TWRA Instream Flow Program
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.
- Hydrology - distribution, conservation, and use of water on Earth and in the atmosphere
- Biology - fish, aquatic bugs, and mussels
- Geomorphology - stream channels changing over time due to weather events
- Water Quality - water temperature, chemicals, and oxygen in water
- Connectivity - from headwater streams to large rivers
This guidance 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 conservative multiple of the September Median Flow and reflect the natural stream hydrograph. A conservative multiple is applied to establish a protective flow. Protective flows are needed when there are multiple withdrawals from a stream or when vulnerable wildlife species require specific aspects of their habitat to be maintained. For example, Boulder Darters (Etheostoma wapiti), a federally endangered species, require fast rocky riffles in small to medium rivers. These habitat components must be protected where this species occurs. A conservative multiple of the September Median Flow attempts to establish an instream flow that protects such aquatic wildlife habitat.
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. In Tennessee, we are increasingly aware of diminished aquatic ecological function resulting from the loss of pervious surface in watersheds. Increased impervious surface from development results in flash hydrologic events that damage stream morphology and habitat. Withdrawal proposals should also quantify the percentage, location, and type of impervious surface upstream of the withdrawal location and characterize its impact on stream flow behavior.
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 maximum of 10% 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.
If, September Median Flow (SMF) = 10 cubic feet per second (CFS), then, using a conservative multiple SMF of 1.5 then 15 CFS is established as the conservation flow at which withdrawal ceases (10 X 1.5 = 15).
Allowing 10% withdrawal of available flow above 15 CFS then at 20.0 CFS, allow withdrawal of .5 CFS (10% of available flow above 15 CFS).
Ex. 20 CFS – 15 CFS = 5.0 CFS. 10% of 5.0 CFS = .5 CFS.
At 25 CFS, allow withdrawal of 1 CFS (10% of available flow above 15 CFS).
Ex. 25.0 CFS -15 CFS = 10.0 CFS. 10% of 10 CFS = 1 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: 12/09/2016
- Instream Flow Prescriptions - how much water should be withdrawn by a city and
- the flows needed to sustain aquatic species
- Collecting data on the five key components of Instream Flow
- Instream Flow Committee, a joint effort between state and federal agencies and non-profits involved in Instream Flow research and policy
- Instream Flow Media Campaign
- Member of Southeastern Instream Flow Network