The River Girl
Here is Seymour-Jones describing Vivien Haigh-Wood Eliot in Painted Shadow:
Vivien was 'fresh, innocent, a chatterbox, pretty', said a friend of Stephen Spender's who used to dance with her at the Hammersmith Palais de Danse before the war. She was nicknamed 'The River Girl', wrote Spender, by 'Eliot's social friends (Lady Ottoline Morrell, St John Hutchinson and his wife, Virginia and Leonard Woolf, the Sitwells, the Aldous Huxleys)', although Eliot himself called Vivien and her friends 'Char-flappers'; and, according to Osbert Sitwell, 'River-Girl' was a term used by the contemporary press to describe 'that kind of young person--the rather pretty young girl who could be seen, accompanied by an undergraduate, floating down the river in a punt on a summer afternoon', It was a light, insouciant name, reflecting Vivien's supposed character, and suggesting the sparkle and mutability of water, that quality which led Brigit Patmore to describe her as 'shimmering' with intelligence. But it is also derogatory, conjuring up the image of the only half-human naiad or water sprite who lives at the bottom of the river and lures men to fall in love with her. 'There seems a trace of mockery in this name,' said Spender, remarking damningly, 'She had a history of illness and "nerves."'
The Oxford English Dictionary does not have an entry for 'River girl' but it does have a usage example for the word 'peek-a-boo' that should help give an understanding of the use of the first term:
1906 Daily Chron. 16 Aug. - The dreamer is embowered in soft cushions, and being punted by a River Girl, in a peek-a-boo blouse--all grace and lissomness and tan and bare arms.