From CRC: "When the Colorado River rises to 20,000 CFS and the benevolent dam keepers at Cameo open the gates, the river upstream picks up steam and “Big Sur” Colorado-style rises from a watery grave."
If ya think about it, dams reduce gradient, decrease flow velocity, and back up water into a pool. In absence of the obstructing dam, you have increased gradient and thus increased flow velocity. This in turn, has far reaching upstream effects, increasing the flow velocity upriver of the opened dam. Increased surface water gradient and velocity creates bigger features!