Any type of Glyphosate product will work. Since you have water in the ditch you are limited to the types of herbicides you can use and they have to be approved for aquatic use. I'd buy a jug of Rodeo and mix 6 ounces of concentrate to a gallon of water. You also will need a sticker so get some cidekick. If you have no place to find it we have Touchdown Pro here which has the sticker in it. Usually retails for $32 per gallon. Other places sell Gly-Pro which also has the sticker in it. Take a look at your local feed stores and if no luck send me a PM.

***If you have woody plants in this ditch try some Habitat Herbicide. BASF makes it and it works great...