivers. We believe a better understanding of variability and uncertainty is critical to the successful implementation of restoration programs for aquatic and riparian systems. Improvements in strategies will require ecological information that accounts for interactions between management and natural factors (National Research Council [NRC] 1992a; 1999).
Extensive modifications of river ecosystems for flood management, water diversion, land reclamation, navigation, hydroelectric production, and other purposes has led to losses in natural physical and biological integrity of these complex ecosystems (Gore 1985; NRC 1992a; Bayley 1995; Gore and Shields 1995; Ward and Stanford 1995). Widespread fragmentation of riverine landscapes and cumulative degradation of channel and riparian habitats threaten the survival of many species, biodiversity of communities, and protection of remaining natural habitats in modified river ecosystems (e.g., agricultural and urban areas) (NRC 1992a, b; Naiman et al. 1995; Galloway 1995; Petts and Calow 1996; Schindler 2001).
The natural variability of river ecosystems in space and time provides an important part of the conceptual foundation for developing restoration approaches (Firth and Fisher 1992; Francis and Hare 1994; Greenland 1998). A review of climate variability, Chapter 2, describes how atmospheric, oceanic, and hydrographic regimes influence physical and biological patterns in aquatic and riparian communities. In Chapter 3, geomorphic and hydrological sources of variability in river systems are examined. Both Chapter 2 and Chapter 3 include discussions of linked, hierarchical processes that influence habitat development at scales ranging from reaches to landscapes.