Oude Maas
During the early Middle Ages the main flow of the Maas followed the current Oude Maas. During the later Middle Ages, a flooding event caused the Oude Maas to connect to the Merwede rivers. From that moment on, the Oude Maas was fed by both the Maas and the Rhine. However, after the St. Elizabeth's floods the Merwede found a new and shorter path to the sea (the current Hollands Diep). As a result, the Oude Maas was devoid from much of the inflow of fresh water and changed into a brackish estuary. When the flow of the Merwede was better distributed among its lower branches (due to artificial means), the Oude Maas was once again a major distributary branch of both the Maas and Rhine.