Tides and Currents in the Pacific Northwest

Seymour Narrows, on the Inside Passage between Vancouver Island and the BC mainland, runs at sixteen knots on a spring ebb — among the strongest navigable tidal currents on the Pacific Coast. Recreational boats transit only at slack water; the slack window is approximately twelve minutes; the timing is published to the minute by the Canadian Hydrographic Service in Tide and Current Tables Volume 6.

That is the extreme end of the Pacific Northwest tidal scale. The working end is Cattle Pass at four to five knots, Deception Pass at eight, Active Pass at eight, Yuculta Rapids at eleven. None of these is optional knowledge for a PNW cruiser. The boat that arrives at any of them an hour late is the boat that doesn’t transit, or the boat that learns the difference between current and tide in the worst possible classroom.

This article is the working version of that knowledge. It assumes you have a chartplotter, a tide app, and a copy of either NOAA Tidal Current Predictions or the Canadian Current Atlas. It explains what those publications are saying.

Tides and currents are not the same thing

This is the first sentence in every honest piece of writing about PNW navigation, and it is the sentence most often skipped over.

• Tide is the vertical rise and fall of water level. Measured at fixed stations, predicted in feet above MLLW (Mean Lower Low Water — the chart datum).
• Current is the horizontal flow of water. Measured at specific reference stations, predicted in knots and direction.

The two are related — they are both consequences of the same gravitational forcing — but they peak at different times and in different places. Slack water — the moment of zero current — is not the same as high or low tide. In the Salish Sea, slack water in a pass typically lags the local high or low tide by one to three hours. The exact lag depends on the geometry of the pass and the volume of water being moved.

The cruiser who plans a transit using only the tide table will arrive at the pass at the wrong time, by an hour or more. Use the current predictions for the pass, not the tide predictions for the nearest reference station.

How tides work — the short version

Tides are driven by the gravitational pull of the Moon (primarily) and the Sun (secondarily) on the Earth’s oceans. The Moon orbits Earth roughly every 29.5 days, and the tidal cycle tracks the Moon’s relationship to the Sun:

• Spring tides occur at new and full Moon, when Sun and Moon align. Tidal range is at its largest.
• Neap tides occur at the quarter Moons, when Sun and Moon are at 90°. Tidal range is at its smallest.

In Puget Sound, spring tide ranges exceed 16 feet at Olympia. Neap ranges drop to 8–10 feet. The difference matters: spring tides drive harder currents, expose more shoreline, and produce the deepest minus tides that strand boats anchored in marginal depth.

Mixed semi-diurnal: the Pacific Northwest pattern

Most of the world’s coasts experience semi-diurnal tides — two roughly equal high waters and two roughly equal low waters per day. The Atlantic seaboard runs this pattern.

The Pacific Northwest does not. The PNW pattern is mixed semi-diurnal: two high waters and two low waters per day, but the two highs differ in height, and the two lows differ in height. Tide tables distinguish:

• Higher High Water (HHW) — the higher of the two daily highs
• Lower High Water (LHW) — the lower
• Lower Low Water (LLW) — the lower of the two daily lows; the chart datum
• Higher Low Water (HLW) — the higher

The practical consequence: between an HHW at 0700 (12 feet above MLLW) and an LLW at 1500 (–0.7 feet) in Seattle, you have a tidal range of 12.7 feet inside eight hours. Anchoring strategies, dock approaches, and shoal-water passage timing all have to account for the asymmetry. A spot that has 8 feet at the LHW may have 4 feet at the LLW. The chart doesn’t tell you that. The tide table does.

Reading a NOAA tide table

NOAA publishes tide predictions for hundreds of stations along US coasts. A daily prediction looks like:

Station: Seattle (9447130)
Date         Time    Height
2026-05-15   0024    -0.7 ft   Low
2026-05-15   0718    12.4 ft   High
2026-05-15   1334    6.1 ft    Low
2026-05-15   1956    0.3 ft    Low

(That second “Low” at 1334 is the Higher Low Water; the 0.3 ft at 1956 is the Lower Low Water — they are unequal. Mixed semi-diurnal in action.)

Heights are in feet above MLLW. Negative values are below MLLW: at 0024 in this prediction, the water is 0.7 feet below the chart datum. A spot the chart shows as 4 feet deep has, at 0024, only 3.3 feet of actual water under the keel.

The arithmetic of actual depth:

Actual depth = charted depth + tide height (signed)

A chart sounding of 6 feet at a tide of +5.0 feet gives you 11 feet of water. The same sounding at a tide of –1.5 feet gives you 4.5 feet. For a boat with a 5-foot keel, that’s a grounding.

NOAA tide predictions for the PNW are accurate to within about ten minutes of clock time and a few inches of water height. They are not weather-dependent — wind and atmospheric pressure can shift the actual water level by a foot or more from the prediction. Use the predictions; allow a small margin.

Currents — the same data structure, different stations

NOAA publishes Tidal Current Predictions separately from tide predictions. Each pass or channel has its own reference station; the prediction gives:

• Maximum flood (current direction “in,” typically toward the head of the inlet or sound) with peak time and peak velocity in knots
• Maximum ebb (current direction “out”) with peak time and peak velocity
• Slack water — the moment of transition, where current velocity is zero

A Cattle Pass current prediction for a typical day looks like:

Station: Cattle Pass (PUG1517)
Date         Time    Event       Velocity
2026-05-15   0312    Slack
2026-05-15   0612    Max Flood   3.2 kn   145°
2026-05-15   0918    Slack
2026-05-15   1224    Max Ebb     4.1 kn   325°
2026-05-15   1530    Slack

The slack at 0918 is what you transit on if you’re coming north into the San Juans. The 4.1 kn ebb at 1224 is what stops you if you arrive late. Plan to be in the pass at slack ± 30 minutes, not at any other time.

For the BC and Salish Sea side, the Canadian Current Atlas is the standard. It publishes current charts as visual flow diagrams keyed to time-from-slack at Race Passage; experienced PNW cruisers use it for visual planning even on the US side.

The named chokepoints

These are the passages that govern PNW cruising. Memorize them.

Deception Pass (north Whidbey Island). 8–9 knots peak. Slack window: about 15 minutes. Bridge clearance: 180 feet, scenic but irrelevant — the current is what matters. Standing waves develop on a strong ebb against a flooding wind. A sailboat under power at five knots will be carried backward through the pass at four knots if it arrives at peak flood instead of slack. Schedule the transit to within 15 minutes of slack water.

Cattle Pass (south end of San Juan Island). 4–5 knots peak. Standing waves at peak ebb in the right wind. Transitable at most current strengths in a powerboat; tough to impossible for a sailboat under sail in a strong opposing current. The most common entry to the San Juans from the south. Time it.

Yuculta Rapids and Dent Rapids (Vancouver Island, north of Stuart Island). 11 knots and 12 knots respectively. Both must be transited at slack — the slack windows are short (10–20 minutes), and the gates are roughly 30 minutes apart, so the transit is timed to catch both within their respective slack windows. This is the standard route for boats heading into Desolation Sound from the south.

Seymour Narrows. 16 knots peak. The strongest navigable tidal current in the world. Slack window: about 12 minutes. Recreational boats transit only at slack. The current direction reverses every 6 hours and 12 minutes; missing the slack means waiting 6 hours.

Active Pass (Galiano/Mayne Islands, BC). 8 knots peak. The complication here is BC Ferries — the Tsawwassen-to-Swartz Bay route runs through Active Pass continuously. Time the transit at slack, monitor Channel 13, and stay clear of the ferry track. Active Pass at peak current with a ferry coming the other way is the kind of situation experienced cruisers do not enter.

Dodd Narrows (south end of Nanaimo). 9 knots peak. Slack window: 15–20 minutes. The standard gateway from the southern Gulf Islands to Nanaimo. Transitable only at slack.

Tacoma Narrows. 5 knots peak. Less critical than the others — boats can power through against the current — but flood vs. ebb makes a noticeable difference in fuel consumption and crew comfort. Time it if you have the choice.

The Rule of Twelfths

The water level does not change at a uniform rate through a tidal cycle. It follows roughly a sine curve: slow at the beginning and end, fast in the middle. The Rule of Twelfths is a working approximation:

In a 12-foot tidal range, the water level changes 1 foot in the first hour, 2 feet in the second, 3 feet in the third, 3 in the fourth, 2 in the fifth, 1 in the sixth.

The same curve roughly governs current velocity in passes: slow near slack, peak at the middle of the cycle, slow as the next slack approaches. For passage planning, this means: if you can’t make slack itself, the hour just before or after slack is the second-best window. Hour 3 or 4 — peak current — is the worst.

Planning a transit

The standard process for any PNW passage that includes a chokepoint:

1. Identify the chokepoints on the route. Cattle Pass on the way north into the San Juans. Active Pass crossing into the Gulf Islands. Yuculta and Dent on the way to Desolation Sound. Use the chartplotter or paper chart to identify them in advance, not on the day.

2. Look up slack times for each, on the day. NOAA Tidal Current Predictions for US passes; Canadian Tide and Current Tables Volume 5 or 6 for Canadian passes. Slack times shift roughly 50 minutes per day; yesterday’s number is wrong.

3. Calculate the transit time between chokepoints. Distance ÷ realistic speed (allow for the worst-case current you might face on the day). A 25 nm leg at 5 knots is five hours; a 25 nm leg at 5 knots fighting a 2 knot current is closer to seven hours.

4. Work backward from the latest chokepoint. If Yuculta slack is at 1430 and the leg from Cape Mudge takes four hours, you need to be off Cape Mudge by 1030. If the leg from your starting anchorage to Cape Mudge takes three hours, you need to weigh anchor by 0730.

5. Build a buffer for weather. A 10–15 knot wind that shifts opposite to the current can produce 4-foot short-period seas in a pass that would be glass at slack with no wind. Check the wind forecast for the chokepoints, not just for the cruising area in general.

6. Have a fallback if the timing slips. Where do you wait if you arrive an hour early? Where do you anchor if you arrive an hour late and miss the slack? The plan needs an answer for both.

Wind against current — the second-order effect

The single most dangerous condition in the PNW is not strong wind. It is strong wind against strong current.

When wind blows against the direction of current flow, the water surface piles up on itself. Wave height roughly doubles compared with the same wind in still water. Wave period shortens, which means the waves break rather than roll. A 15-knot wind against a 3-knot ebb in Rosario Strait produces 4-foot short-period chop — uncomfortable in a 35-foot sailboat, dangerous in a small powerboat. The same 15-knot wind with the current produces 1- to 2-foot rollers.

Always check both the wind forecast and the current prediction for the transit window. The same 15 knots is two completely different sea states depending on the relationship to current.

What the tools do, and what they cannot

Sea.net’s tide tool and marine weather tool pull from the NOAA API and present current and forecast data for the major PNW stations in a passage-planning format. Use them.

The tools cannot:

• Predict atypical weather. A 30-knot frontal wind is unforecast at noon and on you by 1500 — atmospheric models miss this.
• Account for storm surge. A deep low pressure system passing over Puget Sound can raise the actual water level by a foot or two above predicted; a high pressure system can drop it. Both happen.
• Substitute for local knowledge. Cruisers who have transited Active Pass twenty times know things about the current pattern that don’t show up in the NOAA prediction. Talk to them.

A sentence to remember

Slack water is not high tide. The chart is not the tide table. The tide table is not the current prediction. The current prediction is not the wind forecast. All four matter on a PNW transit, and they are not the same thing.

The boat that misses the slack is the boat that learns the difference, in the worst possible classroom.

Related: Anchoring in PNW Waters · How to Cross the Columbia River Bar · Reading Marine Weather · Sea.net Tide & Current Tools