This area is the southern part of the Plio-Pleistocene rift of the Campidano. The latest pulsations of the sea level provoked the closure of the Gulf of Angels by sandbars which emerged from the sea bottom, from the carbonatic island of Sa Illetta and the rocky promontory of Cala Mosca. The anthropogenic actions have strongly conditioned the hydrodynamics: firstly with salt-works, subsequently with the area Macchiareddu and lastly with the Channel Port.
The winter floods and high tides are the most "powerful engine" of water flows. The salinity gradient of the fresh water of the river and salty water of the sea has a great impact, too. Even the dominant mistral wind has a significant effect. The main exchange occurs through the opening of La Scafa, with an underwater section of 280 square meters. The channeling of the Channel Port has prevented any contact with fresh waters and anthropogenic waters.
Nowadays, the remained ancient mouths of the lagoon to the sea, along the sand bar of La Playa, are largely subject to clogging by sand deposits. They constituted the entrance doors to large evaporation tanks of the salt-works of Macchiareddu. Another rare example where the limits of the lagoon are not artificial, but dependent on the water level, vegetation and biocenosis is the area between Cala s'Olgai and the mouth of the Rio of Sestu.
The water supply comes from a water catchment area of over 2,200 square km. The Flumini Mannu drains extended regions of the Sarcidano, the Marmilla and Trexenta. The Rio Cixerri has its sources in the reliefs of the Iglesiente and from the slopes of Sulcis looking to north. Rio Sa Murta, Rio Migoni and Rio Sa Nuxedda flow from the land of Assemini and Elmas. The Rio Santa Lucia (in its first part called Gutturu Rio Mannu), flows from the heart of Sulcis and its eastern slopes.