stream, rivers will sweep and shift laterally and flood periodically, creating seasonal habitats that may be vastly more productive than in-channel environments. Biota influence channel morphology through controlling bank strength (e.g., rooted vegetation), trapping fine sediment, and redirecting flow and sediment (e.g., through fallen wood and beaver dams). Hence, there is a co-evolution of rivers and river ecosystems. This co-evolution can be disrupted either by changes in external drivers (e.g., climate and tectonics) or by changes in internal interactions (e.g., sediment pulses, vegetation dynamics, or the introduction of non-native species, Figure 3-5).

In addition to its role in shaping a river system’s physical habitat, the flow regime is itself an important determinant of the distribution, abundance, and life history traits of river and floodplain (riverine) organisms within its basin. Flow magnitude, frequency, timing, duration, rate of change, and predictability of flow events (e.g., floods and droughts) act

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FIGURE 3-5 The introduction of the beaver Castor canadensis to Tierra del Fuego, an archipelago off the southernmost tip of South America, created extensive damming of streams and converted nearly 30-40 percent of the island’s riparian Nothofagus (or southern beech) forests to floodplain wetlands (Anderson et al., 2009). SOURCE: Photo courtesy of Christopher B. Anderson, University of North Texas.



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