A summer thunderstorm may bring relief from the heat, but it can also send a surge of pollution into the Charles River.

New preliminary data from the Charles River Watershed Association shows that phosphorus levels in the river rise and fall with rainfall and runoff — meaning heavy storms can wash fertilizers, leaf litter and other pollutants into waterways, feeding the algae blooms and low-oxygen conditions that threaten fish and river health.

For Newton residents who kayak, fish, walk the riverbanks or simply enjoy the Charles, the findings highlight a familiar challenge: what happens on land quickly ends up in the water.

"Total phosphorus levels are tied to water flow levels more than anything else," CRWA Communications Manager Carly Sherman wrote in the group's report.

The association spent the past year collecting samples from four locations along the Charles in Waltham, Wellesley, Dover and Medway. Researchers found a strong connection between river flow and phosphorus levels at three of the four sites.

The results depended on location. In Wellesley, higher water flow appeared to dilute phosphorus concentrations. Farther upstream in Dover and Medway, stormwater runoff pushed more phosphorus into the river during rain events.

The Waltham site, closest to Newton, was included in the study, though CRWA did not release detailed findings for that location.

Newton's stormwater system plays a role in what reaches the Charles. The city has about 320 miles of stormwater pipes, 12,750 drains and 384 outfalls that carry water into the river and connected waterways.

Residents pay a yearly stormwater fee of $148.54 to help maintain that system.

Under state and federal stormwater requirements, Newton must reduce the amount of phosphorus entering the Charles River watershed by 2038. The city is already investing in projects designed to capture and filter polluted runoff before it reaches the river.

In June, the City Council approved a $4.3 million low-interest state loan for stormwater improvements at Cold Spring Park, Cabot Dog Park and potentially Braceland Park. The projects are expected to reduce phosphorus runoff by 55 to 66 pounds per year.

The city is also using green infrastructure to work with nature instead of against it. The Cheesecake Brook restoration project near Albemarle Field added underground stormwater storage beneath the new turf field, helping improve water quality while reducing flooding.

"Instead of working against nature, we use what we know about the co-benefits of green infrastructure to take advantage of restoring habitat, water quality, and flood resilience," CRWA stormwater associate Alfredo Con said at a June event highlighting the project.

CRWA said the phosphorus findings are still preliminary and has not announced when the final report will be released.

Meanwhile, Newton is awaiting a response from the EPA after the agency's public comment period closed July 13 on the city's request for more time to complete its phosphorus control plan.