The long Siberian river that flows into the Laptev Sea is the Lena River.
The Lena rises in the Baikal Mountains near Lake Baikal and travels generally northward across eastern Siberia. Near the Arctic coast it spreads into the Lena Delta, a vast network of channels and islands that empties into the Laptev Sea, a marginal sea of the Arctic Ocean. At roughly 4,294 kilometres, the Lena is one of the world’s longest rivers and the longest river entirely within Russia.
The Lena is often confused with the Ob and Yenisei because all three are major Siberian rivers flowing north into Arctic seas. The Ob reaches the Kara Sea, while the Yenisei reaches the Kara Sea farther east; the Lena reaches the Laptev Sea. Its basin is also strongly associated with permafrost, which underlies most of the catchment and shapes the river’s seasonal flooding and delta wetlands.