His stenciling project is one of many nationwide, said Gillian Grossman, the program coordinator for Save the Harbor, Save the Bay, a Boston conservationist group.

Stenciling has become an effective way to let residents know their actions have consequences, she said.

"It helps people realize that what goes down these drains ends up in our waterways, like the Charles River, and eventually in Boston Harbor," Grossman said.

Other Boston-area groups, like the Charles River Watershed Association, have also been stenciling catch basins throughout the year.

The popularity of this movement can be attributed to a shift in focus on the part of local activists.

In past years, environmental groups targeted companies dumping pollutants in both the Charles River and the Boston Harbor.

Now, with the river and harbor much cleaner, storm drain pollution is the main source of pollution in the Charles River, she said. As a result, activists are trying to get residents involved in the movement.

"People don't realize that the drains may empty into the Charles River," said Kris Finn of the Charles River Watershed Association. "Even if people live a mile away, their storm drains may still empty in the river."