Q. What is a minimum stream flow?

A. A minimum stream flow is the amount of water flow necessary to preserve stream values, or the minimum lake elevation necessary to preserve lake values. The water remains in a reach of a river or in a lake to protect fish and wildlife habitat, aquatic life, navigation, transportation, recreation, water quality or aesthetic beauty.

Q. Why is a minimum stream flow important?

A. Flow is important because water is important. Flows in a stream are a “zero sum game” – here is a finite amount of water available at any given moment and if it is being used for one thing; it generally cannot be used for another. Native streamside vegetation in the riparian zone must have natural flow in order to survive and reproduce. The plants, fish, and wildlife in any given river have evolved to adapt to that river’s unique rhythms. Altering natural flow can harm these species. By setting different minimum stream flow values for different seasons (e.g., highest in the spring during runoff), we attempt to approximate natural flow cycles.

Q. What is the difference between minimum stream flow, instream flow, optimum flow, and all the other flows?

A. Minimum stream flow is as explained above – a level below which the amount of flow in a specified stream should not drop. Instream flow is used generally to describe the amount of flow at a given time in a stream. It is also used specifically in law to denote water which is expressly dedicated to remain in the stream channel, which should not be diverted for other purposes. Optimum flow is used by some states and groups to describe a target flow; i.e., what would be the flow level if environmental and habitat issues were our first concern?

Q. How is minimum stream flow determined?

A. The amount of water that can be approved for a minimum stream flow is the minimum – not the most desirable – flow or lake level necessary to protect the fish and wildlife habitat, aquatic life, water quality, recreation, aesthetic beauty, navigation or transportation. The instream flow right is usually measured in cubic feet per second (cfs) while a minimum lake level is measured in elevation (feet above mean sea level). A cfs is 448.83 gallons per minute.(1)

 

Here is a link to an overview of the methodology that the Washington Department of Ecology uses to determine a minimum stream flow.

Q. Does setting a minimum stream flow stop development?

A. Unappropriated water remains subject to appropriation. Ideally, no water right with a junior priority date can deplete the water needed to maintain the minimum stream flow unless allowed as a condition of approval of that minimum stream flow.(1)

 

In addition, improving the efficiency with which existing water supplies are used is often the most environmentally beneficial and cost-effective method of meeting future needs. Given the amount of water used in most western states for irrigation, a relatively small increase in irrigation efficiency can produce dramatic results. In the urban sectors, a much larger constellation of water management tools are available, including conservation, water transfers, wastewater reclamation, desalination, groundwater clean-up and conjunctive use.

Q. Can a stream with a minimum stream flow be dried up?

A. Yes. Water rights with earlier priority dates may fill their needs before the minimum stream flow takes effect. In a drought year or when low flows occur, the senior diversions can legally dry up the stream, leaving no water for the minimum stream flow. However, by facilitating the acquisition of existing water rights through conservation measures, donation, lease, or outright purchase, some or all of the stream flow rights may be senior enough in priority to prevent significant drying.(1)

Sources:
(1) Some materials were drawn from Idaho Rivers United.