Euphrates

4 clues
Major Iraqi river
Lake Assad is on it
River to the Tigris
Syrian irrigation source
Similar answers:
Accelerates
Adrates
Adulterates
Aerates
Ameliorates
Apirates
Arbitrates
Aspirates
Bankrates
Barbarypirates
Baserates
Berates
Beratesmark
Bilkrates
Birthrates
Bohydrates
Borates
Bureaucratese
Calibrates
Carbohydrates