Clark Ashton Smith (CAS) wrote several poems with the title "Lethe" (including a haiku). The one under consideration here is not from any of the collections published during his lifetime, and is not available on The Eldritch Dark, so let's start with the complete poem:
Seekst thou that Lethe of whose depths profound
One drinking shall his sorrows all forget?
Not on this earth are Lethe's waters found:
That pool beyond the gates of Death is set.
I think this is the least interesting poem I have read so far, since CAS hasn't really given us anything particularly original. He presents a fairly straightforward description of the river of forgetfulness from Greek mythology.
CAS was a fan of Edgar Allan Poe, and may have encountered some lines about Lethe in Poe's "The Sleeper", but reading this short piece from CAS mostly leaves me curious as to how he'll make use of the Lethe trope in later, more mature poems.