Forte quadam divinitus super ripas Tiberis effusus lenibus stagnis nec adiri usquam ad iusti cursum poterat amnis et posse quamvis languida mergi aqua infantes spem ferentibus dabat
By chance the Tiber having divinely overflown its banks into a gentle pool was not at all able to be approached to the course of its regular stream, and it offered hope that by bearing the infants, they were able to be drowned by the water however sluggish
I translate:Sic verto:
"It happened providentially that the Tiber overflowed its banks into gentle pools and [the river] could not be approached from anywhere--one could not approach the course of the river proper--and yet, although the water was sluggish, it gave the bearers of the infants hope that they could be drowned."
"ad justi cursum poterat amnis" appears to be a parenthetical remark explaining "nec adiri usquam", which would explain why it's difficult to translate. "nec . . . et" has the sense of "not . . . and at the same time." I'm not sure "languida aqua" is ablative here; I believe it is the subject that gives them hope that the "infantes mergi posse." infantes is also a repeated object of ferentibus. The bearers dump the babies in the shallow pool because they are cut off from the main river by flooding.
Ita velut defuncti regis imperio in proxima adluvie, ubi nunc ficus Ruminalis est (Romularem vocatam ferunt), pueros exponunt.
As if thus having been discharged from the authority of the King close to the floodwater, where now the fig-tree of Ruminalis stands (they bear fruit that was called of Romulus[?]), they exposed the boys.
"Thus as if they had discharged the king's order, they exposed the boys in the nearest floodplain, where now is the fig tree of Rumina [the goddess of suckling] (they say that it is called Romulan)."
"defuncti regis imperio" is a participle phrase. "Ferunt" is a common impersonal way of saying "they say/report." You might also see "fertur" meaning "is said [to be]," with the subject being in the nominative.
Hmm, I think I've discovered that I don't like reading Livy!