The answer is they produce endospores.
Explanation:
The endospores are dormant, non-reproductive, tough bacterial spores that ensure the survival of bacteria in adverse conditions.
During the environmental stress, such as starvation, desiccation, radiation and chemical treatment many bacteria form dormant endospores. These endospores germinate again when the conditions become favorable.
Hence, if drought conditions lead the stream to dry up, the bacteria that live there would produce endospores to survive.
