# What Is the Old Sow Whirlpool? > Old Sow is a large tidal whirlpool that forms in the Western Passage between Deer Island in New Brunswick and Moose Island at Eastport, Maine. Incoming water from the Bay of Fundy squeezes through irregular channels toward Passamaquoddy Bay, where the seafloor... Canonical URL: https://www.argo.net/what-is-the-old-sow-whirlpool/ Byline: ARGO.net Editorial Team Published: 2026-09-02T17:25:09+00:00 Categories: Explainer, Oceans ![A powerful whirlpool churning in turquoise water](https://www.argo.net/wp-content/uploads/2026/09/argo-wave20-53553-pexels-34802203.jpg) **Old Sow** is a large tidal whirlpool that forms in the Western Passage between Deer Island in New Brunswick and Moose Island at Eastport, Maine. Incoming water from the Bay of Fundy squeezes through irregular channels toward Passamaquoddy Bay, where the seafloor and colliding currents create a broad zone of rotating, turbulent water. NOAA calls Old Sow the [largest whirlpool in the Western Hemisphere](https://oceanservice.noaa.gov/facts/old-sow.html). It has been measured at more than 250 feet across, though its size and appearance vary with the stage of the tide, wind and other conditions. Smaller vortices around it are locally called piglets. The feature rarely resembles the perfectly round funnel seen in fictional maelstroms. Boaters are more likely to encounter boils, eddies, standing waves and patches of water moving in different directions. Those motions reveal how a huge tidal flow responds to a complicated underwater landscape. ## Fundy tides drive the current The **Bay of Fundy** is famous for its large tidal range. As the tide rises, ocean water must move through the passages around islands at the bay's western entrance. The narrowing route accelerates the flow, much as water speeds up through a constriction in a pipe. Old Sow develops during the exchange between the Bay of Fundy and Passamaquoddy Bay. The [NOAA Passamaquoddy Bay model](https://gnome.orr.noaa.gov/doc/location_files/passamaquoddy_bay_tech.html) attributes the whirlpool to extreme tidal range combined with variations in local bathymetry, the depth and shape of the seafloor. Tidal currents reverse as the water level moves from flood toward ebb. The whirlpool's position and strength change during that cycle. Maine's official atlas says the feature is commonly most active about three hours before high tide, while calmer periods occur closer to slack water. **Spring tides** can increase the flow around new and full moons, when solar and lunar tidal forces reinforce one another. Wind and atmospheric pressure may alter observed water levels as well, so a clock prediction cannot describe every patch of turbulence. ## The seafloor bends and separates the flow Water entering Western Passage makes a sharp turn near the southern end of Deer Island. It crosses a deep trench before passing an undersea high point. Countercurrents arrive through neighboring channels. Friction with the bottom and the abrupt bend prevent the current from following one smooth path. Part of the flow separates from the main current and begins rotating. Fluid dynamicists call this an eddy. Irregular depth creates vertical motion too, sending water upward over ridges and downward into deeper pockets. The interaction produces **shear**, a sharp difference in speed or direction between adjacent layers of water. Small rotations can grow along that boundary and peel away as piglets. Their brief lives make the surface look disordered even when the tide follows a predictable cycle. **Standing waves** appear where fast current encounters slower water or an obstruction. Unlike an ordinary wave traveling toward shore, the crest can remain in roughly one place while water rushes through it. A boat crossing the feature feels a sudden change in pitch and control. Large circular motion does not require an open hole beneath the surface. Water entering a whirlpool continues through the passage. The visible depression reflects lower pressure and curved flow near the center, not a drain emptying the sea. ## Old Sow changes from minute to minute The whirlpool is best understood as a region of turbulence rather than a permanent object. One tide may produce a recognizable central vortex, while another creates scattered eddies and rough water over a much wider area. Maine's [Old Sow atlas entry](https://www.maine.gov/sos/maineatlas/explore-the-atlas/old-sow-whirlpool) reports a measured diameter of **250 feet** and a funnel drop of 12 feet under notable conditions. These observations describe an exceptional expression of the feature, not a fixed geometry repeated every day. Measurement is difficult because the boundary moves and breaks apart. Aerial photographs can estimate diameter using known landmarks, while current meters record speed at one location. Sonar reveals the trenches and slopes that organize the flow below. ## Small craft face the greatest danger A large powered vessel with an experienced operator can often transit the wider area safely under suitable conditions. Kayaks and small sailboats have less ability to overcome a fast current. Other low-powered craft may also struggle to recover from a steep standing wave. The Canadian Hydrographic Service's [Gulf of Maine sailing directions](https://waves-vagues.dfo-mpo.gc.ca/library-bibliotheque/chs-shc-ATL106-eng-202310-41207002.pdf) identifies dangerous eddies in the passages and warns that Old Sow can be hazardous. Mariners use current tables together with local knowledge. They also check weather forecasts rather than treating high tide alone as a schedule. A whirlpool can rotate a boat away from its intended heading. Crosscurrents may then carry it toward rocks or another vessel. Engine power does not guarantee control if the operator enters at the wrong angle or fails to anticipate the current on the far side. Viewing from shore avoids those risks. Deer Island Point Park in New Brunswick overlooks the area and [Tourism New Brunswick](https://tourismnewbrunswick.ca/listing/old-sow-whirlpool) recommends the period about three hours before high tide. Visibility still depends on conditions because much of the motion occurs as low relief across the surface. ## The whirlpool lifts food toward the surface Turbulence mixes the **water column**. Nutrients from colder, deeper layers can reach the surface along with small organisms. Fish and seabirds feed in the resulting waters. NOAA describes this upwelling as a beneficial environmental effect of Old Sow's circulation. Mixing also transports oxygen and redistributes heat. The biological response changes with the season and river flow. It also depends on the plankton community entering Passamaquoddy Bay. A productive feeding area can appear around a physical feature that is dangerous to navigate. Harbor porpoises use the region. Seals and seabirds also appear, though their presence on a given tide cannot be promised. Fast water may concentrate prey along fronts where currents meet. Predators learn to exploit these temporary boundaries. The same mechanism operates at tidal races elsewhere. Narrow straits with underwater sills create energetic mixing zones when the tidal range is large. Old Sow is unusually prominent because several favorable ingredients meet in one constrained passage. ## Its name comes from sound and folklore One explanation compares the churning noise with pigs feeding, which led to the name Old Sow and the nickname piglets for smaller vortices. Another connects sow to the older word "sough" for a sound associated with suction or drainage. Neither story changes the physical cause and the precise origin is uncertain. Maritime place names often preserve what sailors heard or feared long before instruments mapped the bottom. The dramatic vocabulary can encourage exaggerated pictures of a vessel disappearing into a funnel. Old Sow's genuine hazards begin with **current speed**. Sudden changes in direction compound the risk, especially where the water grows rough. These familiar marine hazards are intensified by the geometry of this passage. ## A natural laboratory on an international border Old Sow sits beside the United States-Canada boundary in waters tied to fishing, aquaculture and coastal communities. Understanding its currents supports navigation and spill planning as well as public curiosity. Tide gauges at Eastport provide continuous water-level observations. Models combine those measurements with shoreline and bathymetric data to predict how water moves through the bay. Local turbulence remains harder to resolve than the broad rise and fall of the tide. More detailed current mapping can show where eddies form under different tidal ranges. It can also improve forecasts for drifting objects or pollutants. The feature illustrates why a water-level prediction does not fully describe conditions in a narrow channel. Old Sow is spectacular because a regional tide becomes visible through local geology. The Bay of Fundy supplies the moving water. Western Passage accelerates that flow before the uneven seafloor breaks it into rotation. What appears chaotic at the surface follows a repeatable set of physical constraints below. **Related reading:** [why ocean whirlpools are compared with black holes](https://www.argo.net/are-there-black-holes-in-the-ocean/) and [how tidal currents change](https://www.argo.net/ebb-flood-and-slack-water-how-tidal-currents-change/). **Related reading:** [why ocean whirlpools are compared with black holes](https://www.argo.net/are-there-black-holes-in-the-ocean/) and [how tidal currents change](https://www.argo.net/ebb-flood-and-slack-water-how-tidal-currents-change/). **Explore this topic:** [What Is the Atlantic Meridional Overturning Circulation?](https://www.argo.net/what-is-the-atlantic-meridional-overturning-circulation/) and [How Big Is the Atlantic Ocean?](https://www.argo.net/how-big-is-the-atlantic-ocean/).