Deepest of the channels, that’s what makes the Boston North Channel so important. This channel connects Boston Harbor to Massachusetts Bay. It is near Boston. Almost all deep-draft ships use the Boston North Channel.
The Boston North Channel is also known as the Broad Sound North Entrance Channel. It stretches 1,500 feet wide. That is approximately 460 meters. The channel begins south of Deer Island Light. This is at the eastern end of the President Roads anchorage.
From there, the Boston North Channel proceeds northeastward to Broad Sound. It passes eastward of Finn’s Ledge. Recently, the channel’s depth was increased. It now reaches 51 feet which is about 16 meters. This change accommodates the larger New Panamax ships. These ships have been in use since the Panama Canal expansion in 2016.
Massachusetts Bay connects to the Gulf of Maine through two channels. One is the North Channel. The other is the South Channel. Boston Harbor has two channels connecting it with Massachusetts Bay. Those channels are President Roads and Nantasket Roads.
Currents in Boston Harbor are mainly tidal. Subtidal currents are wind-driven. They are influenced by the Gulf of Maine. Freshwater fluxes also have an impact. Wind-driven currents matter in Boston Harbor. They also play a role in shallow coastal areas.
There are no major rivers flowing into Massachusetts Bay. The largest river entering Boston Harbor is the Charles River. The Charles-Mystic-Neponset river system’s freshwater input is not enough to drive significant surface currents. The river-driven currents are only a small percentage of the strength of the tidal currents.
Both Massachusetts Bay and Boston Harbor experience strong seasonal changes in water properties. In winter, conditions are well-mixed. However, during summer, there is strong stratification. This is due to riverine input and seasonal warming.
Tidal currents near Point Allerton show strong asymmetries. This happens between ebb and flood tides near many outcrops and islands in Boston Harbor. Wind-driven flow is simulated using two patterns. One pattern comes from NW winds. The other comes from SW winds. These patterns combine to create a current pattern for the user-defined wind field.
Another current pattern mirrors the Gulf of Maine circulation. It flows counterclockwise into Massachusetts Bay. Then it flows back out near Race Point. Spring runoff in rivers that enter the Gulf of Maine increases the counterclockwise circulation in Massachusetts Bay. This is the Massachusetts Coastal Current.