I would not have expected that poem from Poe. Thanks for the find!

It's special that Poe uses the transitory nature of the river to connect the speaker to the object of worship. Consider the childlike interpretation of the universe as something that can reflect our innermost needs and desires; when it rains we feel a sense of sadness or loss and say it reflects our mood, when we traverse mountains we feel the connection with the sublime, etc. This is the same thing that's going on many myths and legends from around the world, like in the Arthurian legends--which as is also recalled masterfully in T.S. Eliot's "The Wasteland." Her image, which "deeply lies" in the stream as in the speaker's heart, allows the worshiper to resemble the "prettiest of brooks." Reading it this way, the "but" in the second stanza carries more weight because it signals the deep but transitory aspect of the connection between the speaker and the source of beauty. It causes the speaker to doubt himself, just as one might doubt whether one's poetry truly reflects the "playful maziness of art."