a river in the northern part of the Mongolian People’s Republic, right headstream of the Selenga River. Length, 452 km; basin area, about 24,600 sq km. It flows mainly in a wide steppe valley in the spurs of the Khangai Mountains. It is fed chiefly by rain. There are flash floods in summer and some flooding in the spring; low water is from November to March. The average annual discharge in the lower reaches is 57 m³sec. The Ider freezes in November and thaws in April. It is not navigable.