Take The 70 Degree Pledge
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FACTS

 What About Feeder Creeks,
Springs and Upwellings?

Yes, there is some cold water inflow below the Mio Dam, but page 16 of the Status of the Fishery Report makes it clear that these inflows are insufficient to offset the massive amounts of water coming in from Mio Pond.  Don't believe any claims about upwelling or feeders significantly mitigating the inflow from the pond.  These waters mix and the result is a warming of the river.  There is far more warm water coming in than cold water seepage, much more.  In addition, the river is dropping in the summer months and the sun is heating the shallow flats and runs. 

The effect is so pronounced that a study demonstrated less than 1% quality habitat - water depths greater than three feet and aquatic temps below 70 degrees - for trout is available during the hottest weeks of the summer.  Relaxing the parameter to 72 degrees produces about 3% quality habitat, that's about 12 acres for the 22 miles of river below Mio.  Even if you find these areas many of them are little more than slivers on the riverbank, 10 or 15 feet long and perhaps two feet wide.  There may be a fish there but as far as survival goes, it's where they are released.  Pull a fish out of 68 degree water and release him in 71 degree water and he's likely a goner.

Why is this not a problem above Mio?  

It can be, but rarely is for several reasons.  First off, the Au Sable above Mio does not have to deal with Mio Pond.  It's the surface water of the pond and the overflow at the Dam that is the major culprit.  (Consumers Energy installed an aerator in 2009, and there are reasons to be optimistic.)  The Upper Au Sable benefits as well from large amounts of cold water inflow from feeders, springs, etc and long stretches of shade.  Some areas get less than an hour of sun a day in High Summer.  This is why the South Branch, tail-water like the Big Water, does not have a temperature problem.

The river here is also very shallow.  This allows for quicker cooling at night.  The North Branch, for example, can get much warmer than the river below Mio, but does not stay that way for long.  Water temps can swing 15 degrees or more on it in a single day.  These are known as diurnal changes and the Au Sable below Mio does not significantly benefit from them.

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