Let me present my hypothesis more in full now.
The entire Channeled Scablands area was covered by water. All the sedimentary layers under the water was not solid rock, but loose wet dirt. Wallula Gap was a dam at the time.
If we look at the diagram below, the flat area in the entire southeast part of Washington state was a lake.
The Channeled Scablands was formed by a breach at Wallula Gap. If we look at the map above, we notice that one of the thinest places holding back the water would be at Wallula Gap. As the water in the Scablands lake receded, it eroded the soil underneath. It eroded it down to the solid rock layer, which is the basalt layer. Any basalt rocks would get eroded very little, including the basalt pillars. Since it was only wet dirt that was eroded, instead of solid rock, it explains why there are not sedimentary rock boulders anywhere. Also, it explains the curved erosion patterns and the standing columns in the Channeled Scablands. It also explains the ripple marks found throughout the region. As water came out of Wallula Gap, the water made a relatively straight and narrow path to the ocean.
My hypothesis also explains why the Scablands is a unique area geologically, at least in the US. Basically it requires a location where it could hold a lot of water, but has a weak point of failure to release all the water.
If we look at a topographical map of the US, we see this is the only location in the US that fulfills this.
My hypothesis would be testable by checking if the layers on each side of Wallula Gap match. They could not match if a river had cut through it 10 million years ago.
There is one major assumption with my hypothesis that is not compatible with conventional geology. That is all the layers between the water and the basalt was loose wet dirt.
Also, there is another question. How could so much water have accumulated in the Scablands area without a breach at Wallula Gap earlier? Why didn’t it get breached earlier?
This is where the Global Flood comes in. The Global Flood created all the sedimentary layers found in the Scablands area. And also, it created all the mountain ranges. The layers were all created first, that is why they are all parallel to each other. Then the layers were uplifted to form the mountains. And in the process, it formed the cavity that formed the Scabland lake. Shortly afterwards, the lake was breached at Wallula Gap.
There are more details about the Global Flood itself and what it did in the Global Flood thread.
I don’t expect many people to accept my hypothesis, at least not in the next 50 years. But, I do believe it has more supporting evidence for it than the ice dam hypothesis.