The Aidar is a river in Luhansk Oblast, Ukraine and Belgorod Oblast, Russia. A left tributary of the Seversky Donets, it is 264 km (164 mi) long and has a basin area of 7,420 square kilometres (2,860 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 litres • sec / km². The river freezes in December.