You are a lucky dog. Period. Let that water run! Let it soak the wood; it'll last just about the same length of time under flood as it will without. If you try to treat wood, are you working to keep the wood from warping with the water, or is it the rot, or is it the bugs like termites? With all the chemicals we are bombarded with in small yet persistent amounts, it might be a good idea to keep more chemicals away from the food one eats (growing up in the DDT/Rotenone/Malathion eras tends to make me draw a line somewheres).

So, just start and use whatever you have handy: block, brick, 2X4 fir, plywood, whatever. if you don't invest too heravily in infrastructure (?) then when you rethink things in a year or six months, it'll be easier to modify things.

To people who do not know Arizona, it is dry dry dry and even, perhaps, dry. Water is expensive as the aquifer has been pumped waaaaaaauyyy low and way down. It is old ocean floor, so clay and sand (think "adobe") hold sway over much of the state. One day's rain in Miami Florida can be greater than one year's rain here. Municipal flood irrigation is a big luxury.

Use concrete blocks

cold_weather is right. I grow vegetables and other plants in an irrigated yard and *nothing* has suffered from the regular flooding. Your plants need the deep watering and I doubt concrete blocks shoved (or mortared) together will let enough water through. Plus, they absorb much more heat than wood (thus baking the roots) and will soak up water the plants need. Go with wood, just *not* pressure treated wood. You'll be glad you did.