Basin plans are thorough investigations into the water problems and potential solutions within a given drainage basin. (The term "drainage basin" refers to all the land that drains to a common body of water.)

Basin plans address issues such as flooding, poor water quality, erosion, and the degradation of aquatic habitat. The plans involve gathering data about the topography of the land, and the way water moves through the soil. The plans also assess how drainage projects and other activities in one area might affect other areas of the watershed.

The plans are reviewed and approved by the elected officials of each participating agency involved. (Basin plans in Thurston County are usually written jointly by the county and cities.) The plan itself does not provide funding or authorization for projects; however, Thurston County commissioners refer to the basin plans when deciding which stormwater construction projects to finance.