How can I control water and algae in drainage ditch?
Throw some bales of barley straw into the water. Barley straw is often used to control algae in fish/garden ponds. I've had pretty good luck using it in my pond. I hear that when the barley decomposes it releases/creates hydrogen peroxide and kills the algae.

Another thing you might try is adding snails and/or tadpoles to the water. They'll eat the algae (and the tadpoles will eat mosquito larvae too.) Adding a few goldfish wouldn't be out of the question either.

Water Hyacinth, Water Lilies, and Lotus are all good plants that will prevent algae from growing by limiting the amount of sunlight that can penetrate the surface of the water. Elodea is an underwater plant that will eat up all the nutrients that the algae need to survive.

The plants are going to set you back quite a few bucks, but the "critters" are pretty cheap. I think you can buy goldfish for fifteen cents each or something like that.

You could go all-out and pick up the plants and critters at the same time and make the ditch into a water garden....but that's a whole other thing.