Jonathan Guy, left, Calli Murray and Cam Nadeau learn the ins and outs of a dragonfly nymph from naturalist Susan Hayward during the Edward Little High School River Day at the David Rancourt River Preserve in Lewiston on Friday. In two days, more than 250 students collected data on the health of the river. Science teacher Karen Boucher, who grew up in Lewiston, has held "river day" each year since 2008. "I see a lot more recreational use on the river now," she said.

These dragonfly nymphs, top three, are a good sign for the Androscoggin River, naturalist Susan Hayward told students at the Edward Little High School River Day at the David Rancourt River Preserve in Lewiston on Friday. Science teacher Karen Boucher concurred: "Macroinvertebrates are a great indicator of the river."

Osman Mohamed, 16, measures the angle of the slope along the Androscoggin River bank during the Edward Little High School River Day at the David Rancourt River Preserve in Lewiston on Friday. Calli Murray, 15, is at right.

Jonathan Guy, 15, gathers Androscoggin River water to sample during the Edward Little High School River Day at the David Rancourt River Preserve in Lewiston on Friday.

Jeff Pengel, right, of Durham helps Cam Nadeau, 15, pour water into a turbidity tube during the Edward Little High School River Day at the David Rancourt River Preserve in Lewiston on Friday. Pengel, a volunteer with the Maine Master Naturalist Program, helped students determine the cloudiness or haziness of the water. Students, from left, are Jonathan Guy, Calli Murray, Nadeau, Osman Mohamed, Joseph Winn, Snow Chan, Maria Hawley and Katelyn Valliere, hidden.