The Ugra () is a river in Smolensk and Kaluga Oblasts in Russia, left tributary of the Oka. The east-flowing Ugra joins the north-flowing Oka at Kaluga and the united river, called the Oka, continues east to the Volga. In the 16th century, the Ugra-Oka confluence was the western end of a line of forts protecting Muscovy from Tatar raids. The river is known for the Great stand on the Ugra River. Its length is and its basin . It is frozen from late November (sometimes as late as January) until the end of March. 60% of its annual flow is snowmelt, mostly in April. A part of the valley of the Ugra located in Kaluga Oblast belongs to Ugra National Park. References External links The Ugra River: photos Rivers of Kaluga Oblast Rivers of Smolensk Oblast