adverb taking the 'to' particle, taru-adjective
murmuringly; with a rippling sound
Formal or literary onomatopoeic word describing the gentle sound of flowing water, such as a stream or brook. Used adverbially with と or as a たる adjective.
小川が潺々と流れている。
The brook is flowing with a murmuring sound.
潺々たるせせらぎが心地よい。
The murmuring of the stream is pleasant.
さらさら is a common onomatopoeia for a light, rustling or rippling sound, used in everyday language, while せんせん is a formal literary term specifically for the sound of flowing water.
The exact derivation is uncertain; the kanji 潺 is used specifically for the sound of flowing water, and the reduplication creates an onomatopoeic literary expression.