Answer:
Mass of water,

Step-by-step explanation:
Given that,
Surface area of the reservoir, A = 30 km² = 3 × 10⁷ m²
Average depth, d = 49 m
The volume of the reservoir is V such that,



We have to find the mass of water is held behind the dam. It can be calculated using the expression for density. We know that density of water, d = 1000 kg/m³
Density,




Hence, this is the required solution.