Complex sentence: They only go on land if the water is too dirty, so they can find cleaner water in another part of the river, or they will go on land because food is scarce, so they can catch a bird or rat.
Split into simpler sentences: They only go on land if the water is too dirty so they can find cleaner water in another part of the river. Or they will go on land because food is scarce so they can catch a bird or rat.