In the back of my mind I recall having heard thet there was some difference between eastern & western species of the march brown. I'll look thorugh my fishing books when I get home to try to answer this question.
With that said, the march brown has been one of my standard early season flies, on streams for several years. It has been a consistently successful pattern for me. Part of this success may be due to its being a reasonable match to food pellets at a hatchery.