text: There are many ways to do this. \n\n1. Engineers pour the concrete into metal forms, and remove the forms when the concrete dries.\n\n2. Another way is to temporarily divert the whole river through a side channel or tunnel, but I think this is usually done only when they want to build a dam across it.\n\n3. They can put a temporary barrier across a part of the river to build the foundations there, then build another temporary barrier across a different part of the river to build the next foundations, and so on till all the foundations are build.  \n\n4. Some rivers flow seasonally, so they just wait till it is not flowing to build the bridge.