On Route 29, Colesville Road is Burnt Mills East park, it is on the same side of the road as the Trader Joe's and California Tortilla. This park has a reasonably sized parking area and a number of nice trails. Facing the trailheads, the trail on the right is closest to the Northwest Branch and it is called the Fall Line trail. It Proceeds downstream and ends when it meets the Northwest Branch trail. On the left is the Northwest Branch trail. In this area it is higher up the hill than the Northwest Branch stream, but it will eventually come down and meet the stream where the Fall Line Trail ends. The Fall line trail is the steeper, rockier of the two trails and is only suitable for hiking, whereas the Northwest Branch trail has been designed to allow biking too.
Another trail in the area is the Copperhead Run trail. There are 3 "chute" trails that connect it to the Northwest Branch.
The Northwest Branch itself is a good-size stream. In many places it is rocky, but other areas are smooth and placid.
One of the interesting, but noisy, sights is where Interstate 495 crosses the stream. The bridge is high above the trees, and although you can hear the traffic, when you are walking south it is hard to see the bridge until you are right under it.