BTW, you guys in MI are very lucky to have both in your neck of the woods.
Sometimes I think we over look just how damn lucky we really are up there! Take that sweetheart of a river for granted, we do.
I just got back from a four night get-away up there again and I couldn't believe the variety of hatching bugs we had...Thursday we suffered through a low pressure system right over my head and one moment the sun came out and the next moment it was pouring rain all damn day. It finally eased up toward evening...But I got a PM from Tim Neal who fished that day and he did very well. I had bugs galore with not a great deal of larger fish feeding on them but I caught enough to keep me out there all day. (Tim and I were out there probably for the most part alone with the river to ourselves while others were sleeping off last nights Hex madness and dreaming of big flies/big fish).
During the rain we had the larger Light Hennies still (invaria), the little sulphers (dorothea), a couple Baetis and maybe the Pseudocleon( I know we changed its name), a few Light Cahill's, there were Iso's, Gray Drakes (Gonzo prefers Grey Drakes :)), in the middle of the night Hex! Little yellow stones and a few of the dorsata hanging around...a few caddis and hell I even had a Dobson fly land on me and that maybe a first for me.
In regards to your question about March Brown/Brown Drakes and where they occur in the river. I am going to go out a bit on a limb here and maybe Tim Neal and/or Gonzo can fill in the blanks, agree with me or not. The Au Sable is not a limited river where it has basically all the same habitat. It has a variety of different charateristics that are repeated over and over again downstream. (I'm talking here about the river in the flies only traditional fly fishing stretches up by Grayling (North, South and Holy Water).
Riffle, runs, pools, nice deep bends, and gravel bottom throughout. Along the edges you get sand, marl banks, and the muckier habitat where the burrowing bugs hang. So, when most folks up there talk about Hex water they are talking way down from where I usually fish, but the habitat for Hex, for example, is throughout the system. Those pictures I posted last year of L. recurvata were taken in the middle of the Holy Water and they were hatching from a smallish patch of muck up under some tag alders along the bank.
So, there is habitat for the March Browns (clinging type) and sandy/soft burrowing habitat for the Brown Drakes all along the river happening over and over again as you move downstream. Where I'm going out on the limb is that for a good many angler's (not all of course) I think the March Brown gets over looked sometimes. I think that some of the success for the smaller Robert's Drakes is due to the March Brown...Everyone talks about the yellow stones/sulphers/yellowish Brown Drake/ etc all being around at the same time, but so is the March Brown.
We have all heard about the massive spinner flights of Hennies (both light & dark), the Brown Drakes, and the Hex...but I think that the March Brown's being overlooked here somewhat has to do with its hatching behavior. They are sporactic hatchers and spit out all day & night a little at a time mixed in with invaria etc...Folks see large hatches and spinner flights of other bugs and get mesmorized by them...I have sat on the bank and watched female March Browns on the down stream side of wind protecting trees depositing their eggs in tiny little areas of broken water all day! One at a time or a few here and there...
The fish see them all the time! Let's not call it selectivity just familiarity...:) The angler is busy out in the middle of the river slashing away and telling his pals about the great sulpher hatch but missing the March Brown activity. A March Brown something-or-other is a great searching fly when nothing else seems to be going on in May/June.
Sorry for the ramble...I'm still on river time and my heads still humming and I wish I was still there...:) Don't tell my wife, I just got home for goodness sakes!