Hello Theresa,

Dredged material can be highly variable in quality and properties. If it has a lot of organic material from waterfowl poop, leaf material, or anything like this, it could potentially be used similar to compost. However, If you are going to use it as compost, I would allow it to sit for at least a couple of months to fully break down. If it has large amounts of manure, it should not be used as fertilizer directly on plants as this could be a biohazard.

When the material is dredged out, you will want to take a look at it before attempting to use it. Some dredged material has a lot of rocks and sometimes even garbage that blows into the lake a sinks. If this is the case, you may not want to use it in the garden. 

Good luck,

Robby