A reservoir formation barrier is an object designed to collect and hold a massive amount of water for use as a supply for drinking water, irrigation, industrial use, or other deliberate use, whether actually used or kept as a fallback supply. The same barrier may serve the dual purpose of reservoir formation and flood prevention.

The retained water is intended as a supply. Actual withdrawal is not required when the water is held as a fallback reserve.

A barrier can also provide flood prevention. The supply purpose distinguishes reservoir formation without excluding that additional function.