Question : how can water supply be increased?

my answer is going to be "water transfer" - which is when water is transferred from an area of surplus to deficit

but i need a negative side to this and in my book it says "river habitats would be at risk when transferring water"

how would river habitats be at risk when transferring water?

(the pic is from my book)

Transfering water from one location to another has two results: lower amount of water in first location and higher amount of water in second location. 

By taking away water from surplus location we leave that location with lower amount of water. Animals and plants require certain amount of water in order to survive.
Fish filter water to take oxygen in order to be able to breathe. If the amount of water is too low than there would not be enough oxygen for all fish to breathe. This would result in a lot of dead fish. Plants have adjusted to be fully submerged and to survive like that. If the amount of water is too low than part of plant could get out of water and die.

Swamps are huge natural reservoirs of water. Taking water away from them reduces greatly the available habitat for animals. Also some types of trees are adjusted for high levels of water. With low levels they would die.

Answerseafloor spreading;