Bannock County, ID Environmental Rules: Stormwater Management (2026)

Bannock County Code § 17.52.540 sets the county's core stormwater standard for development in the unincorporated area: no project may cause downstream property owners, watercourses, channels or conduits to receive runoff at a higher peak flow rate than the same storm event would have produced with the site left in its natural, undeveloped condition. The county engineer and/or building official may inspect drainage facilities while they are under construction, and if the work does not match the approved plans, the county can compel correction, suspend building permits, or pursue enforcement under Section 17.56.030.

A companion provision in the subdivision ordinance, Bannock County Code § 16.28.100, applies the same peak-flow limitation to platted subdivisions and adds design detail: developers must provide on-site or off-site detention for the excess runoff created by new impervious surface, compacted or filled soil, cleared vegetation, or altered drainage ways; detention basins are capped at a maximum planned depth of five feet unless natural ground conditions justify more; outlet structures must limit discharge to the site's natural flow; and an emergency spillway is required unless inflow is otherwise controlled.

Under that same subdivision chapter the developer posts a maintenance bond equal to 110 percent of the facility's development and maintenance cost, held for five years, and remains responsible for upkeep until 80 percent of the development is built and occupied or five years have passed, whichever is longer, before the facilities can transfer to a homeowners' association. Wetlands, riparian buffers within 100 feet of a water body, and steep-slope areas over 30 percent draw additional restrictions under the adjoining natural-features standards in the same chapters.