Hydropower requires damming a river, and streams and rivers are crossed on rocks, beaver dams, on fallen trees or by wading. Given these facts, hydropower requires beavers damming what among 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 