Fact 1: Hydropower requires damming a river.

Fact 2: Streams and rivers are crossed on rocks, beaver dams, on fallen trees or by wading.

Given the two facts above, answer the question "Hydropower requires beavers damming what?" with the following options: 
- lakes with large rocks
 - a hydraulic system
 - organisms and their habitat
 - streams with two branches
 - water with old boards
 - scarce resources
 - fissures of hot, acidic water
 - streams with fallen trees


 

        streams with fallen trees 