Hydrographs and factors that cause a river to flood

Question 1

How do these factors cause rivers to flood?
Trees
Impermeable rock
Valleys
Drains

Answer 1

Trees prevent rivers flooding because trees drink up water; transpiration
Impermeable cause flooding because impermeable rock cannot soak up water therefore causing fluid to build up on top of the ground
Valleys cause flooding because the water builds up and will overflow causing flooding
Drains prevent flooding because the water is able to flow downwards and not build up on top of the ground