Focusing on the location:
* Riverbank: The edge of a river.
* River surface: The top of the river.
* River source: The beginning of a river.
* River mouth: The end of a river.
* Upland: Higher ground away from the river.
Focusing on the composition:
* Water: The fluid that fills the riverbed.
* Sand/soil/rock: These materials might be found outside the riverbed.
* Vegetation: Plants that grow outside the riverbed.
Focusing on the function:
* River flow: The movement of water through the river.
* River life: The organisms that live in and around the river.
* River transport: The use of a river for transportation.
The best antonym will depend on the context of your sentence.