I don't think it's the "Aa" River but : Déversoir d'Aa : https://www.coordinationsenne.be/mailer/JournaldelaSenne_14/JdlS14_T4FRbis.htm

The Aa river was formerly // to the Canal and going to the "île d'Aa" https://uurl.kbr.be/1296963 (and the flow was going to Senne not comming from.

This part is a man made derivation to control over flow not the river itself (was a stream btw).