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Conservancy presentation challenges sediment modeling for Lake Malibu drawdown
Summary
Wild Rivers Conservancy told the committee the Stantec sediment model likely underestimates dry bulk density and phosphorus transport; the group urged direct site sampling before any drawdown proceeds.
At the Community Development Committee's special meeting, Mark White of the Wild Rivers Conservancy presented technical slides questioning assumptions in Stantec's sediment-transport modeling and urged that direct dry bulk-density sampling at Lake Malibu be completed before any authorization is final.
White told the committee that the dry bulk density factor used in Stantec's calculations was not studied directly in the cited analog and that using a bulk density between 1.55 and 1.75 g/cm3 would change downstream phosphorus-load estimates dramatically. "We strongly question the rationale behind using Stantec's dry bulk density number and would advocate for you to rescind your authorization until at the very least this type of dry bulk density study can be done," he said.
White also shared an email from Adam Heathcote (director, St. Croix watershed research station) stating that Lake Pepin is not a defensible analog for Malibu sediments and that site-specific bulk-density measurements would be the most defensible approach. Committee members asked staff about monitoring plans and where modeled phosphorus would travel once it reached the Saint Croix; staff said current modeling addresses load to the dam and that downstream transport remains a key uncertainty.
Committee action to direct staff to rescind authorization followed the presentation; the Conservancy said rescission would allow time for targeted, site-specific sampling and analysis before any drawdown proceeded.
