A hydrological paradox
On the border of China's Heilongjiang province and Russia's Primorsky Krai sits Xingkai Lake, or Lake Khanka. It is the largest freshwater lake in Northeast Asia, with a surface area that fluctuates between 3,940 and 5,010 square kilometers. The lake is exceptionally shallow for its size, with an average depth of only 4.5 meters and a maximum depth of 10.6 meters. Twenty-three rivers flow into this vast, pear-shaped body of water, but only one flows out, the Songacha River.
The Songacha River is the lake's sole outlet, connecting it to the much larger Ussuri River, which itself is a major tributary of the Amur River. This connection creates a peculiar hydrological phenomenon. During periods of intense rainfall, typically associated with summer monsoons, the water level in the Ussuri River can rise dramatically. When the Ussuri's water level surpasses that of the lake, it exerts immense pressure on the smaller Songacha River. This pressure differential forces the Songacha to stop its eastward flow and reverse course, pushing water from the Ussuri back into Xingkai Lake. This backflow event alters the lake's water level, chemistry, and sediment patterns, showing a rare case where a lake is fed, however temporarily, by its own drain.