Problems and constraints in managing rivers
with variable flow regimes
A.J. Boulton, F. Sheldon, M.C. Thoms E.H. Stanley
Many contemporary ecosystem theories developed to explain how rivers function originated from research on temperate, perennial streams (Williams, 1988). Not surprisingly, recommendations for river management and restoration (e.g. Petts and Calow, 1996) and water policies and legislation (e.g. Johnson, 1993) share similar origins. However, uncritical extrapolation of theories developed in permanent lotic ecosystems to intermittent and ephemeral streams can prove perilous and even misleading (Boulton and Suter, 1986; Williams, 1987, 1996). For example, extremes of flooding and drying (variable flows) largely structure stream assemblages and regulate ecosystem processes in most intermittent streams (e.g. Feminella, 1996; Miller and Golladay, 1996; Stanley et al., 1997). Conversely, biological interactions such as predation by fish that often are excluded by a temporary flow regime may be more important in perennial streams. Flooding occurs in most perennial streams as well (Resh et al., 1988) but drying is rare except during severe drought when the fauna is devastated by desiccation (e.g. Hynes, 1958).
Rivers and streams naturally vary in flow although we must specify temporal scale when we use the term ‘variable’. In this chapter, we are discussing whole river systems and adopting a multi-year temporal scale appropriate to the definition of flow regime (see below). Thus, the most highly variable flow regimes usually occur in intermittent and ephemeral rivers, especially those in dryland areas. Here, the coefficients of variation of annual flows are, on average, 467% greater than those from humid and temperate regions (Davies et al., 1994). This hydrological variability seems to be associated with increased habitat and food web complexity (Thorns and Sheldon, 1997). Although it is likely that the persistence of many species in dryland rivers relies on maintenance of intermittency (Walker et al., 1995; Boulton, 1999), there are few scientific data to support this hypothesis because we often lack information about the ecology of the river before regulation. Such data are a fundamental requirement for managing these types of rivers and raises the question: How should we manage rivers with variable flow regimes when we have so little information?
Historically, water management practices in arid and semi-arid zones have been driven by a single priority – human demand for water (Biswas, 1996; Postel, 1996). Demand for water in some dryland areas such as Mexico has escalated as surface waters have become polluted or dried through excessive pumping of groundwater, leading to reservoir construction and other regulatory structures (Grimm et al., 1997). River regulation and, more recently, interbasin water transfers (IBTs) (Davies et al., 1992, 1994) are imposed most intensively upon rivers with highly variable flow regimes (including natural intermittency) to sustain human agriculture and navigation (Walker et al., 1997; Kingsford et al., 1998). The issue is made more complex by a Western human perception that a ‘healthy’ river flows all year round; many of the more ambitious river regulation projects have had technological and intellectual input from experts living in well-watered regions. This is despite evidence