ST. PAUL — The St. Croix River is still in good health but is facing pollution threats, according to two new reports.

Popular for its scenic beauty and recreation opportunities for boaters and anglers, the St. Croix remains one of the cleanest tributaries to the Mississippi River, thanks in part to a large forested watershed that helps filter out contaminants.

But the reports highlight some troubling trends, including increasing development that leads to more polluted runoff entering the river, Minnesota Public Radio News reported.

The St. Croix flows more than 160 miles before joining the Mississippi River at Prescott, Wis., forming a large part of the border between Minnesota and Wisconsin. It was one of the first rivers in the United States to be designated for protection under the federal Wild and Scenic Rivers Act in 1968.

The Minnesota Pollution Control Agency’s study of the river’s health finds much to be positive about, said Pam Anderson, who manages the MPCA’s surface water monitoring program.

“The St. Croix River is a beautiful river, (with) lots of intact natural shorelines,” Anderson said. “We have relatively good water quality. We see low levels of bacteria. We have excellent fish communities and good bug communities.”

Much of that cleanliness can be attributed to the scenic river protections and the heavily forested watershed, said Monica Zachay, program director for the St. Croix River Association. which released its first “State of the St. Croix River” report.

“Forests are really good for water quality,” Zachay said. “They’re our best defense against water quality degradation as when it rains. Those natural ecosystems soak in the water into the ground and they act as natural filters.”

However, the reports highlight some threats to the river, including too much phosphorus from farm and urban runoff, which can cause algae growth. The stretch of the river from Taylors Falls to Lake St. Croix is considered impaired because of phosphorus levels.

Cities and industries discharge stormwater and treated wastewater into the river, which can contain phosphorus and other pollutants. That includes chloride from road salt or water softeners. Levels of chloride in the St. Croix are relatively low but rising, according to the MPCA report.

“We are adding more chloride to the system than should be there, and that’s something that can’t really take out of the water,” Anderson said. “So it’s imperative that we do what we can to minimize what goes into the system.”

Urban stormwater also can raise the temperature of streams, which can threaten fish and other aquatic life.