Bonneville County, ID Environmental Rules: Stormwater Management (2026)
Key Facts
- Runoff retention
- On-site, from all impervious surfaces
- Design storm event
- 5-year storm
- Idaho Falls Impact Area exception
- 1.5-inch flat rainfall, lots under 0.5 acre
- Borrow pit maintenance
- Adjacent lot owner's responsibility
- Standard type
- Minimum; road/bridge rules can be stricter
Summary
Bonneville County requires runoff from impervious surfaces to be retained on-site, with lots designed to hold a five-year storm event, or a flat 1.5-inch rainfall for small lots in the Idaho Falls Impact Area.
DRAINAGE STANDARDS; DRAINAGE DESIGN AND MAINTENANCE.
(A) Runoff from all impervious surface areas on a lot shall be retained on-site.
(B) The site shall be designed to retain a minimum of a five-year storm event. Exception: In the Idaho Falls Impact Area on subdivisions of less than one-half acre lot size then a flat one and one-half inch rainfall shall be used.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2026 S-1: Ordinance 280-26, passed 1-20-2026).
Full Breakdown
064 requires that runoff from all impervious surface areas on a lot be retained on-site rather than discharged onto neighboring property or roads. 5-inch rainfall design figure instead, reflecting the denser development pattern expected near the city. The section also protects natural drainage: modification of a natural drain channel on a property is only allowed if the flow capacity and discharge point are maintained, preventing a landowner from re-routing water onto a neighbor or overwhelming a downstream channel. 064(D) puts maintenance responsibility on the lot owner adjacent to that right-of-way, keeping the pit functioning as designed on the approved cross-section.
Subsection (E) makes clear these are floor standards only. 064 for a given subdivision or road project, so a developer working near county infrastructure should check both sets of standards before finalizing a drainage plan.
Violations & Fines
Failing to retain on-site runoff, undersizing a lot's stormwater retention below the five-year storm design standard, altering a natural drain channel in a way that reduces flow capacity or changes its discharge point, or neglecting a borrow-pit retention area required under § 156.064, is a zoning violation. Chapter 156's general penalty in § 156.999 makes such violations infractions punishable by a $100 fine, with continuing violations counted day by day.
Frequently Asked Questions
Does Bonneville County require on-site stormwater retention?
Is there an exception for small lots near Idaho Falls?
Can I reroute a natural drainage channel on my property?
Sources & Official References
Other rules in Bonneville County
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