I grew up in Catonsville and fished the Patapsco river almost daily growing up. I have not fshed it for several years since getting married. Each year I promiss myself that I'm going to fish the river again but for whatever reason I never seem to get the time. I mostly fish the Potomac and Upper Bay now out of a $40K Bass Boat but I still miss those days when i was younger on the river. You can fish all day on some sections and have the entire river to yourself -- tons of wildlife, deer, beaver, etc...
Growing up my friends and I would spend the day wading the section from the Route 40 Bridge down to Ellicott City. I have also fished on the river north of the Route 40 Bridge (Daniels Area) and south near Elkridge in the State Park.
No matter where you fish on the river you can expect to catch the same thing. Plenty of bluegill and tons of small smallmouth bass. You can use any number of artificial lures for either species (beetle spins, small crank baits and small twister tail grubs in Chart or Chart with Silver Sparkle always worked for me).
Watch the weather before you go fishing. The river will rise rather quickly during a thunderstorm. Several people drowned over a three year period back in the late 80's - early 90's. The river is mostly shallow (knee deep) but there are several holes that get deeper.
You will catch a lot of fish starting in mid to late April through the Fall. Most of the smallmouth will average under 12 inches with maybe one a year over 14 inches. Its still a blast on ultra-light gear. In good water you can have a 50 fish day easily. --- Good Luck
Rich
Westminster, Maryland