Caspian Sea
The Caspian has characteristics common to both seas and lakes. It is often listed as the world's largest lake, although it is not a freshwater lake. It contains about 3.5 times more water, by volume, than all five of North America's Great Lakes combined. The Caspian was once part of the Tethys Ocean, but became landlocked about 5.5 million years ago due to plate tectonics. The Volga River (about 80% of the inflow) and the Ural River discharge into the Caspian Sea, but it has no natural outflow other than by evaporation. Thus the Caspian ecosystem is a closed basin, with its own sea level history that is independent of the eustatic level of the world's oceans.
Azerbaijan
The main water sources are surface waters. However, only 24 of the 8,350 rivers are greater than in length. All the rivers drain into the Caspian Sea in the east of the country. The largest lake is Sarysu (67 km²), and the longest river is Kur (1,515 km), which is transboundary with Armenia. Azerbaijan's four main islands in the Caspian Sea have a combined area of over thirty square kilometers.