The Red River is formed by the confluence of the Da, Thao and Lo Rivers at Viet Tri upstream of Ha Noi. The Chay and Gam Rivers are tributaries of the Lo River, all rising in China. The Day River is the first of several distributaries that form the Red River Delta. It branches downstream of Son Tay and flows south of Ha Noi. The Cau, Thuong and Luc Nam Rivers are tributaries of the Thai Binh River. They rise within Vietnam, where rainfall is lower, and discharge to the Gulf of Tonkin. The Duong and Luoc Rivers, distributaries of the Red River, discharge to the Thai Binh River.