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Boulder Falls closed after large rockfall; geotechnical report finds high hazard, costly mitigation
Summary
A November rockfall that deposited multi‑ton boulders at Boulder Falls prompted a geotechnical review that found exceptionally hazardous slope geometry and recommended that the city not reopen the trail in its prior alignment; mitigation could exceed $7 million and options include moved viewing platforms or permanent closure.
The Boulder Falls trail and viewing area in Boulder Canyon have been closed since a large rockfall event over Veterans Day weekend and a subsequent geotechnical analysis shows a continued high risk of rockfall and severe injury to visitors.
Visitor Infrastructure Senior Manager Hillary Deese told the Open Space Board of Trustees that a consultant’s analysis found multiple hazard mechanisms in the northern cliffs and gully above the falls, including toppling wedges, differential erosion, overhangs and launch features. The consultants applied the Colorado Department of Transportation rockfall hazard framework and rated the slope in the highest hazard categories across multiple metrics.
Deese said rockfall from the event included very large boulders — consultants reported bounce heights up to 17 feet — that traveled down to the creek and, in at least one instance, “bounced” up the opposite bank. Gabion retaining baskets installed in previous decades were destroyed or displaced; one basket “vaporized” under impact and dispersed fill material down the slope, she said. The 2018 post‑flood reconstruction that reopened the trail cost about $1.23 million, Deese added.
Consultants outlined mitigation methods — anchored rock rods and meshes, targeted rock scaling, draped mesh and rock‑fall fences — but Deese said any realistic stabilization would be technically complex, require helicopter and specialty contractors, and demand ongoing maintenance. The consultants’ upper‑end cost estimate for a full suite of protections and stabilization approached $7 million, Deese said.
Given the hazard rating and the site’s heavy visitation (OSMP recorded 261,000 visits at the falls in 2023), Deese told trustees the city cannot restore the trail to its previous alignment. Staff are studying alternatives: relocating the main viewing platform farther back toward the canyon mouth, building a raised/tiered viewing structure at safer locations, or permanently closing the managed trail in that segment. Staff plan to collect additional targeted geotechnical data for specific viewing options and to return to the board in early 2026 with a recommended alternative and cost estimate.
Trustees and staff discussed public outreach and how to explain the science and the public‑safety reasoning to the community. Deese said staff will release more of the technical findings and noted the site remains closed and unremediated. She asked the public to respect the closure for safety and rescue‑personnel considerations.
