From around 1900, after the Treaty of Peking (1860) had assigned Outer Manchuria (Primorskiy Kray) to Russia, the eastern part of the Sino-Soviet border had mainly been demarcated by three rivers, the Argun River from the tripartite junction with Mongolia to the north tip of China, running southwest to northeast, then the Amur River to Khabarovsk from northwest to southeast, where it was joined by Ussuri River running south to north. The Ussuri River was demarcated in a nonconventional manner somce the demarcation line ran along the right (Chinese) side of the river, putting the river itself with all its islands in Russia. "The modern method (used for the past 200 years) of demarcating a river boundary between states today is to set the boundary at either the median line ("ligne médiane") of the river or around the area most suitable for navigation under what is known as the 'thalweg principle'".