Teeming with raw sewage, animal waste and fertilizer runoff, yet responsible for half the Chesapeake Bay’s fresh water, the Susquehanna River is the most endangered river in the United States, according to a report released this week by American Rivers, a national conservation group.

Pennsylvania’s massive Susquehanna, the bay’s biggest tributary, tops a list of 10 American rivers with uncertain futures, including Ohio’s Little Miami River, Tennessee’s Roan Creek and South Carolina’s Santee River. Most of them, said American Rivers President Rebecca R. Wodder, are befouled by raw sewage and urban and farm runoff — problems made worse by population growth and cuts to federal cleanup money.

The Kansas River has been on the list of endangered or threatened rivers several times in the last decade, in 1995, 1996, 1998 and 2002, when it was rated fourth.

The Susquehanna exemplifies “the crisis we’re highlighting in this report, that when it rains in America, raw sewage pours into our rivers and streams — in an increasing amount,” she said.

Uphill and upstream from the bay, thousands of acres of Pennsylvania crop- and pastureland abut the banks of the Susquehanna or the creeks and streams that feed it. The farms are so small and far-flung that state environmental agencies struggle to bring all of them into compliance with laws governing pollutant runoff.

The report also cites runoff from urban centers and the hundreds of sewer pipes that overflow in rainy weather, spewing human waste and garbage into the river.

Fertilizer nutrients and filth from the Susquehanna is a chief cause of the algae blooms and pollution that suck oxygen from the Chesapeake Bay, decimating crab, oyster, fish and aquatic plant populations. A University of Maryland consultant has estimated that the river contributes 40 percent of the nitrogen and 20 percent of the phosphorus — two key ingredients in fertilizers — to the bay.