Melanie, this 'dry creek bed' installed by the county to channel stormwater is probably something you don't want to mess with. Did the county line the swale with rip-rap (6-8" rocks) in the bottom and up the sides to prevent erosion? Check your survey and your county's real estate tax parcel records. They're probably on-line and viewable. The Feds have strict rules regarding stormwater channels, natural or municipally installed and most every waterway has 'buffers' that can't be disturbed. Look for the term " S.W.I.M. " which I believe is Storm Water Integrated Management..a buffer of specific width to each side of any natural or constructed waterway. Even if you don't have this restriction, there's no use trying to fight the leaves. They will blow off or degrade naturally to feed the soil and the trees.

I did something similar on my side yard because our houses drain to the street out front. We put down 2 layers of landscape fabric after getting the slope right, then decomposed granite with bull rock on the edges. Most of the granite has washed away over the last few years & we will be putting down more bull rock to replace it. It flows very well in torrential rain, but I do have to spray the area with vinegar quarterly to keep weed seeds from sprouting in there. We did not try to plant that area, but if we had more space we might have put some large potted plants thru there. I'll get a pic later today so you can see it.

"is there any evidence that a rock creek bed would be superior to a swale for moving storm-water through the property?" No. Rock mulch might be preferable, though, in situations where grass or groundcover can't grow well. However, this is very few situations. Dry stream beds are usually created because someone wants the appearance of such a structure. Unfortunately, many of them are not well done and don't look good.

Will the bed be used to redirect run-off during rainy times and with snow melt or will it remain dry and just be decorative? If intended to be functional, you want to make sure it is sized appropriately to accommodate the water flow. Typically, dry stream or creek beds are wider than they are deep. And usually by a ratio of 2:1. So a 2-3' wide bed should be 12-18" deep. The rocks you use should be in a range of sizes, just as they would be in a real stream bed. You will need a fair number of large rocks (6-10") as edging along the sides but placed rather randomly. And even a few random larger boulders, especially sited at any curves in the stream bed. For that length and width, you will need 2-3 tons of 1-4" diameter river rock and probably another ton of the larger sized rock (~80-100). The boulders you would purchase by quantity rather than weight.....as many as you need. Make sure you lay landscape fabric along your stream trench (anchor it firmly in place with the boulders and large edging rocks) and then a 1-2" layer of coarse sand. Then fill with your river rocks, evening them out a bit across the surface. You can use whatever shade tolerant plants you like but generally you want lower growing plants closer to the edges of the steam bed and taller plants at a bit of distance. Hostas come to mind immediately, with perhaps some Japanese forest grass (Hakonechloa) here and there along the edge. Primulas are another possibility if enough moisture in the area. Other plants I would consider are astilbe and columbine for some height but you can really use any combintion of shade tolerant plants you like. Pinterest is good for some inspiration on planting layouts. The bottom line is you want it to look natural. So avoid any rows of plants or too precise placements :-)