In the sentence, the word "subside" most nearly means the following: abate.
The detail from the passage that best supports the answer to the previous question is this: the description of the flooding and its aftermath.
What is the meaning?
The description of the flooding provides context for the use of the word "subside." It shows that the floodwaters were at one point very high and inundating the town, but they have now receded or abated.
The other options do not fit the context of the sentence because "Elevate" means to rise or go higher, which is the opposite of what is happening to the floodwaters. "Merge" means to combine or join together, which is not relevant to the sentence and "Recur" means to happen again.