Short answer: He runs to the brush (the clearing) by the Salinas River — the same spot where he and George camped in the novel's opening.
Step-by-step context:
- In the barn Lennie accidentally kills Curley’s wife after earlier mishandling a puppy.
- Frightened, he remembers George’s instruction that if he ever gets into trouble he should go to the brush by the river and hide.
- Following that plan, Lennie flees to the pool/brush area by the Salinas River and waits there for George.
Significance: The return to the riverbank/brush ties the scene back to the novel's opening and sets up the tragic ending.