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Updated flood study shrinks some 100‑year floodplains in Baker; county considers targeted mitigation
Summary
State consultants presented draft FEMA floodplain maps that incorporate new survey data and reservoir storage; the new model reduces mapped flood extents in several Baker neighborhoods and suggests targeted conveyance projects (Sixth Street box culvert) could further reduce risk; DNRC recommends maps proceed while offering a mitigation assessment to test project scenarios.
Consultants working with the Montana DNRC presented draft floodplain updates for Baker on Oct. 7 that use modern topography and hydraulic modeling to redraw 100‑year and 500‑year flood extents. The updated study explicitly accounted for reservoir storage at Upper Baker Lake, and that inclusion reduced mapped flood extents in several town areas compared with older maps.
Casey, the mapping engineer, displayed HEC‑RAS outputs that show the largest reductions in the areas immediately downstream of the reservoir and along the Sixth Street corridor. Commissioners discussed the possible effect of planned capital work — notably replacement of the Sixth Street box culvert and channel improvements upstream — and whether to delay the mapping process until projects are finished. DNRC staff advised against pausing: they said the mapping project must maintain momentum with FEMA, but they offered to run limited mitigation scenarios at no cost (a mitigation memorandum) to model how a specific county project would change local flood elevations and extents.
Commissioners directed staff to supply simplified scenarios (for example: partial liner installation, full liner, or concrete channel conversion through the Sixth Street passage) so the consultant can run comparative tests before the maps are finalized and submitted to FEMA for review. DNRC staff also noted that large, comprehensive projects (e.g., the north tributary improvements) are better handled with a Letter of Map Revision (LOMR) after construction and do not typically justify delaying the statewide study.
DNRC and county staff agreed to share model files and survey data once FEMA clears the submittal and to follow up with technical assistance for limited mitigation assessments that will inform capital planning and grant decisions.
