Working with Change: The Importance of Evolutionary Perspectives in Framing the Trajectory of River Adjustment

'Nothing is permanent but change.' --Heraclitus. Research in "pristine" environments provides an intriguing sense of natural river function and evolution (e.g., Collins and Montgomery 2001; Brooks and Brierley 2002). We typically fail to appreciate just how profoundly rivers have been altered by human activities. For example, Brooks et al. (2003) document a 700 percent increase in channel capacity and a 150-fold increase in the rate of lateral channel migration within a few decades of clearance of riparian vegetation and removal of wood from a river in southeastern Australia. Long-term evolutionary insights are required to interpret river responses to human disturbance relative to natural variability. Most rivers have been fundamentally altered from those that existed prior to human disturbance. River systems can be characterized as shifting mosaics of patch dynamic relationships (see chapter 4), for which disturbance is a fundamental requirement for the maintenance of ecosystem integrity. Indeed, change is a natural, vital component of aquatic ecosystem functioning. However, changes to the geomorphic structure of a river can modify and fragment the physical template, severely diminishing its capacity to support ecological systems. For example, substantial declines in the integrity of ecosystems have been associated with habitat change, fragmentation, and loss (Bunn and Arthington 2002; Dudgeon et al. 2006; Postel and Richter 2003).