Once, Tarrigo Lake was an exclusive resort for a select handful of wealthy families; but when a modern dam diverts the water, the lake becomes a bog and the resort is abandoned and forgotten. Two children stumble across it by accident and befriend the elderly brother and sister who have come back to live there.
This magically-written book was a Newbery runner-up.
Elizabeth Wright Enright Gillham (September 17, 1907 – June 8, 1968) was an American writer of children's books, an illustrator, writer of short stories for adults, literary critic and teacher of creative writing. Perhaps best known as the Newbery Medal-winning author of Thimble Summer (1938) and the Newbery runner-up Gone-Away Lake (1957), she also wrote the popular Melendy quartet (1941 to 1951). A Newbery Medal laureate and a multiple winner of the O. Henry Award, her short stories and articles for adults appeared in many popular magazines and have been reprinted in anthologies and textbooks.
https://en.wikipedia.org/wiki/Elizabeth_Enright