 Piling aren't driven down into the bedrock. They're just driven down so that the ends are sitting on top of the bedrock. And all of the lateral stability for the piling was achieved through the sediment that was in place that had been impounded in the river basin over the life of the dam. So as we're seeing dramatic levels of river head cutting as the river's lowering itself down and washing away tremendous amounts of sediment, we're losing stability. The water's flowing too fast.