The Aidar (Ukrainian: Айдар) is a river of the Luhansk Oblast, Ukraine and Belgorod Oblast, Russia. A left tributary of the Seversky Donets, it flows for 256 kilometres (159 mi) and has a basin area of 7,370 square kilometres (2,850 sq mi). The slopes of the valley are partitioned with ravines and gorges. In the spring, snow melt accounts for approximately 70% of the flow . The average flow module is 1.7 liters • sec / km ². The river freezes in December.