You can see three rivers. You'll have to answer correct on the river for the first city to appear.
When you've answered correct, a new city will appear. When you've answered correct on all the cities by that one river, you'll move on to the next river.
There's a total of five cities by each river.
You can always switch river by clicking on the second row of another block.
You can see which river you have to answer on in the top row of the block where the yellow box is located