Identify the major river that has supported agriculture activities in modern day syria for nearly 7000 years.

That river rich in history is the Euphrates River.  The Tigris River and Euphrates River gave the ancient name "Mesopotamia" ("Land between the Rivers") to a region that was an ancient cradle of civilization.  The lengthy Euphrates River originates in eastern Turkey, flows through Syria and Iraq, and then joins the Tigris River in the "Shatt al-Arab" ("River of the Arabs") which flows out into the Persian Gulf.