Why sand dams Sand dam FAQs How is a sand dam different from a ‘normal’ dam? A sand dam does not stop the flow of a river. They are designed to allow 97-99% of river water to continue downstream, by flowing over the dam. This means that downstream users are not affected. Sand dams are also different to standard dams in that they allow sand to build up behind the dam wall, so that water can be stored within it. This means that you can’t see the water being stored until you scoop into the sand, or pump it out using a shallow well. Storing water in sand like this keeps it safe from evaporation, contamination and disease.