Committee hears 2023 water-year briefing: rising streamflow, declining sediment concentrations

Staff presented a 2023 water-year summary of streamflow, sediment and phosphorus from three gauging stations (North Fork, South Fork and Parmenter Street), noting a trend toward higher runoff volumes even during years with below-average precipitation.

Presenting data, Speaker 1 summarized key figures: North Fork sediment load about 196 tons in 2023, South Fork about 113 tons, and Pheasant Branch about 226 tons. Phosphorus loads were reported at roughly 3,600 pounds for the North Fork and about 780 pounds for the South Fork. "The Confluence Pond trapped 48% of the sediment and 33% of the phosphorus in 2023," Speaker 1 said, citing USGS-derived monitoring results and long-term trapping averages.

The presentation highlighted that rising annual flow volumes have not translated to higher concentrations of sediment and phosphorus, a trend attributed in part to the Confluence Pond and other detention features. Committee members asked about dredging history and where dredged material has been placed; staff replied dredged material was taken to an offsite deposit area. The committee discussed whether the South Pond could be altered (deeper or different invert) to improve trapping efficiency, but members noted potential downsides for flood protection and funding complications.

Budget implications were discussed: Speaker 1 recommended considering moving gating-station funding from soon-to-expire TIF revenues to the stormwater utility (an estimated $35,000 per year shift) once TIF funds sunset. Members noted timing uncertainties tied to a possible referendum and parallel-budget needs pending council decisions.

The briefing concluded with committee agreement that the Confluence Pond has materially reduced sediment and phosphorus loads over the long term, and a recommendation to review funding options for continued monitoring and possible pond improvements.