Rivers and lakes form the principal part of the water systems of Azerbaijan, they were formed over a long geological timeframe and changed significantly throughout that period. This is particularly evidenced by remnants of ancient rivers found throughout the country. The country's water systems are continually changing under the influence of natural forces and human-introduced industrial activities. Artificial rivers (canals) and ponds are a part of Azerbaijan's water systems. In terms of water supply, Azerbaijan is below the average in the world with approximately  per year of water per square kilometer. All big water reservoirs are built on Kur. The hydrography of Azerbaijan basically belongs to the Caspian Sea basin.