Julie C. Stromberg

The Natural Flow Regime

H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).

Riparian vegetation instream flow requirements: a case study from a diverted stream in the eastern Sierra Nevada, California, USA.

A methodology is described that allows determination of instream flow requirements for maintenance of riparian trees. Tree-ring data revealed strong relationships between tree growth and stream flow volume for riparian species at Rush Creek, an alluvial stream within an arid setting; these relationships allowed development of models that predict growth rates from hydrologic variables. The models can be used to assess instream flow requirements under the assumption that certain levels of growth are necessary to maintain the population. There is a critical need for development and use of instream flow methodologies for riparian vegetation, since present methodologies focus on needs of aquatic animals (e.g., fish) and may underestimate needs of the entire riparian ecosystem.

Rivers, dams, and willow flycatchers: a summary of their science and policy connections

The southwestern willow flycatcher (Empidonax traillii extimus) is a riparian bird that spends winter months in Central and South America and summer breeding months in riparian zones of the American Southwest. Decline of the willow flycatcher population to less than 1000 breeding pairs prompted the Federal government to declare the species endangered, triggering a major recovery effort. The most important aspect of recovery is management and improvement of the riparian habitat of the bird population. Although the direct management of the species is primarily a biological issue, fluvial hydrology and geomorphology play an important role in understanding the dynamics of the present bird population and in designing a recovery plan because these physical systems are the substrates for the living communities which include the birds. Contributions of geomorphology and hydrology to the recovery plan include the use of watersheds and river basins as planning and evaluation units; understanding the connections between fluvial forms and riparian vegetation; implications for the bird population of the magnitude, frequency, duration, timing, and rate of change for various river discharges. The installation and operation of dams are the most important causes of hydro-geomorphic and ecological change in the region, so that management of these structures offers primary opportunities to improve the physical and biological conditions for the endangered species.