The Oka River is a major western tributary of the Volga, which it enters at the city of Nizhnii Novgorod.The Oka is fed by the Moscow River, which joins it just south of Kolomna. Maintaining the river in a navigable state required a sizeable work force. This 1912 view, taken near the village of Beloomut in present-day Moscow Oblast, shows log supports placed in preparation for pouring concrete to create the foundations for a sluice dam, part of a system of hydraulic works designed to enhance the navigability of the river. In the background are workers and supervisors; in the foreground are project engineers wearing tunics. Because of the great distances and dearth of good roads, river networks were critical for transportation in Russia since the early medieval period and they remained so in the 20th century. The image is by Russian photographer Sergei Mikhailovich Prokudin-Gorskii (1863–1944), who used a special color photography process to create a visual record of the Russian Empire in the early 20th century. Some of Prokudin-Gorskii’s photographs date from about 1905, but the bulk of his work is from between 1909 and 1915, when, with the support of Tsar Nicholas II and the Ministry of Transportation, he undertook extended trips through many parts of the empire. Although he photographed primarily along railways, much of Prokudin-Gorskii’s work for the ministry was also devoted to water transportation.