The streams and rivers of the GCP LCC are delicately balanced ecosystems that link diverse habitats with the people, plants and animals that rely on clean and abundant water supplies to thrive. The natural patterns of seasonal flows in streams and rivers – called instream or environmental flows - are the drivers for many of the ecosystem functions and processes on which the riverine and coastal natural and human economies rely. Extreme droughts and population growth in the GCP LCC region have forced the recognition that water resources are limited and need to be better managed. Excessive extractions and diversions of water alter instream flows and threaten the ecological processes that are dependent upon them. Dams on large rivers, for example, often reduce high flows and maintain unnaturally high baseflows. Climate change is yet another threat to natural flows as temperature and precipitation patterns are predicted to shift dramatically in the future. Failure to prepare for these pressures on the aquatic resources in the face of the uncertainties of climate change threatens the health of the region’s economy and sustainability of aquatic resources.The importance of natural flow regimes to the ecological integrity of rivers has been established for decades, but more specific information is needed to develop and implement scientifically-credible instream flow standards and management practices (Richter 2010). In fact, recent reviews of resources to support state instream flow standards reveal there is little available information that helps define specific ecological responses to flow alteration (Poff and Zimmerman 2010, McManamay et al. 2013). This makes it difficult to specify ecological flow regimes for a river and explain why the regime is critical to maintain and protect the aquatic resources.A holistic suite of flow-ecology hypotheses about how riverine ecosystems respond to altered flow regimes forms the scientific basis for setting ecological limits of hydrologic alteration for streams and rivers. However, very few of these relationships have been identified in the GCP LCC region, which limits the ability of the states to substantiate instream flow standards and water management practices. The suite of regional flow-ecology hypotheses presented here addresses many components of riverine ecosystems that are sensitive to flow alteration. We present example flow-ecology hypotheses for fish, mussels, birds, and riparian vegetation.
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