There is a slight amount of salt in the rivers, but it is very low. In the oceans, this accumulates, but the water evaporates. Hence the salt content goes up w

There is a slight amount of salt in the rivers, but it is very low. In the oceans, this accumulates, but the water evaporates. Hence the salt content goes up whilst the water volume remains constant (ignoring ice melt) which means the salt concentration goes up.