within a certain range of quantity and quality. Because
freshwater ecosystems are dynamic, they additionally
require a range of natural variation or disturbance to
maintain viability, or resilience (Holling 1986, Pickett
et al. 1992). Both seasonal and interannual variability
in flow are needed to support biota and maintain natural
habitat dynamics that support production and persistence
of species (Naiman 1992, Stanford et al. 1996,
Poff et al. 1997). The sizes of native plant and animal
populations and their age structures, the presence of
rare or highly specialized species, the interactions of
species with each other and their environments, and
many ecosystem processes are strongly influenced by
the temporally varying hydrologic regimes that characterize
these ecosystems. Water quality, physical habitat
conditions, and energy sources are shaped by periodic
and episodic water-flow patterns. Natural freshwater
ecosystems, therefore, have evolved to the
rhythms of hydrologic variability.