 He proved that the Strait of Tartary was not a gulf, but indeed a strait, connected to Amur's estuary by a narrow section (later called Nevelskoy Strait).On 13 August 1850 he founded Nikolayevsk-on-Amur, the first Russian settlement in the region.Not knowing of the work of the Japanese navigator Mamiya Rinzō, who had explored the same area forty years earlier, the Russians took Nevelskoy's report as the first proof that Sakhalin is indeed an island.