It dams up the water by blocking the ditch so the water goes out side channels or another ditch. The weight of the water seals the tarp against whatever surface it gets pushed against.
Hard to imagine areas that they buried drainage tile to get it dry enough to plant crops.
I never knew about drainage tile til I got on some ag discussion boards around the mid 2000s.
The tarp has a sleeve manufactured into it across one side. That is where you slip the board or in the case of this one a pole. The dam causes the water to flow out of the ditch and spread across the field. Also on this one this morning I had to put a rock under one end of the pole to keep the water from flowing over the top of the tarp. We have ditch plows that you run down the ditch every 3 years or so as the silt will build up in the ditch. And as you can see in that picture I roundup the ditch to keep the grass and weeds from slowing the water flow. It also helps the guy mowing to see the ditch.
Yep, there are two ditches. One down each side of the valley. There are to head gates at the diversions from the river. Both head gates are on my property. Being at the top of the ditch does have its benefits. The guys at the bottom of the ditch do complain from time to time. But there are a lot of other people between me and them. We all do try to get along. Historically there have been shootings over irrigation water.
Those gates are called weirs. They are generally used to divert from a larger ditch into the smaller field ditches. I have 5 of them on my property. Mine are all made of concrete. I put boards across them to block the flow and divert water. When not in use I pull the boards (not the most fun job) to allow the water to flow freely down the ditch.
Every person on the ditch has a schedule and a certain time to do their irrigating. There are ditch riders that maintain the ditches, and are on the watch for people that steal water. A VERY serious offense that can result in some pretty terrible consequences if you make a habit of stealing water.
The location on the ditch does not always determine the priority for water though. Water rights were filed as the country was homesteaded. One or more landowners would get together to dig a ditch with a point of diversion from the creek or river and file the application with the State Engineer (at least in Wyoming and I think it is real similar in most other states that recognize the prior appropriation doctrine). After constructing the diversion and means of conveyance (ditches, flumes, siphons, etc.) they had to file a proof showing that they were beneficially using the water for the use or uses applied for (irrigation, domestic, stock water, mining). For irrigation, the quantity allowed was 1 cubic foot per second per 70 acres under irrigation. So long as the creek or river is not under regulation, everyone can take their full appropriation or more of water (free river status). If any person on the creek system with a priority dated water right calls for regulation, the water commissioner has to start turning off the most junior appropriations upstream of that point of diversion until the senior appropriator receives their appropriation amount. Everyone on the system usually comes out better if they can hold off calling for regulation, so cooperation can pay off.
