Answer:
Three positive effects are -
a) Water diverted into canals for agriculture
b) For generating electricity
c) for preventing floods.
Explanation:
Water within a water body is retained, diverted or stopped basically for three main reasons -
a) The water from main water body is diverted towards canal or regions that are devoid of water to serve the people and fulfill the requirement of agriculture.
b) The water is stored in elevated reservoir so that electricity can be produced from it.
c) Water is some time stopped through barrages or dams to prevent floods.