The one place in the library with no current station in it, where the stream lags the tide by hours, and where depth and air draft matter more than the current does.
That is a question about depth rather than current. The channel is a dredged cut and it shoals between dredgings, so what matters is your draft against the dredged depth plus the height of tide at the moment you are in it. A boat that does not fit does not fit at slack either — waiting for the turn does not help, only a bigger tide does. Read the current Notice to Mariners and controlling-depth survey before committing a deep-draft boat.
No, and this is the mistake that catches people. The channel is driven by the difference in water level between Skagit Bay at the south end and Padilla Bay at the north — not by the tide at either end — and the stream that difference produces lags the tide by about five and a quarter hours. Timing a transit off a La Conner tide table will be wrong by hours rather than minutes.
North or south along the cut, and which one depends on whether Skagit Bay or Padilla Bay is standing higher at that moment rather than on whether the tide is rising or falling. Because the two ends belong to different tidal regimes, the channel can be setting different ways at its two ends at the same time, so a single direction for the whole channel is an average and not a promise about the water where you are.
There is not. The nearest published current station is about four miles outside the entrance, which is why the stream here has to be computed rather than looked up. TidePush drives a physical model of the channel with the head between the two tide gauges and calibrates it against a 2006 USGS survey — the only current measurements ever made inside the cut.
It is a dredged federal cut with a twelve-foot project depth, so what decides whether you fit is the height of tide at the moment you are in it, added to that dredged depth, against your draft. A boat that does not fit does not fit at slack either — no departure time changes it, only a bigger tide. Note that the project depth is a design figure rather than a recent survey.
About seventy-five feet at the mean higher high water mark, and because clearance moves opposite to the tide it varies by roughly fourteen feet across the range — the tide that gives your keel the most water gives your mast the least. Cables have also been reported over the channel about three feet below the span; that figure is reported rather than surveyed, and for a tall rig it rather than the bridge is the limiting obstruction.
Every other timed passage in TidePush is a published current station: an agency measured the water and predicts it, and the app reads that prediction. The Swinomish has no station in it at all. It is also the only place in the library where three different constraints bind at once — the stream, the depth under your keel, and the air over your mast — and where the second of those usually matters more than the first.
In short: The channel connects two bays whose tides do not keep the same time. Water runs through it because one end is higher than the other, and that is why the current does not turn when the tide is high.
The channel is a cut between two bays that keep different time. Skagit Bay at the south end has the larger tidal range and runs later than Padilla Bay at the north. Water flows through the cut because one end stands higher than the other, so what drives the stream is the DIFFERENCE between the two — the head — rather than the tide at either.
That difference peaks well away from local high or low water, and the channel has enough water in it that the stream takes time to respond to the push and time to stop once moving. The result is a stream that lags the tide by around five and a quarter hours, which is why a tide table for La Conner is the wrong instrument for this decision.
It also follows that the two ends need not agree. The cut can be setting north at one end and south at the other, so where the app names a direction it is describing the channel’s average behaviour.
In short: Nobody publishes a current forecast for this channel, so TidePush calculates one from the tide gauges at each end and checks it against the only survey ever done inside the cut, in 2006. It is a model, not a measurement.
TidePush synthesises the water level at each end from published harmonic constants, takes the difference, and drives a physical model of the channel with it: the head accelerates the water, friction resists it, and the balance between them sets both the peak speed and the lag. The constants are calibrated against a 2006 USGS survey, which is the only occasion the current in this channel has been measured. The model reproduces both the peak speed and the delay that survey recorded.
An earlier version of the app took its channel velocity from a NOAA hydrodynamic forecast, read at the grid element off La Conner. That seemed the obvious source until it was checked against the survey: the finest published element of that model is around a hundred metres and the maintained channel is thirty metres wide, so it could not have been resolving the cut. Its agreement with the survey was poor and the disagreement was structured rather than random, which is the signature of a model resolving the wrong thing rather than one that is merely noisy. It now contributes only as an optional correction to the water level at the ends, where its resolution is adequate.
THIS IS A MODEL AND THE APP SAYS SO. Being the only available answer is a reason for care, not confidence: the calibration rests on one survey from 2006, and nothing has measured the channel since. Treat the direction and the timing as good, and the exact speed as an estimate.
In short: Whether you fit depends on the tide at the moment you are actually in the channel. If your boat is too deep, no departure time fixes it — only a bigger tide.
On most gated passages the question is when to go. Here it is often whether to go at all. The cut carries a twelve-foot project depth, and the water you actually have is that depth plus the height of tide at the moment you are in the channel — not at the moment you cast off, which for a boat coming from Everett or Seattle can be several hours earlier and several feet different.
Two caveats belong with that figure. The project depth is a design target rather than a recent survey; dredging runs on a cycle and the bed shoals unevenly between rounds. And the model assumes a flat bed both along the channel and across it, where a real dredged cut shoals at its edges — so a boat off the centreline has less water beneath it than the number allows for. This is the largest unquantified assumption in the depth check, and it is stated rather than buried.
In short: The bridge stays put and the water moves, so a high tide that helps your keel is the same tide that squeezes your mast. Cables have been reported below the span, and for a tall rig those are the real limit.
Air draft is the mirror image of depth, and the same tide drives both in opposite directions. The span offers about seventy-five feet at the mean higher high water mark, and swings roughly fourteen feet across the tidal range. TidePush computes the clearance for the minute you pass beneath it and shows it on the card.
It does not check that figure against your rig, because it does not know your air draft, and it says so rather than implying a verdict it has not reached. The more serious point is that cables have been reported over the channel about three feet below the span. That report is not a survey, and for a tall rig the cables rather than the bridge are the real limit.
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