Consider the flow over a dam as shown in fig, on the upstream side of dam, the depth of water will be rising. if there had not been any obstruction in the path of flow of water in the channel. the depth of water would have been constant as show by dotted line parallel to the bed of channel.

Due to obstruction the water level rises and it has maximum depth from the bed at some section.

Let, $h_1$ = depth of water at the point where water starts rising up.

$h_2$ = maximum height of rising water from bed.

Then $(h_2 - h_1)$ = Afflux

Afflux : It is defined as maximum increase in water level due to obstruction in the path of flow of water.

The profile of rising water on the upstream side of dame is called as back water curve.