How does Atwood incorporate foreshadowing in the story? What are the effect of this?

“Death by Landscape” by Margaret Atwood tells the story of Lois, presently an older woman, who recounts the haunting tale of the disappearance of her friend. Lois and Lucy were best friends that spent each summer together at Camp Manitou until one summer when Lucy disappeared while on a canoeing trip. Lois has to deal with the death of Lucy and the implication that she was the cause of her friend's death.
Before we learn anything about what is haunting Lois, the story of her misfortune is set up in the introduction of the story. Atwood sets up the story of something haunting Lois with the line,

Despite the fact that there are no people in them or even animals, it's as if there is something, or a someone, looking back out.

At this moment, we don’t understand the full meaning of this sentence, but it creates a sense of foreboding about what Lois feels looking at her in the paintings. It foreshadows the guilt she feels about Lucy’s death and the dreadful feeling of company she feels consistently following her through her life. That sense of never being alone is explained at the end of the story, when Lois says that Lucy follows her like a “shadow.”
There are a few instances of foreshadowing in the story of camp before Lucy goes missing. The first is when Lucy tells Lois that she doesn’t want to go home after camp is over. Lucy has experienced a hard set of circumstances at home recently, her parents had split up, and she disliked the people they had married. She tells Lois,

“It would be nice not to go back.”
“To camp?” Said Lois.
“To Chicago” Said Lucy, “I hate it there.”

The problems that Lucy faces, and her desire to not only break the rules but also get away foreshadows her disappearance—she wants to go somewhere else, anywhere else, rather than go home.
Later that day, when they were on their hike up to the lookout point, Lucy tells Lois that she wants to jump down into the water below. Lois tells her shes “nuts,” but this scene too foreshadows that Lucy might do something crazy—which could cause her disappearance.
The reason Lucy’s sudden disappearance haunts Lois is that she feels responsible. Despite the way she tells the story, others blame her of pushing Lucy off the cliff, and Lois is so distraught that she starts to question even her telling of the events. It is easy to understand then why she compulsively buys paintings that remind her of the scene, and why the ghost of Lucy follows her everywhere, through her entire life, and now fills the void of loneliness in her life.