Twin Falls County, ID Environmental Rules: Stormwater Management (2026)

The core rule sits in Title 10, Chapter 2, the county's subdivision regulations. Section 10-2-9 puts the burden squarely on the developer: grading has to be done and drainage structures installed as the county decides are essential to move storm water off the site. Section 10-2-10 backs that up procedurally, making the subdivider responsible for all engineering work while the County Engineer reviews what is submitted before the plat can move forward. Where a subdivision is crossed by a watercourse, drainageway, channel, or stream, section 10-2-7(J) requires an easement wide enough to carry the water and to build and maintain the waterway itself, and section 10-2-6(5) ties any drainage design back to the requirements of the canal company or irrigation district serving the land.

Planned unit developments carry a tighter standard: section 10-3-11(A)(4) requires that storm water generated inside a PUD be contained on-site rather than pushed onto neighboring land, and section 10-3-14(C) requires exposed ground within a PUD to be covered with vegetative material, stone, or similar cover and graded to handle expected surface runoff, controlling soil erosion as part of the same requirement. Outside subdivisions and PUDs, commercial zone development answers to section 8-6E-5(C), which requires on-site detention of storm water runoff engineered to standard practice and consistent with local, state, and federal rules before the county signs off.