system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do aquatic animals stay in the same stretch of river? If so, wouldn't they have to constantly swim against the river current?
gpt
Yes and no! River ecosystems are actually deceptively complex, there's a bunch of different types of organisms living in all different parts of the river. The easiest way to describe it would be to divide the river into three parts: The top layer of water (where the air and water meet), the middle, and the bottom (which includes all the sedeiment and dirt on the bottom of a riverbed). Different types of organisms exist in each of these sections, and some are more influenced by river flow than others. 

Some animals, such as the planktons, are lazy. They will just float along with the flow of the river and accept wherever it takes them, and because they are usually photosynthetic, they don't really have to worry about gathering food and so can just float along wherever the current takes them.

Neustonic animals are the one's you're thinking of, and yes, they're able to fight currents to move around feely in river ecosystems. They can float above or under the surface, and if they wanted to stay in the same place then they'd have to use energy to swim against the river current. These can be fish, or even water striders which float along the top of the water.

Benthic organisms are those that live within the riverbed sediment, and will remain fairly stable in the same stretch of the river. Most of them have structures that help them cling to the bottom of the river bed, allowing them to stay in place.

Like another commenter has already said, many insects lay their larvae in the water to spend the first half of their life, before growing into fully realised insects and leaving the river later on. These can float along, or stay put, all depending on the type of insect and where larvae eggs are laid.