There's a lot that goes into an impoundment actually holding water. If you built a correctly constructed damn across a ravine, you would then have to contend with if the 'walls' of the ravine held water, or leaked. The other consideration would be how much water inflow was there to counteract leakage or natural seepage.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."