In addition to the comments by Fred Cowles and Chuck Cubbage I would like to add the views of Howard Tanner, Director of the DNR '75-83. Dr. Tanner was one of the speakers at the Fresh Water Forever Forum at St. James Catholic Church in Grand Rapids Thursday night.
He believes that Granholm should recombine the DNR and the DEQ, though "perhaps not as it was". Citing his reasons he said that our sewers dump into our rivers and discharge in the Great Lakes. Oversimplifying, sanitary engineers want to know how much effluent can dumped in a stream before the smell becomes objectionable. A fisheries biologist wants to keep the water clean enough to support fish and other life. These two need to be under one director so they can sit down together and work out their differences satisfactorily for both.
In response to a question, he added that thermal loading of water (thermal "waste", i.e. nuclear plants, etc.,) is another good reason to combine the DNR/DEQ.
These look like good reasons from where I sit, and I would add that its less expensive to run one coordinated organization, than to run two that don't communicate well.