Instructions
Go to the Upper Zora river after destroying the Twilight Insects and restoring light to Lanayru Province.
Walk toward Iza's cabin. You'll know it by the presence of a Twilight Portal above it, marked as a purple spot on your map.
Defeat the three Shadow Beasts that appear. Iza will thank you for your trouble and invite you inside.
Agree to help her remove the rocks that are blocking the river. She will give you bombs and arrows. Open the item menu, equip the Hero's Bow to one item slot and move to the bombs. Press "Z" on the Wiimote to combine the bombs with the arrows to make explosive arrows.
Shoot the rocks downstream with two Bomb Arrows to clear the blockage, then ride the boat downriver.
Destroy the rocks that Iza's employee shows you when you get downriver. Follow her to the mouth of the river.
Play the boat ride minigame again and shoot each of the hanging pots with an arrow. If you get 25 or more points, you receive the Giant Bomb Bag as your reward. It can hold twice as many bombs as the standard bag.