In Bridge to Terabithia by Katherine Paterson, Leslie drowns in the creek in the woods.

Jess, Leslie's best friend, decides to spend the day in Washington, DC with one of his teachers. He comes home happy and satisfied with the outing, only to find his family upset. His mother starts sobbing when she sees him. His sister says, "your girl friend's dead, and Momma thought you was dead, too."

Jess is told that Leslie drowned and was found in the creek. He refuses to believe it because she was a strong swimmer. His father explains, "That old rope you kids been swinging on broke. . . . They think she musta hit her head on something when she fell." 

Jess has a hard time coming to terms with what he's heard and believing that Leslie is dead until Bill, Leslie's father, mentions they're having her cremated. 

Leslie and Jess are based on Paterson's son David and his childhood friend Lisa Hill, according to an interview with NPR. When they were eight years old, Lisa was struck by lightning and killed. Though she didn't drown like Leslie, Leslie's death is still based on what happened to Lisa.

No one can say for sure why Leslie drowns. This is how it happened:

Jesse has big crush on his teacher, Miss. Edmunds. When Jesse goes with Miss Edmunds to the National Gallery of Art, Leslie goes to Terabithia by herself. The creek is overflowing because of the rains and she swings on the rope above the creek to cross it. The rope breaks and she falls into the creek. Although Leslie is a good swimmer, she can’t make it to shore and she drowns.

Her death brings the story to the climax, or the central crisis. Through her death, Jesse has to come understand himself and overcome his fears.