It’s not a structure, not like a dam that you would imagine. It’s a catch basis and in dry season or even a reasonable rain would just look like a field, dip in the ground.
We’ve just had a very abnormal amount of rain. If we get some wind in it will start to dry things out.
I live in the desert and am familar with retention basins. But I was looking for any type of man-made structure (a berm or concrete weir for example), but haven’t found any yet.
The retention dam is somewhere upslope from these coordinates:
Latitude: 39.259382
Longitude: -119.509091°