We reported on 1/25/2010 about a big storm that dropped so much water so fast that the dam below the Arcosanti site broke.
The dam has a spillway at the south end, but the force of the water was so strong that it deposited large rocks and bolders into the spillway and just about blocked it. We can see the large silt deposits that now cover a large part of the field.
Water pressure from continuing rainfall after this storm ate away at the bottom of the gap …
… and left a canyon like cavity in the dam.
Some repair has been done to fill the broken of the dam part way.