In the region, the hydrogeological formations vary significantly. Thus, layers appear with high, low or minimum permeability.
High permeability characterizes the Cretaceous and Eocene limestone formations, the land and river silting of the modern landfils, and the lenticular sand layers of the landfills in the lagoon, as well as the formations of the Triadikos. Permeability is relatively high and micaceous sandstones and conglomerates exist in northeastern Aitolikon lagoon.
Low permeability is present in the pleistocene deposits to the north of the Messolongi lagoon. Practically, water impermeable layers can be considered as the flysch, the marly lacustrine deposits as well as the recent stage lagoon sediments.
The above characteristics of the hydrogeological formations contribute to the creation of karst aquifer systems and coastal alluvial aquifers. In the wider region, and specifically in the watersheds of the rivers Acheloos and Evinos, the drainage appears to be quite low with increased surface runoff, providing the river Achelous with 500-900 m3/s, the minimum flow of the river Evinos being 7 m3/s.
Also in the lagoon terminate the streams developed in the mountain east of the lagoons. These creeks (about 10) on their way to the alluvial coastal area of the lagoons present steep slopes and carry large quantities of water, especially during the rainy months. The total surface runoff is estimated to be 26 million m3 per year. These four tributaries end in the central part of the lagoon of Messolongi and in the lagoon of Etoliko.
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