Inspector Knacker has a point. You can't infer intention from what's there. There's a ditch from which they excavated the hill. Were they digging a ditch or building a hill? No-one can say but I'd go for the ditch being a quarry for materials primarily - but digging it out also added to the apparent height of the hill. You get a double whammy digging down and then piling what you've dug up above - same sized hill, half the effort.

Also, what's this reservoir stuff? If the water table drops you dig a small hole down to it don't you, not a huge one? It'll never run dry no matter how much water you take out of it.