Beaver is a borough in and the county seat of Beaver County in the U.S. state of Pennsylvania. It is located at the confluence of the Beaver and Ohio Rivers, approximately northwest of Pittsburgh. As of the 2010 census, the borough population was 4,531. The borough is a Tree City USA community.Robert Linn was the mayor of Beaver for 58 years (1946 - 2004), making him one of the longest serving mayors in the United States. The borough was the home of influential U.S. Senator Matthew "Boss" Quay.HistoryThis area around Beaver was once home to Shawnee Indians, who were later displaced by groups such as the Mingoes and the Delawares. It was part of the Ohio Country that was in dispute during the French and Indian War.Beaver became the site of Fort McIntosh, a Revolutionary War era Patriot frontier fort. After the war, the fort was the home of the First American Regiment, the oldest active unit in the US Army. The fort was abandoned in 1788 and razed a short time later. By then, the frontier had moved westward and there was no further need for a permanent garrison to protect the area.