That Mythological River That You Forgot

I wrote on another blog about forgetfulness in old age and referenced a poem by Billy Collins on the subject. In the poem, he alludes to 

Whatever it is you are struggling to remember
it is not poised on the tip of your tongue,
not even lurking in some obscure corner of your spleen.

It has floated away down a dark mythological river
whose name begins with an L as far as you can recall,

I linked that allusion to this post, hoping that a reader there might come here if they also have forgotten that river.

The mythological river starting with L is Lethe from Greek mythology. It is the perfect river to choose because it is known as the "River of Forgetfulness".

It is one of the five rivers of the Greek underworld. The newly dead were required to drink its waters to completely wash away their memories and earthly life before entering the afterlife or being reincarnated.

The name comes from the Greek word lethe, which translates to "forgetfulness" or "oblivion".

In philosophical traditions like Orphism and Plato's Republic, the river was contrasted with Mnemosyne, the "River of Memory". Initiates were taught to drink from the latter to retain their wisdom and identity.

Did you once know the other 4 rivers of the underworld?
Styx (shuddering/hatred), Acheron (woe), Cocytus (lamentation), and Phlegethon (fire).