In Chapter Five, if the lake has more water than the pond why can't Nya's family just stay year round?

Nya's family cannot stay be the lake because of the animosity between their tribe (Nuer) and the Dinka tribe.

Because of the fighting. Her tribe, the Nuer, often fought with the rival Dinka tribe over the land surriunding the lake. Men and boys were hurt and even killed when the two groups clashed.