The term 'River in Siberia' can be summarized by two crossword solutions: Lena (4 letters) and Kolyma (6 letters). The Lena River is the 11th longest river in the world, stretching 4, 400 km across Russia and emptying into the Laptev Sea of the Arctic Ocean. It is the largest river in Siberia and is known for its spectacular scenery. The Kolyma River is a 2, 129 km long river in northeastern Siberia that flows into the East Siberian Sea. It is one of the most important rivers in the region, providing transportation, fishing, and hydroelectric power.