I just launched in North Chelmsford (Wotton St.) last Sunday. It was my first time fishing the river. The Chelmsford launch has a nice pot hole on the driver's side of the ramp closer to the water. There is no dock so you just have to beach your boat. I have not yet tried the Lowell Ramp (Pawtucket Blvd.). It is a bigger facility and has a floating dock to tie off when launching. There is also a ramp behind the White Horse Inn in Methuen on 110. There is a $10 fee for launching and you pay at the restaurant. I have not trued there either as the place is not open in the morning. I will get by there at some point and will post whatever info I can.
I checked in at the White Horse today and you are allowed to launch outside of business hours BUT not too early as it would piss off the owner. Any time after 8 am is ok apparently. You can just pay at the restaurant when you pull your boat.