Morgan Run is not one of the three listed. As Ryan said, those creeks are a good deal farther to the west. I'm well into my 50s and have been fishing Morgan since I was a kid (I grew up in Reisterstown, just across Liberty Reservoir, so during fishing season I was over there at least once a week). While Morgan Run does not have anything approximating a self-sustaining population of wild rainbows, Liberty Reservoir has a year-round population of stocked rainbows that thrive over the summer months owing to lake's depth. There aren't a lot of them, but they do run up into Morgan Run and a few other tributaries in the autumn and winter, presumably in an effort to spawn. (I've been known to refer to them as Maryland steelhead, and they're only slightly more common than unicorns.) It's a long shot, but not beyond the realm of possibility that a spawning pair of lake rainbows somehow found each other and made a nest between London Bridge and Klee Mill, or there abouts.
Many years ago, I'm guessing late 1970s, old federal brood fish were stocked each autumn for a couple of years at Klee Mill. It was only a few fish but most of the females still had plenty of eggs in them and went through the motions of spawning. I don't recall ever seeing any young of year rainbows from that effort, though.
Maryland does not spawn out its own fish, but rather buys eggs from locations across the country. Back in the 1970s and at least even into the 1990s (and maybe even now), Maryland got its rainbow eggs from Washington State. These fish tended to be highly migratory. In Morgan Run, they would provide good fishing from late February through maybe early June, but as soon as that water temperature hit a certain point, they all headed downstream and ended up in the lake.
Morgan has had a population of wild browns for decades, but it struggles, or at least it used to, because upstream from Route 97 just outside Westminster, the stream ran through a lumber mill and a lot of rain would filter through the sawdust before entering the creek. A soaking rain when eggs were in the rocks could really do a number on a year class because because of the sawdust juice. Meanwhile, several nearby streams with no sawmills have much healthier wild brown populations (as I was able to confirm on a few trips this past autumn in pursuit of Maryland steelhead; no steelies, but plenty of resident browns).
Another curious aside about Morgan Run: Back in the early 1980s, Maryland fisheries came into possession of four landlocked Atlantic salmon. The fisheries guys regarded them as curiosities, and they were raised in a cooperative hatchery facility managed by a fishing club. When the Atlantics hit about 14 inches, they were turned loose in Morgan Run. You can bet I fished for them and hoped for an autumn return, but I never so much as saw them (outside the hatchery) and never heard of anyone catching them. Possibly the raccoons had a feast.
Short answer: A wild rainbow in Morgan Run is a distinct possibility, but the odds are very long.
