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Alaska Tides for Boaters: Tide Tables, Currents and Big Ranges
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Alaska has some of the biggest tides in North America. In upper Cook Inlet the range can approach 40 feet, and many other harbors see ranges well over 10 feet. For boaters, tides control when you can launch, how steep the gangway is, how fast the current runs through narrows, and whether a cove is a safe anchorage or a mudflat.
How do tides work in Alaska waters?
Most Alaska coastal waters have mixed semidiurnal tides: two high and two low tides per lunar day (about 24 hours 50 minutes), usually of unequal heights. Ranges are largest around new and full moons (spring tides) and smallest around the quarter moons (neap tides).
Reading a tide table
Use the NOAA Tides & Currents website (tidesandcurrents.noaa.gov) or an app that uses NOAA data, and choose the station closest to where you'll be. Predictions list the time and height of each high and low.
- Heights are measured from Mean Lower Low Water (MLLW). Charted depths use the same datum, so a 0.0 ft tide means water is at chart datum.
- Minus tides drop below MLLW. A -3 ft tide means 3 feet less water than charted depths show, which can strand boats and expose rocks.
- Times may need converting. Check whether predictions are in local time with daylight saving time applied.
- Subordinate stations are adjusted from a reference station and may be less exact.
Estimate the water level between high and low: the rule of twelfths
Tide doesn't rise at a steady rate. A rough rule for a tide that takes about six hours to change:
| Hour after low (or high) | Share of total range that changes | Running total |
|---|---|---|
| 1st hour | 1/12 | 1/12 |
| 2nd hour | 2/12 | 3/12 |
| 3rd hour | 3/12 | 6/12 |
| 4th hour | 3/12 | 9/12 |
| 5th hour | 2/12 | 11/12 |
| 6th hour | 1/12 | 12/12 |
With a 20-foot range, the water can change about 5 feet in each of the middle two hours. That's why a launch ramp that was usable an hour ago may be out of the water now. Treat the rule as an estimate; local geography and weather change actual levels.
Regional tide differences
| Area | General character |
|---|---|
| Upper Cook Inlet (Anchorage, Turnagain and Knik arms) | Extreme ranges, fast currents, bore tides and dangerous mudflats. Not a casual small-boat area. |
| Lower Cook Inlet and Kachemak Bay (Homer) | Very large ranges; strong currents off points and in the inlet |
| Prince William Sound (Whittier, Valdez) | Moderate to large ranges; currents strongest in narrows and passages |
| Southeast Alaska (Juneau, Ketchikan) | Large ranges on spring tides; strong currents in narrows and channels |
| Kodiak and the Gulf coast | Generally smaller ranges than Cook Inlet; strong currents in some straits and passes |
Tidal currents, rips and narrows
Big tides move a lot of water, and where it squeezes through narrows or around points, currents can run several knots. The roughest water often forms where a strong current runs against the wind.
- Plan transits of narrows near slack water, using NOAA current predictions where available.
- Remember that slack current doesn't always coincide with high or low tide.
- In wind-against-current conditions, waves become short and steep, which is dangerous for small boats.
Tides at the dock and at anchor
- Floating docks: Most Alaska small-boat harbors use floats that rise and fall with the tide, so dock lines don't need adjusting. Gangways to shore, however, become very steep at low water. Plan carts and loading accordingly.
- Fixed piers and grids: On fixed structures, leave enough slack for the full range and check lines often.
- Anchoring: Calculate scope for the highest tide you'll see, and make sure you'll still float at the lowest. Check the swing radius at low water for rocks.
- Beaching and tide grids: Tide grids let boats dry out for bottom work, but timing must be precise. Ask the harbor for rules and reservations.
Safety: mudflats and bore tides
Glacial silt mudflats, especially in upper Cook Inlet and Turnagain Arm, can trap people like quicksand, and the incoming tide arrives fast and cold. Never walk on these mudflats. Turnagain Arm's bore tide, a wave that moves up the arm with the incoming tide, is spectacular to watch from shore but hazardous to small craft.
Frequently asked questions
Where do I find official tide predictions for Alaska?
NOAA's Tides & Currents website publishes predictions for Alaska stations. Choose the station nearest your location and check the time zone setting.
What does a minus tide mean?
It means the water level falls below Mean Lower Low Water, the datum used for charted depths, so there is less water than the chart shows.
Why is the gangway so steep at low tide?
Floating docks drop with the tide while the shore end of the gangway stays fixed. With Alaska's large tidal ranges, gangways can become very steep near low water.