Setbacks in Lemhi County, Idaho

One rule for the Salmon and Lemhi Rivers, a different tiered rule for everything else

Lemhi County's development code treats the Salmon and Lemhi Rivers as a specific, named category: new development must stay at least 25 feet from the average annual high-water mark, and the regulatory "stream corridor" for these two rivers extends to cover the full mapped special flood hazard area plus associated wetlands and riparian areas (see the county hub for that fuller corridor discussion). That's the rule most buyers hear about, since river frontage on the Salmon or Lemhi drives so much of the county's land value.

Every other stream in the county -- a smaller tributary, a seasonal drainage, an unnamed creek crossing a ranch parcel -- falls under a separate, tiered standard in the same development code, and which tier applies depends on the physical shape of the channel itself, not the name of the waterway. For a stream with a well-defined channel, or one with a poorly defined or braided channel, the setback is the same 25 feet from the average annual high-water mark used for the named rivers. But for an incised channel or ravine where the stream bottom is narrower than 50 feet, the setback increases to 30 feet -- and, critically, it's measured from the top of the bank rather than the high-water mark. Those are two different reference points on the ground, and confusing them on a narrow, steep-sided drainage could put a proposed building envelope in the wrong place entirely.

Why the reference point matters more than the number

On a wide, gently sloped stream, the average annual high-water mark and the top of the bank are often close enough together that the distinction barely matters. On a narrow, incised ravine -- common in Lemhi County's mountainous terrain, where small drainages cut steep-sided channels well below the surrounding grade -- the top of the bank can sit meaningfully farther from the water's actual edge than the high-water mark would. Getting this wrong in either direction has real consequences: measuring from the high-water mark on a channel that actually qualifies as an incised ravine could put a structure 5 feet closer to the drainage than the code allows, while over-measuring from the wrong point could needlessly shrink a buildable envelope on an otherwise developable lot.

Before finalizing a site plan near any stream, drainage, or ravine on a Lemhi County parcel -- not just the Salmon or Lemhi Rivers -- have the county (or a qualified surveyor) confirm which channel category applies and which specific point on the ground the setback should be measured from.

Irrigation ditches: no county setback, but check state law separately

One specific exception is worth flagging directly for a ranching-and-irrigation county like Lemhi: the county's own building permit materials note that irrigation ditches are not subject to any county setback requirement at all. That doesn't mean an irrigation ditch crossing or bordering your property is unregulated in every sense -- the same materials direct applicants to Idaho Code 42-1102, the state statute governing ditch and canal rights-of-way, for the separate question of what rights a ditch operator or easement holder may have to access and maintain that ditch across your land.

In practical terms: the absence of a county-mandated building setback from a ditch does not mean you can build right up to it without consequence. Confirm both the county's setback position and any existing ditch easement or right-of-way recorded against the parcel's title before finalizing where to place a structure near an irrigation ditch.

Practical steps

1. Identify every waterway on or near the parcel -- named river, tributary, seasonal drainage, or irrigation ditch -- since each can carry a different setback rule or none at all.
2. For the Salmon or Lemhi Rivers specifically, confirm the setback against both the 25-foot high-water-mark standard and current floodplain mapping (see the county hub).
3. For any other stream, ask Lemhi County Building Department to classify the channel type (well-defined, poorly defined/braided, or incised/ravine under 50 feet) before assuming which setback and measurement point applies.
4. For a parcel with an irrigation ditch, confirm there is no county setback but separately check the recorded title for any ditch easement or right-of-way, and review Idaho Code 42-1102 for the ditch operator's statutory rights.