Another problem with ice dam theory

Here is another problem I see with the ice dam theory.

The flood was supposedly strong enough to erode hundreds of feet deep of solid rock. When it approached Wallula Gap, all the water in the flood had to funnel through this small passage, yet it was only able to erode it one mile wide. It should’ve eroded it at least as wide as Grand Coulee Canyon, which is 5 miles wide.

Further, the flood was hitting the gap head on. You would expect water to more rapidly erode what it pushes than what it flows over. How could a mountain that was in the direct path of the flood be impacted so little, while at the same time hundreds of feet of solid rock under the flood be so easily eroded?