Dear Molinari!
There are some books that deal with this topic:
(1) F. Matz, The
Personifications of Nature in Greek Art, Göttingen 1913
(2) F. W. Hamdorf, Greek cultural
personifications of the pre-Hellenistic period, Mainz, 1964
(3) H. P. Isler, Acheloos, Berne 1970
(4) C.
Weiss, River deities in pre-Hellenic times, Würzburg 1984
(5) Dohrn, ancient
river gods(6) Sylvia Klementa, Gelagerte Flußgötter,
Cologne 1993 (Late Hellenism and Imperial Age)
(7) Reinhard Falter, A
Collection of Depictions of
River Gods on Coins of the
Roman East, in "
Salus Provinciarum, Schriften
des Instituts für Naturphilosophische Praxis Nr. 3, 2009".
Klementa and Falter are in my
library. I think
Weiss sounds
good for your project.
In classical fine arts the anthropomorphic representation of the river occurs in about the second quarter of the 5th century B.C., if the interpretation of the corner figures in the east
gable of the temple of
Zeus of Olympia as Alpheios and Kladeos mentioned in Pausanias (5, 10, 6f.) is correct (Sylvia Klementa, Gelagerte Flußgötter 1993). Then the representation as recling deity would be owed to the available triangular space in the
gable.
But there are also other opinions:
(1) The origin in the reclining figures (
Satyrs) of the Dionysian entourage.
(2) Curtius explains the reccling
type with the fact that springs and places with flowing water are comfortable sites for laying down.
(3) The resting
Herakles, who is connected to the water in various ways and also served as a fountain figure, is also mentioned as an ancestor.
(4) A relative of the reclining
river god is the Great God of
Odessos, whose connection to the water (
amphora etc.) is unambiguous and so this relationship is not accidental.
I'm interested in your results.
Best regards
Jochen