Steven Franz, age 9, of Merrillville, Indiana, for his question:

How do they hold back the water to build a dam?

Naturally the river bed must be dry while a sturdy dam is built across it. Sometimes they hold back half the stream at a time. They dump piles of dirt and rocks across half the river. This forces all the water to squeeze through the other side. They build the first half of the dam on the dry side. Huge wide pipes are set inside, through its sturdy wall. Now the pile of rocky dirt behind the half finished dam is removed. Another pile is dumped across the other side of the river. The water can flow through the pipes. The second half of the river is dry and they can finish the dam.

Sometimes they dig a channel to lead the river around the place where they plan to build the dam. This leaves the river bed dry. When the dam is finished, they lead the water from the channel back to its old river bed    and over the dam.