This article addresses population movements across the Amur and the Ussuri River borders between Russia and China. It analyses the history of border crossing in this region from Russia's acquisition of the Amur and Maritime provinces from the Qing Empire in 1860 to the present time, with a focus on the 1920s and 1930s. The article's first part demonstrates that the movement of people (settlers, work migrants, refugees) across the two river borders went in both directions. The second part asks when the formerly porous river borders became sealed through strengthened military control. By analysing the mechanics of border crossing, such as the clandestine passages of Mennonites, a Russian-German Protestant sect, from Soviet territory into Chinese Manchuria over the Amur in 1929 and 1930, as well as the escape stories of other refugees from the Soviet Union, the article shows in its third part that the 'sealed' borders could nonetheless be transgressed.