Total Maximum Daily Load (TMDL)

DEQ manages the Total Maximum Daily Load program for the State of Oklahoma. A TMDL is the process of establishing the sources of impairment in a given waterbody, whether from point sources (discharges) or non-point sources (runoff), determining the amount of reduction necessary to meet water quality standards in that waterbody, and allocating the loads to the various contributors of pollution. A margin of safety is also included in the TMDL calculation process.

Water quality modeling is the linkage between the sources of pollution and the instream water quality of a given waterbody. A model is a representation of the water-quality processes that occur in a given waterbody. Water quality modeling can be resource-intensive. Different levels of complexity may be used depending on the level of confidence required in a given situation.

Draft TMDLs

No draft TMDLs are currently available for review.

Completed TMDLs

EPA approved TMDLs for the State of Oklahoma are available for viewing or download at the following web page:

Completed TMDLs by Planning Basin

Lake Thunderbird TMDL Project

Lake Thunderbird TMDLs for Nutrients, Turbidity, and Dissolved Oxygen received EPA approval on November 13, 2013. Documents and information related to development of the Lake Thunderbird TMDL can be found on the Lake Thunderbird TMDL Project Page.

Lake Thunderbird TMDL Report

Lake Thunderbird TMDL Project Page

The TMDL Process

Water Quality Monitoring – Surface water quality data is collected by State, federal, and local agencies.

Waterbody Assessment – Analysis of water quality data to determine if a waterbody is achieving water quality standards

303(d) List of Impaired Waterbodies – Waterbodies in the State of Oklahoma that do not meet water quality standards are compiled in the State’s 303(d) list.

TMDL Prioritization – DEQ prioritizes watersheds containing impaired waterbodies for TMDL development. A rating system is utilized to determine a priority ranking value for each impaired watershed.

TMDL Development – A Total Maximum Daily Load value for each cause of impairment in a waterbody is calculated using appropriate water quality models.

Public Review – TMDL reports are published for public review and comment. DEQ accepts any comments submitted during the specified review period.

Report submittal – DEQ submits the TMDL report to EPA for review and approval.

EPA approval – EPA approves the TMDL report if the TMDL calculations are technically sound and the report satisfies required elements.

Water Quality Modeling

DEQ employs the use of a number of different types of surface water quality modeling software. Model selection is based on the needs and complexity of a given project.

Load Duration Curves – Simple

QUAL2K – Stream and river water quality model developed at Tufts University for EPA

SWAT – Soil & Water Assessment Tool

LOADEST – Load Estimator software developed by USGS

HSPF – Hydrological Simulation Program – FORTRAN

BATHTUB – One-dimensional steady-state lake model

EFDC – Environmental Fluid Dynamics Code

ELCOM-CAEDYM – Estuary, Lake and Coastal Ocean Model – Computational Aquatic Ecostystem DYnamics Model

Contact Watershed Planning

An interactive map of Oklahoma’s waterbodies is available at