The joined Delta of Hong River (Red River) and Thái Bình River is a flat, triangular region of 15,000 square kilometers. The Hong River Delta is smaller but more intensely developed and more densely populated than the Mekong Delta. Once an inlet of the Gulf of Tonkin, it has been filled in by the enormous alluvial deposits of the rivers over a period of millennia, and it advances one hundred meters into the Gulf annually. The ancestral home of the ethnic Vietnamese, the delta accounted for almost 70% of the agriculture and 80% of the industry of North Vietnam before 1975.