Ada County, ID Environmental Rules: Erosion Control (2026)

Section 8-2-30(E)(6), the Hillside Overlay's Sediment standards, states flatly that "all construction must meet current EPA and IDEQ erosion and control rules," layering federal and state sediment-control law directly into the county's hillside grading permit. " Alterations of major watercourses are prohibited outright except for approved roadway crossings and drainage structures, and natural or improved open channels in those watercourses must be preserved rather than piped, again except where a roadway crossing requires a conduit. These sediment rules sit alongside the chapter's broader vegetation and stabilization standards, which require an effort to conserve topsoil removed during grading for later revegetation, and require every disturbed area to carry vegetation or a chemical or mechanical stabilization method sufficient to hold the soil as each grading stage finishes.

Large hillside tracts must be divided into workable units that can be finished within a single construction season so the county is not left with bare, exposed soil through the winter-spring runoff period, and any disturbed soil surface not stabilized before December 1 needs a temporary erosion treatment if the permanent surface, such as a roadway or driveway, is not ready by then. Roadway drainage facilities cutting through hillside developments must also be designed to handle the 100-year design storm, and watercourse discharge points below culverts must be riprapped or otherwise stabilized to keep the outflow from eroding the channel.