Lets talk about floods. For example floods are coming over to a place which is behind a forest. Obviously the speed would decrease because the water needs to uproot trees to move forward. Trees act as a barrier so if we deforest then we could have dangerous floods!!!

Now drought.
Maybe you would have known that rain also depends upon trees. So less number of trees rain would decrease and due to lack of water we suffer with drought.
So you may have understood the reasons.
Shawez :)
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