The creation of the lake "Trichonida" is the result of intense geological phenomena that took place 15-20 millions of years ago. The present lake is part of an older and larger lake that covered the whole valley of Agrinio in the prehistoric era.

According to tradition, inside the lake there used to be an ancient city that sunk. This city had three cones that gave -according to a theory- the lake its name (tri (three) - choni (cones)). According to another theory, which is also the most dominant, the name of the lake derives from the ancient city "Trichonio" situated by the lake, near the village "Gavalous".

Lysimachia is situated in the Prefecture of Aitoloakarnania. It covers an area of 13,2 Km², it is about 17 km wide, and has a maximum depth of 9 meters. The river of "Ermitsa" spills into the lake and at the same time the waters of the lake find their spillway to the river of Acheloos.