When George and Lennie approach the river, why does George warn Lennie not to drink to much water?

George warns Lennie about the water because it doesn't look good, and the water isn't running as it ahould be. He doesn't want Lennie to get sick from the water.

George unslung his bindle and dropped it gently on the bank. "I ain't sure it's good water," he said. "Looks kinda scummy.

George knelt beside the pool and drank from his hand with quick scoops. "Tastes all right," he admitted. "Don't really seem to be running, though. You never oughta drink water when it ain't running, Lennie," he said hopelessly. "You'd drink out of a gutter if you was thirsty."