This picture was taken on the Rio Grande River Valley, on the left side of the image is the river, the right side is the flood control ditch. I am guessing the purpose is when the river rises too high some underground pipes divert the extra water to this so it can drain into the flood control.
This image there is a second, black pipe visible that goes under the ditch road and empties into the ditch.
I would love to know what the grey pipe is called so I can look up a diagram of how they work exactly.
I'm pretty sure that 'stand pipe' is correct. It allows for pressure equalization. There's a lot to this topic. And I find it fascinating.
You could dig in over at https://www.abcwua.org/, they are the ones in charge of municipal water issues.
Also Cooperative Extension Service : Acequias of the Southwestern United States: Elements of Resilience in a Coupled Natural and Human System https://pubs.nmsu.edu/acequias/index.html
https://www.mrgcd.com/ - Middle Rio Grande Conservancy District
https://en.wikipedia.org/wiki/Acequia
https://waterdesk.org/2024/10/can-new-mexicos-ancient-water-system-survive-climate-change/