The Mystery Polluter
You need to have first unlocked "The Thunder Helm" sidequest first. You can learn how to start it here. Talk to Dalia, a Gerudo child, who is in the northwest corner of Gerudo Town.
She will talk about trying to grow plants, but to her distress, the water is contaminated. Your job is to figure out the source of the pollution so that she can grow plants in peace.
First, collect 10 Wildberries. This will save you the labor of having to get them later, that's all. Wildberries grow where it is cold and can also be found growing near the stables in the Hebra region.
Then, climb up the waterfall that Dalia is standing by. If you wait a few seconds, you will see what looks like melon rinds floating in the water. Follow the melon rinds to their source by going straight west.
You will see a Gerudo named Calyban snacking on melons. She will offer to stop eating the melons if you bring her 10 Wildberries. This is why we told you to gather them up earlier, to save you the effort of having to come back again.
Deliver her 10 Wildberries. She will leave the area, saying that she wants to help clean up the mess she made. Return to Dalia. She will give you a Hydromelon as thanks and with that, this sidequest is complete.