USGS presentation: Pheasant Branch supplies about 15% of Lake Mendota phosphorus; Confluence Pond traps roughly 30%

Todd Stuntebeck, a hydrologist with the U.S. Geological Survey(USGS) Upper Midwest Water Science Center, presented the agency—s monitoring data for Pheasant Branch and other tributaries to Lake Mendota at the Middleton stormwater committee meeting. He said USGS—s Parmenter Street gauge has daily flow records dating to 1975 and that the agency—s measurements focus on phosphorus because of its role in algal growth.

Stuntebeck said the four primary tributaries feeding Lake Mendota delivered roughly 55,000 pounds of phosphorus over the 2024 water year and that, using a 30-year average and modeled estimates to fill data gaps, Pheasant Branch accounts for about 15% of the annual load. "Pheasant Branch contributes about 15% of the overall, annual load to the lake," he said.

Why it matters: phosphorus drives nuisance algal blooms and beach closures in the Madison-area lakes. Stuntebeck emphasized that a combination of weather variability and long-term land-use change complicates trend detection, so the USGS uses flow-normalized analyses and multiple gauges to separate climate-driven spikes from underlying trends.

Key findings and methods

Stuntebeck explained USGS monitoring practice: continuous stage measurements at gauges paired with periodic discrete discharge measurements and automated refrigerated samplers for water chemistry. Samples are selected toward event peaks when most load is delivered; per-sample analysis for total phosphorus, orthophosphate and suspended sediment costs about $100 (roughly $30 per analyte). He said the USGS typically submits about 100 samples per site per year (more in wet years) and that samples are analyzed at the State Lab of Hygiene or the Madison Metropolitan Sewage District laboratory.

On trapping, Stuntebeck reported that the Confluence Pond—s average trapping efficiency (2012—2024) is roughly 30% for phosphorus (about 3,300 pounds per year) and about 55% for sediment (about 700 tons per year). "On average, between 2012 and 2024, we trapped about 3,300 pounds per year," he said, and he noted substantial year-to-year variability: dry years can show trapping approaching 45—50%, while very wet years can reduce trapping to roughly 20%.

Limitations and interpretation

Stuntebeck cautioned the committee that some patterns visible in recent years are regional and not unique to sites with digesters; he said he could not attribute observed reductions solely to a nearby manure digester because similar downward trends appear at ungauged or non-digester basins. He also said that dissolved phosphorus comprises a large share of load (about 40% of trapped phosphorus in his analysis) and that particle-trapping structures will not remove dissolved forms.

Costs, contracts and next steps

Stuntebeck summarized funding arrangements: the City contributes about $15,000 per year toward the Parmenter gauge through the Capital Area Regional Planning Commission (CARPC); USGS provides matching funds; sample analysis historically has been paid by DNR but the five-year CARPC agreement expires at year-end and contract terms and per-site costs are being renegotiated. He warned committee members to expect updated estimates when the next contract is proposed.

The committee asked for follow-up analyses and noted interest in additional nitrogen monitoring in some basins and in revisiting trapping and dredging studies for the Confluence Pond. Stuntebeck offered to provide data and to coordinate on contract discussions.

What's next

Committee members said they would consider adding a sediment survey and dredging discussion to a future agenda and requested clearer cost projections for monitoring and sample analysis before contract renewal. The committee also discussed communicating monitoring results to residents but raised timing concerns about linking older results to recent fee increases.

Sources: USGS presentation to Middleton stormwater committee; USGS data and analysis presented by Todd Stuntebeck.