This is the Swamp Locks Gatehouse in Lowell Massachusetts which controls one section of the water flow that used to power the mills. There are several such Gatehouses around the city, all serving different sections of canal. When the water is low, you can see the rocks surrounded by just a trickle. But when the water is high, it rushes by through the sluiceways and around the open gates. The canal boats for tours are anchored on the other side of this building and the water there was barely rippling. It was 2 different worlds, just steps apart. When the mills were at full operation the noise from them must have drowned out the pounding of the water.
This is one of the mill buildings as seen from the backside of the gatehouse (which would be to the left in this image), the locks that served it are just barely visible where the water turns out of sight. The city is criss-crossed by canals and locks from the heyday of the mills.