Mile 85.8 - Past the Big Slackwater parking lot and boat launch is the guard lock for Dam 4. It was here that boats would leave the canal and be pulled along the river using the bank as the towpath. So the canal stops here and doesn't reappear until lock 41 at mile 88.9. It was the dam itself that caused the slack water.
An artsy shot of the lock.
On the other side of the guard lock the canal ends at this inlet gate. Water from the river would enter this gate to fill the canal and was controlled by the sluice gates you can (barely) see here. You could see them better if the gate hadn't been cemented in.
Water from the Potomac enters here and flows to the inlet gate. The slab on the top is the bridge over the towpath and the dangling logs a sort of "filter".
This is your final warning! Stop! Read! Head! The detour route is again displayed with the bad news that you have some backtracking to do.