If reverse Superman flew up and dropped a mountain in the spillways so the dam could be overtopped, the dam would begin to erode.
Whichever part of the dam was lowest would see the fastest flowing water and would erode the most quickly. As a chasm was cut down through the dam, the water would flow faster and faster through it - if the height of the water above the bottom of the cut is 1 foot, only the top foot of water is "pushing" the water over; if the cut is 10 feet deep, now the top 10 feet of water is "pushing" water out the hole, and it will be pushed much faster.
This will turn into a feedback loop, and the water will rapidly cut a channel through to the bottom of the dam. As water flows through it, faster and faster, it will weaken the sides of the chasm, and more of the dam will collapse. This will let more of the water flow through, but slower, eventually limiting further erosion.
This is essentially the same thing happening in the spillways right now, but slower, because the spillways are designed to resist erosion. Once the vicious cycle gets started, though, the end result is the same.
Whichever part of the dam was lowest would see the fastest flowing water and would erode the most quickly. As a chasm was cut down through the dam, the water would flow faster and faster through it - if the height of the water above the bottom of the cut is 1 foot, only the top foot of water is "pushing" the water over; if the cut is 10 feet deep, now the top 10 feet of water is "pushing" water out the hole, and it will be pushed much faster.
This will turn into a feedback loop, and the water will rapidly cut a channel through to the bottom of the dam. As water flows through it, faster and faster, it will weaken the sides of the chasm, and more of the dam will collapse. This will let more of the water flow through, but slower, eventually limiting further erosion.
This is essentially the same thing happening in the spillways right now, but slower, because the spillways are designed to resist erosion. Once the vicious cycle gets started, though, the end result is the same.