The Psou (ფსოუ; ; ) is a river in the West Caucasus, bordering the Gagra Range to the east. It flows along the southern slopes of the Greater Caucasus Mountain Range and forms a part of the border between Georgia (Abkhazia ) and Russia. Its source in the Aigra Mountain, and it flows into the Black Sea. The Psou is 53 km long, and the drainage basin is approximately 421 km2. Between the mouth of the river and the mouth of the Mzymta is a "sandy depositional foreland", which is approximately 8 km in length and 2 km wide. The principal tributaries of the Psou are the Besh and the Pkhista. Between 1913 and 1955 there was a hydrological station in operation at Leselidze, roughly 1.5 km upstream of the river mouth. The Psou gained notoriety as a smuggling route out of the country, by-passing Russian controls on the border. As of 2008 it was still designated as a transboundary river which lacked an international cooperation agreement as part of the UNECE Water Convention.