Well, with the Mississippi delta at least, all (or most) of that shredding is due to the sheer humungous amount of water dumped into the Gulf by the river. The course of the river changes, sometimes quite radically, over the centuries, depositing sediment into different places, cutting new channels, etc. You also have to take into account the damage wreaked on the coastline in both places by storms. With that in mind, I don't know that there's any sure-fire way to 'get' that effect other than to just draw it that way. If you really get stuck, you could always take bits and pieces of real-world maps, merge them together, and trace the result.