Lisa, I read your post and I think I understand what you are looking for b/c I've done something similar. I've found my favourite light summer dresses at "outdoor" shops. We have one I love called "SAIL", but unfortunately it is not in Winnipeg. But you can order online. Here is a thought:

You do have Mountain Equipment Co-op.... You might have luck there. Their website has some good ideas, though sometimes the dresses are a wee bit short:

Ah I'm glad you like the Universal Standard and All Saints options, Lisa, and you are very welcome!

I've also embraced dresses over the last few years, for the summer. And skorts - but they tend to be above the knee, which you said you don't want.

I'm also someone who wants a summer dress to touch my body as little as possible. Both because it's cooler, and because I don't want to feel body conscious when I'm hot and sweaty. I've found a few great ones from Eileen Fisher - and her fabrics seem to wash so well. Also Boden - bonus with them is they often come in tall, which means you can get the length you want, even if it means hemming after the face. I've got some t-shirt dresses from BR.

Another brand I didn't see mentioned yet is Lole. I haven't looked at them lately, but in the past I've noticed some great casual dresses.