Freshwater Partners presents updated Yellowstone channel-migration map, urges planners to factor erosion risk into development

Jeanette, a representative of Montana Freshwater Partners, presented the updated 2024 Yellowstone Channel Migration Map to the Livingston City Land Use Board on Feb. 24, 2025, describing it as an update to the 2009 map that incorporates movement from the 2022 flood and a DNRC grant the county received in 2023.

The map shows the historic migration zone of the Yellowstone River, a calculated “erosion hazard” buffer (described as the average annual lateral migration rate multiplied by 100), avulsion-hazard areas where the river could cut new side channels, and a new geotechnical overlay that flags over-steepened bluffs. Jeanette said the map’s purpose is to give landowners and planners additional information about lateral river movement and erosion — distinct from floodplain mapping — so they can make informed decisions about property siting and bank stabilization measures. “Floodplain maps are regulatory. Channel migration maps are not,” Jeanette said during the presentation.

Why it matters: channel migration identifies where the river has historically moved and where it is likely to move in the future, highlighting erosion risks that can undermine riprap, roads, bridges and utilities and that are not always evident on FEMA floodplain maps. Jeanette used Livingston’s east end as an example, showing concentrated armoring and confined reaches near the hospital and bridges, and recommended preserving open spaces such as Miles Park as a flood/erosion relief zone where possible.

Details from the presentation: the mapping used aerial imagery going back to the 1940s to trace banklines over time, producing directional vectors that show both direction and distance of lateral movement; yellow arrows show historical migration; red arrows show movement during the 2022 flood. The erosion buffer is an analytical construct based on the average annual migration rate multiplied by 100; Jeanette cautioned this is a planning buffer and not a literal “100 years” guarantee. The map also identifies existing armoring (riprap, dikes, flow deflectors), and the project team marked restricted-migration areas adjacent to armoring.

Jeanette highlighted problems that follow repeated armoring and channel confinement: increased velocity and erosive force, higher maintenance costs, and instances where riprap is later undermined in floods. She said Park County has added roughly 8.5 miles of bank armoring since about 2020 and that the Yellowstone reach through Livingston is a confined stretch with stacked bridges and rail constraints that increase risk of undercutting and avulsions. She noted that the interactive map and GIS data are publicly available (the map can be found through the Montana State Library or the Upper Yellowstone Watershed Group interactive viewer) and that visitors can search an address to see the overlays.

Board members asked how channel migration maps relate to FEMA floodplain maps and to future FEMA updates; Jeanette explained FEMA’s flood maps are based on hydraulic modeling and streamgage data and are a separate, regulatory product. She said draft FEMA map updates for the area are expected to be released for public review starting in 2026 and may be finalized later; channel-migration zones can extend beyond FEMA-regulated floodplain boundaries and therefore show additional erosion exposure not captured by FEMA mapping.

Permitting context mentioned during questions: Jeanette summarized the common permits a landowner would need for bank work in Montana — a Section 404 permit from the U.S. Army Corps of Engineers (for fill below the ordinary high water mark), a 310 permit administered through the county conservation district, a 401 water-quality review through DEQ, and coordination with Montana Fish, Wildlife & Parks; regulated floodplain work also requires a local floodplain permit and a no-rise certification where applicable.

Jeanette encouraged the board, developers and landowners to use the map as an early screening tool to avoid creating a false sense of security through localized armoring and to consider open-space, conveyance-improvement and bridge/culvert strategies to reduce long-term cost and risk. She offered contact information and said Tony Thatcher and Karen Boyd (the project geomorphologists) are available to answer technical questions.

No formal board action was taken during the presentation; the map was presented as information for planners and the public.

Ending: Jeanette closed by noting interactive and downloadable GIS resources are online and by offering to follow up with board members who had technical questions.