If you think about it the same amount of water must go through each bit of a river in any given time (excluding any addition water input from tributories etc.). So in order to make a river shallower (ford it) without there being a continuously growing lake next to it the same amount of water must still go through it per hour as before. To do this the river may either be made wider or it can be made to go faster. (Think of it as a cross section of a rectangle, to keep the same cross section but make the shape flatter, it must be made wider. Alternatively the size may be made smaller by the squashing, but more water must go through the smaller space to compensate).
Sorry if that was a very long winded way of saying very little but there you go.