reared E. doro*thea, and it is a yellow species as an imago. Thoughtlessly I have not photographed it though I have had several opportunities. IF the size gets reduced down to 6-9 mm, I would go with a Serr*atella of some sort.

The only Serratella species that I'm really familiar with is deficiens. Because the ones that I see are nearly black, I think that "colors" my impression of Serratella and I tend not to think about the other species.

Another possibility might be that this could be part of an extended invaria hatch. In places with really heavy populations, like some of the spring creeks in Southcentral PA, they hatch well through July and get smaller as the hatching period progresses. The later-hatching invaria are often confused for dorothea, but Luke Jacobus once told me that dorothea dorothea is mostly found in mountainous (or formerly mountainous) areas. An accurate size and perhaps the name of the stream it was associated with would help.