Instream Flow Unit

Instream flow is the amount of water needed in a stream to adequately provide for downstream uses occurring within the stream channel. Instream uses may include some or all of the following: aquatic habitat, recreation, wetlands maintenance, navigation, hydropower, riparian vegetation, and water quality.

Division of Water Resources' role

The Division of Water Resources (DWR) has a lead role in the Department of Environmental Quality (DEQ) on issues pertaining to the allocation of water. DWR works closely with other state and federal agencies involved in fisheries, water quality, and recreation when considering what flows are necessary to protect natural resources. Flow requirements are often thought of as minimum flows or releases, but they can also include maximum flow limits for peaking hydropower dams, seasonal releases for fish spawning, or weekend releases for recreation.

Other instream uses

Other instream uses of water will depend on the stream, the intended use, and also may vary by season. Flows which provide adult fish habitat may be significantly different than those needed for successful reproduction during spawning season. Flows for recreational canoeing are more important during warm months, especially on weekends and holidays. Some streams are not suitable for canoeing, and thus would not include this as a component of its overall instream flow need.

Flow volume

Flow recommendations are always made in terms of volume of water per unit of time, usually cubic feet per second (cfs). As a point of reference, 1.547 cfs equals approximately 11.5 gallons per second (gps) or approximately one million gallons per day (mgd). It is important to realize that this is not the same as the current or speed of the water, often measured in feet per second (fps). The resource manager needs to know the volume of water to determine if enough of the stream channel will be submerged for aquatic habitat, if a wastewater discharge can be adequately assimilated, or if enough water is available for offstream use.

Low flow periods

Most instream flow impacts are more pronounced during low flow periods. It is during these periods when streams are already at low levels that the demand for water is often highest. The drier, and usually hotter, the weather is, the greater the demand is for irrigating lawns, crops, and golf courses, and for electricity.

Permitted discharges

All point source dischargers in North Carolina have conditions in their permits which are based on stream flows. The permit limits provide wasteload assimilation through effluent dilution and reoxygenation of the stream. All wastewater discharges are required to be treated so that water quality standards will still be met when the stream flow is as low as the 7Q10, the lowest flow expected to occur on a particular stream for 7 consecutive days once every 10 years. Obviously this is a very low flow which under natural conditions is the result of drought. Flows less than the 7Q10 may be the result of drought, but also can be caused by water withdrawals or dams which impound water. When stream flow falls below the 7Q10, water quality violations may occur. The Division of Water Quality (DWQ) sets water quality standards and permit limits, and is responsible for enforcement.

Offstream uses

Contrasting with instream uses are the offstream demands for water. These include water withdrawals for municipal and industrial water supply, agriculture, aquaculture, and golf course irrigation. Such withdrawals may use a simple pipe, a low diversion weir, or a large dam. A simple pipe withdrawal that removes water from a stream without the necessity of an impoundment is called a run-of-river withdrawal. Structures placed in streams to withdraw water for any purpose may require a permit from the U.S. Army Corps of Engineers ("the Corps") under Section 404 of the Clean Water Act.

No instream flow study is required if the water withdrawal for the proposed project, either singularly or in combination with other near-by withdrawals, is less than 20% of the 7Q10. Withdrawals of this size relative to stream flow are considered to have minimal impacts on physical aquatic habitat. However, to avoid cumulative impacts, project developers should still contact DWQ regarding potential concerns for downstream assimilative capacity and maintaining water quality standards.

Withdrawals that are 20% of the 7Q10 or more will require additional analysis. The location of the proposed project and the habitat rating of the downstream aquatic habitat will determine whether a desktop analysis or site-specific instream flow study is used to determine the flow. The N.C. Wildlife Resources Commission does the habitat rating.


Construction of a dam will usually require permits under sections 401 and 404 of the federal Clean Water Act. As mentioned earlier, 404 permits are administered by the Corps. Section 401 permits are administered by DWQ and may include instream flow requirements. Like run-of-river intakes, withdrawals from an impoundment to serve a public water supply must be approved by DWR's Public Water Supply Section (PWS). Instream flow requirements or withdrawal limitations may be included as part of the environmental review under the N.C. (State) Environmental Policy Act (SEPA). This applies to both new and expanding withdrawals.

Dams used for hydroelectric power production may affect instream flows by diverting water out of the natural stream channel by canal or pipe to a powerhouse where turbines are spun, power is generated and the water is then returned to the stream. Although not all hydroelectric dams use this type of diversion, those that do may divert water for a distance of several miles. Hydroelectric dams can also change stream flows if they store water behind a dam and release it at certain times to meet the demand for electricity. This type of project, known as "peaking," can dramatically fluctuate stream flows in a very short span of time.

Instream flow concerns for hydropower projects operated by public utilities are addressed during licensing or re-licensing under the Federal Power Act.  State and federal review agencies, including DWR, provide recommendations to the Federal Energy Regulatory Commission (FERC) regarding instream flows.  DWR typically acts as the coordinating agency within the DEQ for matters relating to FERC licensing.  If a project is not subject to FERC jurisdiction, environmental review may take place either during the project's application for a certificate of public convenience and necessity (CPCN) from the N.C. Utilities Commission or under the Dam Safety Law, administered by the Division of Land Resources

In terms of height and storage, the Dam Safety Law applies to dams that are 25 feet or more high, and with an impoundment capacity of 50-acre feet or more. Dams that do not meet both of these criteria are exempt from jurisdiction under this law unless their failure could result in loss of life or significant property damage. Federally owned and operated dams, as well as dams federally regulated for hydro- or nuclear power are also exempt from this law. Additional catagories for exemption may also apply.

The 1995 General Assembly amended the Dam Safety Law to specify the minimum flow for small hydropower producers that divert water around 4000 feet or less of the natural streambed. At new projects, the required minimum flow to any bypassed reach is the 7Q10 flow or 10% of average flow, whichever is greater. A bypassed reach occurs when the project diverts water around the natural stream channel between the dam and powerhouse discharge.

Riparian rights

North Carolina water law is based on the "riparian rights" concept, rather than appropriated water rights. (Appropriated water rights refer to the "first in time, first in right" allocation system used in the western United States.) According to this concept, a riparian owner is entitled to the natural flow of a stream running through or along his land in its accustomed channel, undiminished in quantity and unimpaired in quality, except as may be occasioned by reasonable use of the water by other like owners. Some types of water resource projects are subject to state or federal regulations that establish parameters and procedures to determine what is a "reasonable" use. If a water resource development is not subject to any of these regulations, then a water use dispute could be handled as a civil law matter between riparian owners.

Regulations and Contacts

If you have drought-related questions, please visit SaveWaterNC and the Drought Monitor. Please contact the Division of Water Resources Instream Flow Unit if you have other questions concerning stream flows.

The following regulations and the associated agency with regulatory authority may be useful for addressing instream flow issues: