Those grantees built up a muddy causeway that Natives had used to cut across part of the Back Bay into a Mill Dam, creating the Mill Pond. The Mill Creek cut across the peninsula to connect the pond to the main harbor on the east side. Tides filled up the pond and then let its water run out. Later people built a windmill in the area as well.
There were two bridges over the Mill Creek, called “the draw bridge” and “the mill bridge.” (I don’t recall ever reading about the drawbridge being raised, but it might have been.) The creek defined the boundary of the North End.
In 1769, Boston’s selectmen determined that the heirs of the original grants had let the mills deteriorate. They therefore took over the property and assigned George Leonard to refurbish the mills and grind all the grain that the town owned. (Such practices should give pause to anyone who still thinks that early America had a laissez-faire economy.)
In the early 1800s the new Mill Pond Corporation filled in the pond with earth from Beacon Hill and Copp’s Hill. Soon that shallow area was dry land, to be developed as part of West Boston.