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What did Gennady Nevelskoy's expedition discover about Sakhalin?

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Topographic map of the Amur River basin and its drainage network, covering parts of northeastern China, eastern Russia, Mongolia, and North Korea. The map uses color shading to indicate elevation, shows major rivers and tributaries, surrounding mountain ranges, swamps, marshes, and lakes, and includes a locator inset of the region within Asia. Major cities, railways, and dams are also marked. — Amur River The Amur River basin and its drainage network.

During his expedition between 1849 and 1855, the Russian naval officer Gennady I. Nevelskoy proved that Sakhalin is an island.1

This discovery was significant because it demonstrated that the Amur River is accessible from the south, rather than only from the north as had been previously supposed by the Russians. This finding facilitated further systematic study of the river system as Russia sought to establish transport links to a year-round port on the Pacific Ocean.1