It's not clear what the correct name of the stream downstream (NNE) of the confluence of Western Run and Beaverdam Run is.  It's not labeled on a 1957 USGS Cockeysville quad, so no help there, but Maryland's "Merlin" GIS system says that it's Beaverdam Run (as of June 2024). I'm mildly surprised by this because Western Run is the larger of the two. (By the way, the old NCR railroad bridge over Western Run still stands, but the deck is gone and it's now a tip-toe crossing on the remaining girders. Not recommended.)

It really still IS a stream and not part of Loch Raven Reservoir: I've paddled it and it's flowing, i.e., it's not stillwater until it turns right (east) and meets the reservoir. The location of the transition point depends on how full the reservoir is and how much rain has swelled the two incoming streams. The presence of the old millrace on the east side indicates that it once had enough gradient and flow to generate usable power - that had to be before the reservoir existed.