The Ussuri River is a tributary of Heilongjiang and the border river between China and Russia. The upper reaches are formed by the confluence of the Ula River and the Daubi River. Both rivers originate from the southwestern slope of the Sikhote Mountains and flow northeast to Khabarovsk and Heilongjiang. It is 909 kilometers (565 miles) long and has a watershed area of 187,000 square kilometers (72,200 square miles). The main tributaries are Song Acha River, Muling River, Naoli River and so on. A long section of the main stream caused a Sino-Soviet border dispute in 1969. In 1977, the two sides formulated navigation rules in the Sino-Soviet border waters. The Ussuri River has a freezing period of about 5 months every year.