Instream flow means flowing water in a stream or river. It's important to maintain instream flows because humans and aquatic animals need water to exist and function on a daily basis.
Tennessee is now a member of the Instream Flow Council and the TWRA is presently working on standards for setting Instream Flows in Tennessee.
This policy statement is a default position in the absence of application of a recognized Instream Flow methodology. Instream Flow methodology should consider whether or not a stream is free flowing or regulated, whether or not the stream trends towards zero annual flow, the cumulative impact of multiple withdrawals, and should determine whether the stream hydrology is losing or gaining. TWRA’s preference is for a collaborative Instream Flow study process. Several state, federal, university, and conservation organizations offer planning expertise. Both the Tennessee Water Quality Control Act and the Tennessee Wildlife Code require that water withdrawal not result in a condition of pollution or harm to aquatic habitat and that resulting Instream Flow provide for the protection of fish and aquatic life. Numerous smaller rivers and streams in Tennessee are no longer suitable for providing sustained water withdrawal at low flow for increasing multiple use demand. Examples of sustained multiple use demands exceeding available low flow include Doe Creek, Little Pigeon River, Big Creek, Little River, Harpeth River, and the Piney River. Consideration of water withdrawals requires application of avoidance and minimization of impact as defined by Tennessee’s Anti-Degradation Statement.
Protection and conservation of fish, aquatic life, and aquatic habitat require that, as a result of withdrawal, Instream Flow not be less than the September median flow or less than a more conservative multiple of the September Median Flow and reflect the natural stream hydrograph. Permit applicants proposing to harvest water should provide a water harvest and storage plan that minimizes negative effects on downstream hydrology, aquatic life, geomorphology, connectivity, and water quality. The default approach: a) Establish an Instream Flow below which no withdrawal occurs using the September Median Flow or a multiple of the September Median Flow; b) At flow above September Median Flow or a multiple of September Median Flow, a percentage withdrawal is allowed; c) Percent withdrawal will follow the natural hydrograph; and d) Percent withdrawal will avoid hydrologic flat-lining at low flows. As an alternative to the default position, permit applicants proposing sustained withdrawal should provide an Instream Flow study based on an accepted assessment methodology. The Instream Flow study should be reflective of the ecoregion in which the stream is found. The study should take into account that the September Median and 7Q10 will be different depending on which ecoregion the stream is located. The default position is not applicable to flow regulated by impoundment. The default position is not applicable to streams trending toward zero annual flow or a 7Q10 approaching zero flow.
September Median Flow (SMF) = 4.0 cubic feet per second (CFS)
Using a conservative multiple SMF of 1.5
4.0 CFS x 1.5 = 6.0 CFS
6.0 CFS established as conservation flow at which withdrawal ceases
Allow 50% withdrawal of available flow above 6.0 CFS
At 8.0 CFS, allow withdrawal of 1.0 CFS (50% of available flow above 6.0 CFS)
8.0 CFS – 6.0 CFS = 2.0 CFS 50% of 2.0 CFS = 1.0 CFS
At 12.0 CFS, allow withdrawal of 3.0 CFS (50% of available flow above 6.0 CFS)
12.0 CFS -6.0 CFS = 6.0 CFS 50% of 6.0 CFS = 3.0 CFS
*7Q10: seven consecutive lowest flow days in a ten year period
Reference: Instream Flows for Riverine Resource Stewardship. Instream Flow Council (2004)
Last Revision: 1/28/2011