Checking the books shows that there are 2 mayflies that are called March Browns. Both belong to the same family, have similar behaviors and frequent the same water types but are of different genera & species.
The American March Brown (Stenoma vicarium) is the eastern variety, while the western variety is the Western March Brown (Rhithrogena morrisoni). From the info I have on them the major difference is in size. Adult American March Brown's are ~ 12-15 mm while the Western March Browns are ~ 8-12 mm. This info is from "An Anglers Guide to Aquatic Insects and Their Imitations" by Rick Hafele & Scott Roederer.
I agree with Greg on the small stoneflies being the first bugs out in the spring on the moving water.