I theorize that this painting may be of Clark Lake. It would make sense considering that Heald lived on Clark for much of her life. The lake also has an island, which Clark Lake has. It also makes sense for Heald to do these sentimental paintings of Clark Lake because the lake would have a special meaning to her, being named after her brother.
To explore the potential location further, I took an expedition onto Clark Lake in a kayak and took photos to compare to the painting. Of course, after almost one hundred years, Clark Lake probably looks quite different, and I don't think the photos I took are from the same angle, but I was convinced that this painting was of Clark Lake. I also noticed that the lake is surrounded by the types of trees seen in the painting, such as birch trees and Norway pines.