Arethousa was by origin an Arkadian Nymphe who fleeing from the amorous pursuit of the River-God Alpheios was transformed into a flowing spring. In this form she continued to elude him, flowing beneath the sea until she emerged again on the island of Syrakousa in Sicily as the island' s spring.
"They say that there was a hunter called Alpheios, who fell in love with Arethousa, who was herself a huntress.

Arethousa, unwilling to marry, crossed, they say, to the island opposite Syrakouse called Ortygia, and there turned from a woman to a spring.
Alpheios too was changed by his love into the river ... That the Alpheios passes through the sea and mingles his waters with the spring at this place I cannot disbelieve, as I know that the god at Delphoi confirms the story.
For when he despatched Arkhias the Korinthian to found Syrakouse he uttered this oracle:

'An isle, Ortygia, lies on the misty ocean over against Trinakria [Sicily], where the mouth of Alpheios bubbles mingling with the springs of broad Arethousa.' For this reason, therefore, because the water of the Alpheios mingles with the Arethousa, I am convinced that the legend arose of the river's love-affair." - Pausanias, Guide to Greece 5.7.2