I wouldn't cover it at all. Mostly because my birds spend lots of time outside the run and have made a hot mess out of that skirt. They scratched at the river rock and shot it all over the place.You wouldn't fill with river rock? Or you would fill with river rock nor cover ends with stone? *newbie still trying to figure out how to build my run*
It would have been easier to just leave it alone and let the grass grow through.