Updated Yellowstone channel-migration map shows erosion and avulsion risks, warns against riprap as default fix

Jen Blank and Wendy Weaver of Montana Freshwater Partners presented an updated 2024 channel-migration map for the Upper Yellowstone River to the Park County Planning Board on Feb. 20, describing lateral migration hazards, avulsion pathways and a new geotechnical-hazard overlay.

Blank said the product replaces a 2009 map and covers the Yellowstone from Gardiner to Springdale; it was funded through a Department of Natural Resources and Conservation grant awarded to Park County in 2023. The mapping identifies where the river has occupied its floodplain historically, computes average annual lateral migration rates and produces an "erosion hazard" buffer (the presenters said the buffer is the average annual migration rate multiplied by 100 to provide a consistent planning margin).

Blank described three main map products: the historic channel-migration zone (where the river has moved over the aerial record), erosion-hazard corridors (the averaged-rate buffer), and avulsion-hazard areas (low-lying swales and past channels where the river could cut a new channel). The 2024 update also adds a geotechnical overlay that flags over-steepened banks that are prone to landsliding or rapid lateral retreat.

Blank stressed the map is nonregulatory and intended for planning and risk-communication. She said the map is available as downloadable GIS data and as an interactive viewer via the Montana State Library and that the county GIS office already has the files.

Presenters warned that hard bank armoring (riprap, dikes and levees) is often a short-term or expensive response that can accelerate erosion on adjacent reaches and can be flanked by the river, leaving property owners liable for ongoing maintenance. Blank said many recent post-flood riprap installations have subsequently failed or worsened erosion at neighboring parcels.

Wendy Weaver described the Park Co. Water Initiative, a Freshwater Partners effort funded by a Bureau of Reclamation WaterSMART grant, which assembled watershed partners and identified more than 50 candidate projects aimed at increasing watershed resilience. Weaver told the board that federal funding freezes had put some near-term grant activity on hold.

Board members and county staff discussed permitting and regulatory limitations: county floodplain permitting evaluates inundation (FEMA floodplain criteria) but does not regulate channel-migration hazards directly. County staff confirmed adjacent-landowner notice requirements for floodplain permits but agreed channel-migration mapping is a separate, nonregulatory planning tool.

Blank and Weaver encouraged landowners and the county to use the mapping before subdividing riverfront parcels and offered technical contacts (including Tony Thatcher, consulting geomorphologist) for parcel-specific questions and project development. The presenters said the interactive map, full report and downloadable GIS layers are available on the Montana State Library site and the Freshwater Partners/Upper Yellowstone Watershed Group web pages.

Ending: Blank and Weaver offered to provide presentation files and to brief other county and city boards; the planning board asked staff to share the map and report with the board and with real-estate and permitting offices.