High Quality Content by WIKIPEDIA articles! The Beaver Kill, sometimes written as the Beaverkill or Beaverkill River, is a tributary of the East Branch of the Delaware River, approximately 44 miles (71 km) long, in the U.S. State of New York. The kill drains a 300-square-mile (780 km2) area of the Catskill Mountains and has long been celebrated as one of the most famous trout streams in the United States. Its preservation helped establish many of the basic conservation principles of rivers in the United States.