Lewis Short
rētae (noun F) :
* Trees standing on the bank or in the bed of a stream, acc. to Gabius ap. Gell. 11, 17, 4.— From this is said to come the verb retare, to clear, free from obstructions, in an old edict: flumina retanda; v. Gell. l. l.
Charlton T. Lewis, Charles Short, A Latin Dictionary