Freshwater Partners presents updated Yellowstone channel migration map to Land Use Board

Montana Freshwater Partners representative Jeanette presented the updated 2024 Yellowstone Channel Migration Map to the Livingston Land Use Board at its Feb. 24, 2025 meeting, describing how the map shows historic channel movement, areas likely to erode in the future and locations prone to avulsion during large floods.

The presentation explained that the map is an update to a 2009 product, was funded by a DNRC grant awarded in 2023 and incorporates aerial imagery dating to the 1940s. Jeanette said the map distinguishes the historic migration zone (where the channel has been), a calculated erosion buffer (100 times the average annual lateral migration rate) and avulsion hazard areas where the river is likely to cut new side channels. "Floodplain maps are regulatory. Channel migration maps are not," Jeanette told the board, adding that the migration map is intended to inform decisions rather than impose regulation.

Why it matters: the map highlights where lateral movement and bank erosion, rather than simple inundation, threaten homes, roads, bridges and utilities. Jeanette cited the 2022 flood as a driver for the update and showed how the channel shifted during that event (red vectors on the map) compared with longer-term movement (yellow vectors). She also noted that since about 2020 Park County has added roughly 8.5 miles of bank armoring, which tends to narrow the river channel and increase velocity and erosion pressure elsewhere.

The presentation described new features in the 2024 map: a geotechnical hazard overlay marking over‑steepened bluffs (with a 2:1 setback applied), an inventory of existing armoring (riprap, dikes, flow deflectors) and the restricted migration areas behind armoring. Jeanette explained methodology: analysts mapped banklines from successive historic aerial photos, calculated average annual migration rates, multiplied that rate by 100 to generate an erosion buffer, and separately identified avulsion-prone low-lying swales. She said the mapping team included Tony Thatcher and Karen Boyd and that the map is available as downloadable GIS data and an interactive web mapper hosted via the Montana State Library and the Upper Yellowstone Watershed Group.

Board members asked how the channel migration map relates to FEMA floodplain maps. Jeanette said FEMA's 100-year flood maps use hydraulic modeling and streamgage data and are updated separately; channel migration zones can lie outside the regulated FEMA floodplain and therefore are complementary but distinct. She said a FEMA map update and public draft are expected to be released in the coming year, with finalization later in the multi‑year process.

Local implications cited during the meeting included the East End of Livingston, where the hospital, two stacked bridges and railroad infrastructure constrain the channel and create avulsion pathways. Jeanette noted Miles Park as an example of using open space to provide a "pressure relief valve" for the river. She also described undersized culverts and conveyance constraints under the highway and said those features can trap floodwaters in low-lying parcels; planners could consider improving conveyance or preserving open space to reduce damage.

Questions from board members prompted a brief overview of permitting for bank work in Montana: Jeanette said landowners typically need a Section 404 permit from the U.S. Army Corps of Engineers (work below the ordinary high water mark), a conservation district (310) permit, a DEQ 401 water quality review and, where applicable, a local floodplain permit. She emphasized that erosion mitigation is often costly and may not be a practical long-term solution for buildings located in active migration zones.

The presentation closed with practical notes: the entire GIS dataset and an interactive map are publicly available; users can search by address (for example, Carter's Bridge) and review historic channel positions; and Freshwater Partners and Tony Thatcher are available for follow-up. Jeanette provided contact information and the board moved next into zoning business.

The map and data are intended as planning tools for the city and county; Jeanette and the board discussed potential next steps including targeted feasibility or conveyance studies and grant applications to fund detailed engineering for constrained reaches.