Of course they only do for little rivers, where the water isn't very deep.

And they don't always do even there.
Sometimes in the river Thora, where Milly and Olly's stepping-stones were, when it rained very much, the water rose so high that it dashed right over the stepping-stones and nobody could go across.

Milly and Olly saw the stepping-stones covered with water once or twice while they were at Ravensnest; but the first evening they saw them the river was very low, and the stones stood up high and dry out of the water.

Milly thought that stepping-stones were much nicer than bridges, and that it was the most amusing and interesting way of getting across a river that she knew.

But then Milly was inclined to think everything wonderful and interesting at Ravensnest--from the tall mountains that seemed to shut them in all around like a wall, down to the tiny gleaming wild strawberries, that were just beginning to show their little scarlet balls on the banks in the Ravensnest woods.