What are two different ways dams positively affect ecosystems?

The telegraph idk what else to tell you you are probably just gonna copy it which btw i would do the same thing but it says i have to have at least 20 characters so yeah there you go 

1. d: oceanic crust is thicker and contains a lot of granite2.d: where two continental plates are colliding  3.b.contour boundaries4.b.plates that are attracted to each other by gravity5.a.as two continental plates collide along a convergent boundary, mountains are formed.