In the west mountain ridges the rivers of the province start from melting snow and glaciers that then flow through the plains. The systemization and control of these rivers, and the exploitation of groundwater have given rise to the so called oasis, and to a particular form of occupation of the land.
These oases are formed by the following rivers: Mendoza, Tunuyán, Atuel, Diamante, Colorado, Malargüe and Desaguadero-Salado Rivers; and also by their respective hydrogeological basins. This systemization is the origin of an irrigation net that includes a series of river beds, most of them artificial, with different hierarchies which bring agriculture to life. These oases take up a small percentage of the surface of the province; approximately 5% of the land; but in them 98% of urban centres are gathered, with more than 1600000 inhabitants.