I have been trying to figure out the pictured stenonema for about 3 years. This species emerges as early as the last week in march in Southeastern Minnesota. I have found nymph specimen on several creeks over three years but the numbers of this specie of mayfly on one particular creek is way off the charts this year. Thanks for your help

Hsershen-

If you are talking about the one on the left (the only one I can see clearly) with lighter abdominal segments 5-7, it's a Hendrickson (Ephemerella subvaria).  Here is the link to an article I wrote for the inaugural issue of Hatches Magazine on The Hendrickson Hatch.