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Instream Flow:

The following webpage discusses instream flow models and methods used by the DEP’s Water Allocation Permits Division. The webpage discusses PA/MD Instream Flow Study Model, The Tennant Method, and the Nature Conservancy’s Ecosystem Flow recommendations for the river basins of Pennsylvania.

PA-MD Instream Flow Study:

The PA-MD Instream Flow Study (PA/MD IFS) is a model that was created to determine the impacts of water withdrawals on trout fisheries. Following a 4-year, multi-agency study of about 100 streams in Pennsylvania and Maryland, the Susquehanna River Basin Commission (SRBC) released a report detailing the study findings. Additionally, the report also explains how to use the PA-MD IFS Model.

SRBC acted as project coordinator, with DEP, SRBC, Pennsylvania Fish and Boat Commission (PFBC), U.S. Army Corps of Engineers, Maryland Department of the Environment and the Chesapeake Bay Program providing funding.

The model can be used to evaluate instream flow needs or passby flow requirements for streams that contain brook/brown trout (wild or stocked). The stream must have a drainage ≤100 square miles and must be in the Ridge and Valley, or unglaciated parts of the Appalachian Plateaus, physiographic provinces (provide link to Plate 1 of the study).

The model should not be used to evaluate streams that are designated as Warm Water Fishes (WWF) in PA Code Title 25, Chapter 93, Water Quality Standards.

The PA-MD IFS Model is used to determine passby flow requirements for streams and rivers located downstream of public water supply agency intakes. Passby flows will be based upon the following in priority order:

Exceptional Value (EV) waters‑ Withdrawals may not cause greater than a 5% annual loss of habitat as determined from the PA/MD IFS report.

High Quality (HQ) waters‑ Withdrawals may not cause greater than a 5% annual loss of habitat; except an annual habitat loss of 7½% may be allowed if the following conditions are met:

  • The water supply agency is in compliance with the DEP’s water conservation guidelines, and
  • No feasible alternative source is available, and
  • Available sources are used in a program of conjunctive use approved by the DEP and combined alternative source yields are inadequate.

Cold Water Fishery (CWF) waters:

  • Streams that meet PFBC Class B Wild Trout Streams standards‑ Withdrawals may not cause greater than a 10% annual loss of habitat. Class B streams are those that support a brook, brown or combined brook and brown trout biomass of at least 20 kilograms/hectare.
  • Streams that meet PFBC Class C or D Wild Trout Stream standards ‑ Withdrawals may not cause greater than a 15% annual loss of habitat. Class C streams are those that support a brook, brown or combined brook and brown trout biomass of at least 10 kilograms/hectare. Class D streams are those that support a brook, brown or combined brook and brown trout biomass of less than 10 kilograms/hectare.

It has been the Water Allocation Permits program practice to use the PA/MD IFS recommendations for the protection of aquatic ecosystems downstream of public water supply agency intakes (particularly during low flow conditions).

The PA/MD Instream Flow Study Report (pdf)

The PA/MD Instream Flow Study Model

The Tennant Method:

The Tennant Method (also known as the Montana Method) is an instream flow method specifically designed for aquatic life. The method is quick and easy, requiring little or no field work.

The Tennant Method is based on extensive and detailed field studies that were conducted for over 17 years in 21 states (Camp, Dresser, & McKee, 1986 and PFBC, 1992). Waters in the Eastern and Western United States were evaluated during the studies (Camp, Dresser, & McKee, 1986 and Tennant, 1975) and included different morphological stream types such as high gradient brooks and low gradient rivers. Biological, chemical, and physical data were collected from streams/rivers at different flows throughout the study (Tennant, 1975). Tennant Method recommendations are used when they PA-MD Instream Flow Study Model can’t be used.

In 1992, the PFBC recommended the following passby flow and conservation release requirements based on the Tennant Method:

  • 25% annual Average Daily Flow (ADF) on all streams with reproducing trout populations or on streams designated for special protection (EV and HQ in Chapter 93 of the DEP’s regulations).
  • 20% annual ADF on streams that are designated in Chapter 93 as Warm Water Fishes (WWF), Cold Water Fishes (CWF), Migratory Fishes (MF), and Trout Stocking (TSF).

It has been the Water Allocation Permits program practice to use the PFBC recommendations for the protection of aquatic ecosystems downstream of public water supply agency's intakes and reservoirs (particularly during low flow conditions).

1975 Tennant Method Paper

1992 Position Paper: Fish and Boat Commission Review of Water Allocation Permits

Literature Cited:

Camp, Dresser, and McKee. 1986. Minimum instream flow study. Final Report to the Commonwealth of Virginia State Water Control Board. 392pp.

Pennsylvania Fish and Boat Commission (PFBC). 1992. Position Paper: Fish and Boat Commission Review of Water Allocation Permits.

Tennant, D.L. 1975. Instream flow regimes for fish wildlife, recreation, and related environmental resources. U.S. Fish and Wildlife Service. 30 pp.

The Nature Conservancy Ecosystem Flow Recommendations:

The Nature Conservancy (TNC) has conducted ecosystem flow recommendation studies in the following river basins:

Susquehanna

Delaware

Upper Ohio River Basin in Western Pennsylvania

Tributaries of the Great Lakes in New York and Pennsylvania

Potomac

The objective of each study was to determine the ecosystem flow recommendations needed protect the species, natural communities, and key ecological processes within the various stream and river types in each river basin of Pennsylvania. The flow recommendations for each river basin include a range of flow conditions that are vital for ecosystem protection. The recommendations include extreme low and drought flows, seasonal (and monthly) flows, and high flow. The flow recommendations also include the magnitude, timing, frequency, and duration of the different flow conditions.