Most likely imagery. The poet is trying to paint a very detailed image in your mind as you read.
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The figurative language used in the given description is metaphor. A metaphor is a figure of speech that compares two different things by stating one is the other. In this case, the comparison is being made between the black water above the weir and starry velvet in the night. The use of the word "like" indicates the comparison being made. It helps to create a vivid and imaginative image in the reader's mind by associating the smoothness and darkness of the water with the texture and appearance of starry velvet.
