When george and lennie approach the river, why does george warn lennie not to drink too much water ?

George warns Lennie not to drink too much because it will make him sick..... just like the night before.

“Lennie!” he said sharply. “Lennie, for God’ sakes don’t drink so much.” Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. “Lennie. You gonna be sick like you was last night.”