From Boston.com

This is the first time in recent years that blue-green algae has been spotted in the Lakes District of the Charles River, which includes Newton, Waltham and Weston, the association said. However, there have been a number of algae blooms in the river south of Cambridge over the past six years.

The public is being advised to avoid contact with the water as much as possible and wash off after any contact. The advisory does not close the river to boats.