Well, for the depth to have lowered, all that water would have had to have gone somewhere. If it had been boiled off, the increased atmospheric pressure would have made the earth a very uncomfortable place to live. But the increased humidity would have actually helped the reforestation of earth.
I actually think it's more likely the polar ice shelves were vaporized, resulting in world wide flooding. The arctic, both north and south are actually deserts. It doesn't rain there... The air is so cold that any moisture it had was dumped long before it gets up there and the snow that's currently there is from the rare storms from since whenever... it's so cold the snow doesn't melt.
So, considering the grand canyon was there and it was a prime target for bombardment, the glaciers and permafrost and ice floats get boiled off. Catastrophic world wide flooding occurs... and since the area is a desert, the ice and snow will take thousands if not tens of thousands of years to come back.