It's not horseweed. I pull that out all the time. Leaves/stems are not hairy, but have a slight sheen (I took the above picture in the rain, so ignore the shinier raindrops). Stem is square in cross-section. Leaves are opposite.

Thanks for the response. I checked Senecio vulgaris and it's definitely not that. The flowers aren't composite. The leaves are soft. It started in the spring as a rosette similar to a dandelion, but the leaves were thinner at that stage and more sharply toothed. I've tasted it and it's milder than dandelion.

What a great idea for that space! You might want to peruse some sites to get a visual look at the plantings others have used (see my link below). Or just study creek beds (although your plants won't be the usual 'water' plants due to dry conditions (unless you want to lay a drip line along the banks of your stream). In real creek beds I think you'll find that there are all sizes of stones from boulders to gravel, grasses and wildflowers of different heights. It sounds like your choices will have to like a fair amount of shade. Also, I would recommend laying some thick landscape cloth in the 'creek bed' prior to adding the rocks. That way any weeds that eventually form between the rocks cannot establish very deep roots and can be more easily removed. Of course a little Roundup sprayed onto weeds might be just as easy. Here is a link that might be useful: google image pages of 'dry creek beds'

Rose of Sharon, Hibiscus syriacus, I think.