Question

The normal water level in the river is 0. Now it is 2 feet below normal.

Answer 1

You need to know how much rain per hour there will be.

Answer 2

The question can't be answered with the information given.

First of all, we don't know how fast the rain is falling ... how many inches
per hour.

But even of we knew that, that would only tell us how much water would
pile up in our yard or on our street.  You're talking about a river, and
that's a whole different ball game. 

The rain that falls right here is going to flow down the river to somewhere
else.  But the rain that falls up-river from us is going to flow down to where
we are, and the river is going to rise.  The amount it rises depends on . . .

-- how fast the rain is falling,
-- how much area it's falling on,
-- how much forest there is in that area and how many houses and
         parking lots where the ground can't soak up the water and hold it,
-- and also on how wide the river is and how fast the water can flow through it.