Wat. Code, § 16101

(a) Each county, city, or special district that is a permittee or copermittee under a national pollutant discharge elimination system (NPDES) permit for municipal separate storm sewer systems may develop, either individually or jointly with one or more permittees or copermittees, a watershed improvement plan that addresses major sources of pollutants in receiving water, stormwater, urban runoff, or other surface runoff pollution within the watershed or subwatershed to which the plan applies. The principal purpose of a watershed improvement plan is to implement existing and future water quality requirements and regulations by, among other things, where appropriate, identifying opportunities for stormwater detention, infiltration, use of natural treatment systems, water recycling, reuse, and supply augmentation; and providing programs and measures designed to promote, maintain, or achieve compliance with water quality laws and regulations, including water quality standards and other requirements of statewide plans, regional water quality control plans, total maximum daily loads, and NPDES permits.

(b) The process of developing a watershed improvement plan shall be open and transparent, and shall be conducted consistent with all applicable open meeting laws. A county, city, special district, or combination thereof, shall solicit input from entities representing resource agencies, water agencies, sanitation districts, the environmental community, landowners, home builders, agricultural interests, and business and industry representatives.

(c) Each county, city, special district, or combination thereof shall notify the appropriate regional board of its intention to develop a watershed improvement plan. The regional board may, in its discretion, participate in the preparation of the plan. A watershed improvement plan shall be consistent with the regional board’s water quality control plan.

(d) A watershed improvement plan shall include all of the following elements relevant to the waters within the watershed or subwatershed to which the plan applies:

(1) A description of the watershed or subwatershed improvement plan area, the rivers, streams, or manmade drainage channels within the plan area, the agencies with regulatory jurisdiction over matters to be addressed in the plan, the relevant receiving waters within or downstream from the plan area, and the county, city, special district, or combination thereof, participating in the plan.

(2) A description of the proposed facilities and actions that will improve the protection and enhancement of water quality and the designated beneficial uses of waters of the state, consistent with water quality laws and regulations.

(3) Recommendations for appropriate action by any entity, public or private, to facilitate achievement of, or consistency with, water quality objectives, standards, total maximum daily loads, or other water quality laws, regulations, standards, or requirements, a time schedule for the actions to be taken, and a description of appropriate measurement and monitoring to be undertaken to determine improvement in water quality.

(4) A coordinated economic analysis and financing plan that identifies the costs, effectiveness, and benefits of water quality improvements specified in the watershed improvement plan, and, where feasible, incorporates user-based and cost recovery approaches to financing, which place the cost of managing and treating surface runoff pollution on the generators of the pollutants.

(5) To the extent applicable, a description of regional best management practices, watershed-based natural treatment systems, low-flow diversion systems, stormwater capture, urban runoff capture, other measures constituting structural treatment best management practices, pollution prevention measures, low-impact development strategies, and site design, source control, and treatment control best management practices to promote improved water quality.

(6) A description of the proposed structure, operations, powers, and duties of the implementing entity for the watershed improvement plan.