taru-adjective, adverb taking the 'to' particle
Literary onomatopoeic word describing the gentle sound of flowing water, such as a stream or brook. Used as a taru-adjective or adverb with と.
淙々と流れる小川の音が心地よい。
The murmuring sound of the brook is pleasant.
淙々たる渓流のほとりで一休みした。
We took a rest by the babbling mountain stream.
さらさら is a common mimetic word for a light, rustling sound, often used for leaves, sand, or thin streams. 淙々 is more specific to the sound of flowing water and is literary.
せせらぎ is a noun meaning the sound or sight of a shallow stream, more common in everyday language. 淙々 is an adjective/adverb describing the sound itself.
Reduplication of the kanji 淙, which represents the sound of flowing water. The reading そう is the on'yomi of the character. The word is a literary onomatopoeia.