There was once a time where every river and stream had some sort of spirit, fairy, or other creature associated with it.
"Wherever there was a spring, there was life; where-ever there was life, there was a spirit; and each river and loch, each burn and tarn, each bubbling spring had its own deity." The Silver Bough: Volume 1 by F. Marian McNeill (1957)
Sometimes water sprites were kind and helpful, but there are also ones that were a lot darker.
"I am reminded of a walk I was privileged to take with Mr. William B. Yeats on Lady Gregory’s estate at Coole Park, near Gort (County Galway); for Mr. Yeats led me to the haunts of the water-spirits of the region, along a strange river which flows underground for some distance and then comes out to the light again in its weird course, and to a dark, deep pool hidden in the forest. According to tradition, the river is the abode of water-fairies; and in he shaded forest-pool, whose depth is very great, live a spirit-race like the Greek nymphs. More than one mortal while looking into this pool has felt a sudden and powerful impulse to plunge in, for the fairies were then casting their magic spell over him that they might take him to live in their under-water palace for ever." The Fairy-Faith in Celtic Countries by W. Y. Evans-Wentz (1911)
There are many sprites that were often talked about in Scotland, and one was Cuachag. She was said to be a dangerous river sprite living in Glen Cuaich.
“Many a river, too, has its spirit. Glen Cuaich, in Inverness-shire,’ writes Professor Watson, ‘is – or was till lately – haunted by a being known as Cuachag, the river sprite.” The Silver Bough: Volume 1 by F. Marian McNeill (1957-1968)
It was said to be a fauth, which is a name for evil water creatures.
"It was a river sprite, which haunted Glen Cuaich in Inverness-shire, which is connected to it by name. Like all the Fuachan, it is a dangerous spirit." A Encyclopedia of Fairies by Katharine Briggs (Published in 1976)
(I decided to try drawing my take on a river sprite.)