Read "Lethe" at The Eldritch Dark:
This poem from Clark Ashton Smith (CAS) shares a title with several other poems from the same writer, and the mythical river of forgetfulness is frequently invoked throughout his poetic corpus.
CAS included this poem in a section of his Selected Poems (1971) called "Strange Miniatures", which is a sub-section of "Experiments in Haiku". As with other verses that he included in that broad grouping, "Lethe" does not adhere to the traditional definition of the haiku form in English*: three unrhymed lines of five, seven, and five syllables. However, at sixteen syllables, "Lethe" is close to the traditional length of an English haiku, and it certainly adheres to Kenneth Yasuda's concept of a "haiku moment" (which I described in my blog post on CAS' poem "The Limniad").
*The Poetry Foundation provides a succinct description of the haiku form in their excellent Glossary of Poetic Terms: