Storing Freshwater In The Salty Sea

Rivers are big and full of freshwater, which is great because we humans can only drink freshwater. But do you know what's their problem? Well, those rivers tend to lead to the ocean! And that means all of that sweet sweet freshwater vanishes into the salty salty sea! This accident of circumstances is, of course, unacceptable. So what if you can keep all that freshwater from leaking out there by damming the very sea itself?