The Day River () is a river in Vietnam. It was formerly known as Hát River ( or ) or Gián Khẩu River (). The river is a distributary of the Red River, draining into the Gulf of Tonkin.
The river has a length of 240 km and has a drainage basin of more than 7,500 km2, flowing through Hanoi, and the provinces of Hòa Bình, Hà Nam, Ninh Bình and Nam Định.
See also Kẽm Trống Bay
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