Last Updated 07/21/2026
Stormwater Management Manual for Eastern Washington
Executive Summary
The Stormwater Management Manual for Eastern Washington (SWMMEW) provides guidance on the measures necessary to control the quantity and quality of stormwater runoff. Local jurisdictions use this manual to set stormwater requirements for new development and redevelopment projects. Land developers and designers use this manual to design permanent stormwater control plans, develop construction stormwater pollution prevention plans, and determine stormwater infrastructure for their project. Businesses use this manual to help design their stormwater pollution prevention plans.
The greatest use of the SWMMEW has been through National Pollutant Discharge Elimination System (NPDES) stormwater permits. The Municipal Stormwater General Permit for eastern Washington incorporates and references the SWMMEW. The Industrial Stormwater General Permit, Construction Stormwater General Permit, Boatyard General Permit, and the Sand and Gravel General Permit reference the SWMMEW. Since 2005, Ecology has reissued or issued for the first time all of these NPDES stormwater permits. The 2024 revisions to the SWMMEW will continue to help permittees comply with these permits.
Objective of the Manual
The objective of this manual is to provide guidance on the measures necessary to control the quantity and quality of stormwater produced by new development and redevelopment. The goal of the measures is to comply with water quality standards and contribute to the protection of beneficial uses of the receiving waters. Application of appropriate Core Elements (CEs) and Best Management Practices (BMPs) identified in this manual are necessary but sometimes insufficient measures to achieve these objectives. See 1.1.4 Effects of Urbanization on Stormwater for further information about the impacts of development on water quality.
Water quality standards include:
- Chapter 173-200 of the Washington Administrative Code (WAC), Water Quality Standards for Groundwaters of the State of Washington
- Chapter 173-201A WAC, Water Quality Standards for Surface Waters of the State of Washington
- Chapter 173-204 WAC, Sediment Management Standards
More Stringent Requirements
Federal, state, and local permitting authorities with jurisdiction can require more stringent measures that are deemed necessary to meet locally established goals, state water quality standards, or other established natural resource or drainage objectives. Water cleanup plans or Total Maximum Daily Loads (TMDLs) may also identify more stringent measures needed to restore water quality in an impaired water body.
Applicability
This manual is applies to all of eastern Washington. This includes the area bounded on the west by the Cascade Mountains crest; on the north by the Canadian border; on the east by the Idaho border; and on the south by the Oregon border. At the southern end of Washington’s Cascade Mountain range where the crest does not follow county borders, this manual is applicable to all of Yakima and Klickitat Counties.
- Adams
- Asotin
- Benton
- Chelan
- Columbia
- Douglas
- Ferry
- Franklin
- Garfield
- Grant
- Kittitas
- Klickitat
- Lincoln
- Okanogan
- Pend Oreille
- Spokane
- Stevens
- Walla Walla
- Whitman
- Yakima
Comparison of Manuals for Eastern and Western Washington
After crossing from western to eastern Washington from any mountain pass, one thing is apparent: eastern Washington is very different from western Washington in terms of climatic conditions, vegetation, and landscape. In many regards, eastern Washington is more similar to Idaho and Montana than to western Washington, with its cold, snowy winters; hot, dry summers; strong winds; rugged mountain landscapes; high desert plateaus; rolling hills of wheat and fruit;
lakes; and abundance of flat agricultural land.
This manual was developed to address conditions that are unique to eastern Washington with respect to designing stormwater BMPs such as the following:
- The Phase II Municipal Stormwater National Pollutant Discharge Elimination System Permit for eastern Washington requires retention of the 10-year stormwater runoff on-site, resulting in minimal discharge to municipal drainage systems. Some jurisdictions require retention of larger storms.
- Many developed sites in the region drain directly to underground injection control (UIC) wells.
- Storms generally occur as short-duration, high-intensity events.
- In some areas with intense agricultural use, groundwater levels can fluctuate due to irrigation in the summer months, with reductions of several feet in the winter when irrigation is shut off.
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The information on this page is provided for general informational purposes only and is not legal advice. Regulations change frequently — AQUALIS makes no warranty as to the accuracy or completeness of this content, and any reliance on it is at your own risk. Consult a qualified professional or your local regulatory authority to confirm requirements applicable to your specific property.
