Why do boats float higher in seas than in rivers?

When a boat is placed in water, it displaces an amount of water equal to its volume. In other words, it takes the place of a volume of water equal to its volume. Since the density of seawater is higher, a boat will displace a lesser amount of water than it would in a river and will, therefore, float at a higher level. This effect can be dramatic in the Dead Sea, which has one of the highest concentrations of salt.