I was a bit surprised to see that there wasn't any water in the arroyo. What was going on?!? I then realized that the flow had only started about a minute before, and soon a lobe of water began flowing downstream. I captured that on video as well.
I then followed the flow upstream to see what was happening. There was good flow coming from the detention basin to the west. That was meeting up with flow coming from the detention basin to the north. In some places there was a well-established channel. In some places there was insufficient gradient, so the flow spread out in sheet flow. I made my way up to the concrete channel that leads from the north detention basin. It was flowing pretty good. There was definitely some erosion going on. I then got something of a shock when I realized that the new development to the north would also be draining into the detention basin. That will be adding a significant volume of water to the mix! But then again, the outflows are pretty much restricted, so the flow rate might not be significantly higher, but the flow duration might be. The other modification to my hypothesis is that there may not be significant erosion in the future due to control points. There probably will be channel establishment just below the detention basins, but due to grade controls, it shouldn't be extreme. The waterfall will act as a control point since it will take a lot to erode bedrock. Just below the waterfall there is a narrow bedrock canyon which will also act as a control point. Finally there is a stretch of arroyo that is fairly flat just below the canyon. Velocities should be low since it appears so flat. The channel then goes under a road bridge which I noticed has a concrete lip underneath it. That will also act as a grade control to prevent significant erosion. So basically my study will focus on the development of channels over time as they react to greater volumes. There should be a geomorphic response since you're now putting flow down a channel perhaps 5 times a year when in the past there might have only been flow down the channel once every 5 years.