What is instream flow? At its most basic level, the term means water flowing in a stream. Most streams have some level of flow, but flow is no guarantee that all is well for the organisms (including humans) that depend on the river’s resources. Natural resource managers are faced with the complicated task of protecting and restoring public values to rivers while honoring existing uses.
Why is instream flow important? Natural stream and river systems provide many beneficial values and services, including flood mitigation, groundwater recharge, navigation, nutrient transport and recycling, pollution attenuation, water supplies, biological productivity, aesthetic values, and recreational opportunities such as fishing, boating, swimming, and wildlife viewing. Instream flow is necessary to sustain these and other utilitarian and intrinsic values. A good understanding of how instream flow levels and regimes relate to these values, and the scale of alteration from the natural condition, is necessary for informed river management.
How does one determine how much flow a river needs? There is usually not just one flow level that a river needs to stay healthy. If the objective is to preserve riverine values, that can only be done by preserving the processes and functions of the river ecosystem. The structure and function of riverine systems are based on five riverine components; hydrology, geomorphology, biology, water quality, and connectivity. Inter- and intra-annual flow prescriptions are needed to preserve the ecological health of a river. And some flow needs, such as those that flush sediments from stream substrates or maintain channel integrity, may be quite high.
Isn't instream flow really an issue of “water for fish” vs. “water for people”? Aren't people more important than fish? If you're a normal person, you'd answer "yes, people usually are more important than fish". However, the issue of instream flow isn't that simple. It actually boils down to a value judgment of what we want our world to look like. Fish are in fact just one of many organisms that live in streams but they often offer a gauge of overall environmental health. Instream flow is an issue of water and river management - seeking ways to maintain healthy, diverse ecosystems that contribute to a high quality of life while sustaining our basic life functions and economies. Accomplishing this goal is never easy, as it involves integration of scientific knowledge and societal demands within a set of legal limitations. But informed and effective instream flow management should afford a healthy, enjoyable existence for people while maintaining healthy, diverse aquatic resources. It's much more complicated than "keeping a little water in the creek for the fish".