The extreme flooding in Belgium which killed at least 31 people has started a debate about the river systems and protections.
Euronews went to the town of Pepinster, near Liege, to see the clean-up work.
There we met an expert hydrologist who says the town's position on the confluence made it vulnerable.
Benjamin Dewals, a hydrologist at the University of Liege, said: "On the one hand you have the River Vesdre which is here and this river is controlled to some extend by two large dams which are further upstream, some 20 kilometres upstream
"Then you have the tributary which is river Hoëgne and this tributary, it's totally uncontrolled. There is no dam at all, the flow is natural on this river."
When houses collapse during floods one of the big issues is that the debris blocks and diverts the flow of the river, he added.
"They are likely to create some kinds of dams at some locations. They are likely to dam the river, for instance in between bridge pillars," said Dewals. "This may completely change the flow of the river. It tends to increase flow velocity in places that can lead to erosion, scour, and extra damage. This is not really incorporated in the current, standard, inundation mapping methods."
When asked what needs to happen during the rebuilding, Dewals believes it cannot be done in the way it has been.
"You need to consider flood resistance in the way you design the buildings, in the way you design the district and this should involve some thinking with urban planners that take full consideration of such events which are very likely to happen more often in the future," said Dewals.
A debate is now raging in Belgium about whether the dams like the Vesdre dam, some 26 kilometres upriver should have been storing so much water behind them when warnings of heavy rainfall had been given.
Water is held there to provide enough drinking water in the summer.
Experts say lowering the water level would only have had an effect on the towns immediately beneath.
Climate activists like Lucy Gilliam, policy officer at Transport and Environment, warn we are going to have to get better at judging these things.
"A lot of climate change is already baked into the system," she said. "We've already put significant amount of gases into the atmosphere so we now need to prepare. It means that we need to think about having places where water can go, so when we get this extreme rainfall that it can soak into the ground."
While the mourning and clean up continue, new long term flood control plans are starting to be formulated.