Heck, just noticed I've put "anomaly" instead of "analogy" 

Returning the the dam/culvert analogy..... Some people (??) want to reintroduce Beavers to create natural dams and hold back "flooding" above the headwaters. Well, we've beaten the Beavers to it!!! On the Severn there's the dam on it's tributaries, Lake Vyrnwy and the Clywedog, on the Dee there's Bala Lake (OK, it was natural but it was enlarged with a dam) as well as Llyn Celyn, on the Wye there's the Elan Valley reservoirs.

Soooo, all those are supposed to regulate the flow of their rivers and maintain a stock of water for drinking etc. The thing is, when they're full, they're full?