In a time before extensive road networks existed, rivers were used for inland transportation. They are also a source of food (fish, irrigation). But the additional reference to “springs of water” indicates that we need to think here of drinking water. Drinking water, of course, is essential to the life of people, animals, and plants.
10 The third angel blew his trumpet, and a great star fell from heaven, blazing like a torch, and it fell on a third of the rivers and on the springs of water.