You are beginning with a false premise "The irrigation canal has no fish or crawdads in it, because its only full for a short time."
Abstract.—Irrigation canals can be a major source of mortality for fish in the Rocky Mountain region. Our study looked at how fish were affected by the irrigation canal system in the Smiths Fork, a tributary to the Bear River in western Wyoming. Source
In most cases it is not known whether fish in canals are actively reproducing or whether they are only periodically recruited from the natural waterways. Source
Even if the canal water has some physical means of preventing fish from entering, the influx of water could have profound effect on many creatures the mink eat, who may live in the area of canal.
Mink eat just about any type of animal that lives in and near water, including fish, frogs, ducks, crayfish, eggs, lizards, grubs, earthworms, mice, and muskrats. Source
Mice, lizards, and earthworms, may have been dwelling in burrows in the canal that suddenly flooded, and now they are homeless and easy prey.