Lake Peipus, (Estonian: Peipsi järv, Pihkva järv; Russian: Псковско-Чудское озеро (Pskovsko-Chudskoe ozero), German: Peipussee) is the biggest transboundary lake in Europe on the border between Estonia (part of European Union) and Russia.
The lake is the fifth largest in Europe after Lake Ladoga and Lake Onega in Russia north of St. Petersburg, Lake Vänern in Sweden, and Lake Saimaa in Finland.
Lake Peipus is a remnant of a body of water which existed in this area during an Ice Age. It covers 3,555 km2, and has an average depth of 7.1 m, the deepest point being 15 m. The lake has several islands and consists of 3 parts:
- Lake Peipsi/Chudskoe (Estonian: Peipsi järv, Russian: Чудское озеро) is the northern part of the lake with the area of 2,611 km2 (73%).
- Lake Pihkva/Pskovskoe (Estonian: Pihkva järv, Russian: Псковское озеро) is the southern part of the lake (area 708 km2 or 20%).
- Lake Lämmijärv/Teploe (Estonian: Lämmijärv, Russian: Тёплое озеро) is the sound connecting both parts of the lake (area 236 km2 or 7%).
The lake is used for fishing and recreation, but suffered from some environmental degradation from Soviet era agriculture. Some 30 rivers and streams discharge into Lake Peipus. The largest rivers are the Emajõgi and the Velikaya River. The lake is drained by the Narva River.