Pristine, Yes! But I Don't Get It

I am curious why the water doesn't just leak into the ground?  I build a pond in my back yard and when a branch falls from the trees above penetrating the lining, the water leaks into the ground and empties my pond.  Yet people built a dam a century ago and the water happily collects into a reservoir.  Can anyone explain the geology of this?

Nikon D610, 1/60 second at f/22, ISo 100, 17mm (17-35mm) - 9586