In each of the state’s 15 major river basins, state officials keep track of where and how phosphorus, a major water pollutant, enters the waterway. They use categories including developed lands, agriculture, wastewater and natural resources.

In many of the basins, agriculture and developed lands are major sources of phosphorus. But in the Winooski River basin — even before the floods of July 2023 — the largest source of phosphorus came from eroding river banks. 

The Winooski River basin is the largest of the state’s 15 watersheds. Its waters pass through 50 towns before it empties into Lake Champlain, which means that activities in each of those towns can impact both the Winooski River and the lake. 

Every five years, state officials with the Department of Environmental Conservation publish water quality data for each watershed in sprawling documents called tactical basin plans. Each plan details the progress each watershed has made toward improving water quality. 

The draft plan for the Winooski River basin is the latest to be published. It became available on Oct. 10, and the public can comment on the plan until Nov. 10. 

River erosion accounts for nearly 34% of the river’s phosphorus load, according to the plan. The watershed’s rivers contribute nearly 57,600 kilograms (about 127,000 pounds, or 57,600 one-liter soda bottles) of phosphorus per year to Lake Champlain. 

The data included in the plan was collected before the flooding in July 2023. Basin planning is a yearslong process, and state officials were finalizing a draft of the Winooski plan when the flooding hit. 

Still, officials estimate that the Winooski River carried 130% more phosphorus into Lake Champlain in the seven days following the storm than it typically carries in one year. 

Phosphorus, a naturally occuring nutrient, can cause toxic cyanobacteria blooms when too much of it exists in water bodies. The blooms often close Burlington area beaches in the summer months and have caused problems in lakes and ponds across the state.

To meet federally mandated water quality goals for Lake Champlain, state officials who work on watersheds in the greater Lake Champlain Basin typically focus their plans on reducing phosphorus. 

Forest management also contributes a significant amount of phosphorus in the Winooski River Basin, at 38,084 kilograms of phosphorus (about 38,084 one-liter soda bottles) per year, followed by developed lands at 42,733 kilograms per year, agriculture at 32,048 kilograms per year and wastewater at 24,358 kilograms per year. The agricultural sector has made the most progress toward water quality goals, according to Keith Fritschie, the planner assigned to the Winooski River basin. 

Currently, more than 1,000 clean water projects are in progress in the basin. Fritschie said that many of the projects are focused on restoring rivers to their natural state watershed after they have been altered for human uses. 

“A really important co-benefit of those sorts of water quality focused projects is flood resilience,” Fritschie said. 

In 2022, the Environmental Protection Agency concluded that the state was on track in its efforts to implement the five-year goals described in the previous version of the Winooski River basin plan.