A river-bed refers to the bottom or channel of a river or stream. There are several synonyms that can be used to describe this geological feature. One term is "river channel", which specifically refers to the path that a river takes as it flows. Another synonym is "stream bed", which denotes the bottom of a smaller watercourse. "Watercourse" can also be used to describe any channel that carries water. "River course" is another term that can be used to describe the path of a river, while "bedrock" refers to the underlying layer of solid rock that supports the river-bed. Overall, these synonyms can be used to provide a better understanding of the complex and dynamic nature of this important natural feature.