You might notice some soupy green mats in the lower basin of the Charles River.
Cyanobacteria are one component of these soupy mats, and they make a toxin that can result in a rash, or an upset stomach, if you happen to ingest the water.
So what's causing the harmful cyanobacteria bloom, and what could prevent these blooms?
Marc Nascarella is the state Department of Public Health's chief toxicologist. He says cyanobacteria is always present, but it spikes when there's runoff laced with stuff they like to eat, such as phosporus and nitrogen.
"Those nutrients provide almost like a Petri dish for the bacteria to grow in the waterbody,” says Nascarella.
Yesterday’s storms could be both a help and a hindrance when it comes to the cyanobacteria bloom in the Charles.
That’s because the bloom in the lower basin might get flushed out into Boston Harbor with all the rain, says Elizabeth Cianciola, an aquatic scientist with the Charles River Watershed Association.
On the other hand, Cianciola says, such storms also increase runoff upstream, eventually returning more bacteria-producing nutrients into the river.
She says one key to a healthy Charles is to cut down on nutrient pollution from the runoff.
"Not only does reducing the stormwater runoff remove the nutrient pollution that fuels the growth of the cyanobacteria blooms,” Cianciola says, “but it also recharges the flow in the Charles River. "
The Watershed Association says some of the dozens of cities and towns along the Charles are reducing nutrient pollution from runoff into the river. But they want to see a lot more communities pitch in, and help make the Charles healthier
The DPH says they'll keep testing to give the all clear on the cyanobacteria.