**ALASKA BORE TALES**

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Photo -- NOAA Central Library

A LOCAL GUIDE TO BORE TIDE SIGHTINGS

by Marianne Molchan-Douthit

July, 1985

REVISED APRIL 2008

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Updated and Distributed by:

National Oceanic and Atmospheric Administration

National Ocean Service, Office of Coast Survey

Alaska Region Navigation Manager

4230 University Dr., #102

Anchorage, AK 99508

\(907\) 786-7004

**AN INTERNATIONAL PHENOMENON**

They have been described as breaking waves as high as 26 feet, sounding
like locomotive trains, foaming and frothing along at speeds between 6
and 24 mph. Tidal bores are anything but *boring*. There are more than
60 locations worldwide where tidal bores can be seen. The highest bore,
called the pororoca, is located in the Amazon basin and can be 25 feet
tall. This particular bore will also span across several miles of river.
The effects of this bore have been noted as far as 500 miles up river.
China\'s Tsientang River bore is considered one of the strongest bores,
barreling up the river at between 18 and 24 mph near the city of
Hangchow. The British Severn River bore has been known to carry surfers
upstream for miles. This particular bore also disturbs the
bottom-dwelling eels as it rushes up the river, which allows local
residence to catch the eels in their nets when the river begins to flow
downstream again. In North America, bores have been recorded in the Bay
of Fundy (Nova Scotia), the Colorado River (Mexico), and Alaska. The
Colorado River bore has practically disappeared due to other reclamation
projects which include a series of dams in the Colorado River to
regulate the flow.

**THE ALASKAN TIDAL BORE**

The only locations in the United States where tidal bores occur
regularly are Turnagain and Knik Arms. The Turnagain Arm bore not only
exists, but according to Susan Bartsch-Winkler (USGS), who has studied
the sedimentology of the intertidal zones of both Knik and Turnagain
Ams, bores in Turnagain Arm are a daily occurrence. On the days with
small tidal fluctuations you have to look a little harder, but there
will be a bore down a channel somewhere in the arm.

To understand the nature of a bore, one must first examine the
environment necessary to create such a phenomenon. Both Turnagain and
Knik Arms are uniquely suited for the formation of tidal bores for two
major reasons:

1\. They are both adjacent to a body of water with a large range in tide
(Cook Inlet, 35 ft).

2\. Their configurations are, by comparison to Cook Inlet, narrow,
shallow, and gently sloping, forcing the rapidly rising tide waters to
form a tidal flood with a raised abrupt front.

The extreme range of Cook Inlet tides can be attributed to the natural
resonance of the inlet being nearly equal to the daily tidal interval of
12 hours and 25 minutes. Every basin of water has a natural resonance,
that is, a natural vibration associated with the time it takes the water
to slosh back and forth from one end of the basin to the other. Both
Cook Inlet and the Bay of Fundy\'s tidal ranges are amplified to
phenomenal heights due to the combined tide and resonant effects. Half
of the 60 known bores in the world are associated with bodies of water
that have similar resonant effects.

**SOME CHARACTERISTICS OF ALASKA\'S TIDAL BORES**

\*It takes 5 1/2 hours for the tidal bore to travel from the mouth of
Turnagain Arm to the end of the arm (it forms at the mouth within one
hour after each predicted low tide in Anchorage).

\*The tidal bore in Knik Arm is not as common as the Turnagain Arm bore
but is definitely worth noting. In 1974, NOAA set up a \"bore watch\" in
Knik Arm to verify or disprove statements that the Knik Arm bore had
disappeared. NOAA Ship RAINIER\'S personnel were able to witness bores
on two separate occasions just north of Goose Bay, in Knik, and near Bay
City. These bore tides were noted on days with minus tides of (-)4.5
feet or less. The bores were apparent approximately 1 1/2 hours after
predicted low tide in Anchorage.

\*The Knik Arm bore has also been viewed from the village of Knik
approximately three hours after predicted low tide in Anchorage.

\*The size and speed of the Alaska bore tides, and the Knik Arm bore
tide in particular, have changed since the 1964 earthquake, due to the
drastic change in bottom configuration. The land subsided and created
deeper channels which produce smaller bores.

\*In Turnagain Arm, bores range in size from 1/2 foot to 6 feet high and
travel at speeds between 10 and 15 mph. Knik Arm bore size and speed are
on the low end of those figures.

\*A bore tide can be seen nearly every day somewhere in Turnagain Arm
just after low tide. Its size depends on the range of the tide for that
day. The most dramatic bore tides occur during days with extreme minus
tides (between -2.0 feet and -5.5 feet).

\*Opposing winds can enhance the bore tide (i.e., winds blowing from
Portage to Anchorage).

\*The ever-changing channels throughout the arms dictate the size of the
bores and the best points from which to view them.

\*As you follow the bore up the channels from the Indian Creek side of
Turnagain Arm, you can see the bore breaking on the other side of the
arm near Hope.

\*The larger bores can be heard with a sound similar to that of a train.

**TIDAL BORE TALES**

For those Alaskans who are fortunate enough to either live or spend a
great deal of their time along Turnagain Arm, tidal bore stories are a
common topic of conversation.

One such person is Doug Fesler, who works for the Alaska State Parks
Service and has lived in Bird Creek since 1975. He remembers seeing a
large bull moose out on the tidal flats of Turnagain Arm one afternoon
in October, 1978. The bore was really large that day and could be heard
and seen rushing toward the moose. Frightened and confused the moose
tried unsuccessfully to out run the bore. The frothing wave picked him
up and carried him along with it for a few hundred yards until the moose
simply disappeared below white water never to surface again. The power
of these waves should not be underestimated.

Doug has also seen windsurfers barely escape the bore at Beluga Point.
Sea gulls often fly along the bore tide looking for small fish in the
white water. Beluga whales have been seen playing in the wave as it
forms near the mouth of the arm.

Many a hooligan fisherman has been surprised by the bore tide as it
breaks along the shores of Turnagain Arm. It is highly recommended that
recreators stay clear of this powerful phenomenon. The best advice is
not to be in the water or near the shore shortly after low tide,
especially during an extremely low tide.

**PLANNING TO SEE A TURNAGAIN ARM BORE TIDE**

Planning a bore tide sighting is easier than one may think. With some
general knowledge about bore tides and a tide table for Anchorage, one
can witness many bores in Turnagain Arm throughout the year. Following
are some guidelines to make your bore tide field trip worthwhile:

1\. Choose the new or full moon periods during the month to catch
extreme tides.

2\. Once you identify specific time periods with extreme tides, look for
a day when the predicted tide range for Anchorage includes large
negative values for low tide.

3\. Find yourself a good vantage point between McHugh Creek and Indian
Creek or at the Hope campground for watching the tidal bore.

4\. The bore should reach the pullout near Indian Creek along the Seward
Highway approximately 1 1/2 hours past predicted low tide in Anchorage.

5\. Knowing the bore travels about 10 to 15 mph, you can estimate when
it will be at other locations in the inlet.

6\. You should be at your chosen vantage point at least 1/2 hour before
you expect the bore to arrive.

7\. The water will seem calm and uneventful just prior to the bore\'s
arrival.

8\. Look and listen for a series of undulant waves two to three feet
apart which may be breaking along the shore or breaking across the
channel or channels.

9\. Enjoy the view.

10\. One of the best ways to view the tidal bore in either Knik Arm or
Turnagain Arm is by air. This way you can watch it move up each channel,
change speeds, disappear and reappear as it travels along. The drawbacks
to viewing the bore by air are that you cannot really judge the height
of the bore, nor can you hear it roar along. Following the bore from the
road one can see the bore change in size as well as hear it.

**EXCELLENT DATES FOR TIDAL BORE VIEWING 2008**

January 23

February 20

March 10

April 7

May 5-6

June 4-5

July 3-4

August 1, 30

September -

October 17

November 15

December 14-15

NOAA 2008 tide predictions for Anchorage, AK:
<http://tidesandcurrents.noaa.gov/get_predictions.shtml?year=2008&stn=1036+Anchorage>

**REFERENCES**

Bartsch-Winkler, Susan; Schmoll, H.R.; 1984: [Guide to Late Pleistocene
and Holocene Deposits of Turnagain Arm]{.underline}: Alaska Geological
Society Field Guide. pp. 17, 63-64.

Carter III, Samuel; 1966: [Kingdom of the Tides]{.underline}, pp. 71-75.
Hawthorn Books Inc.

Jeffers, K. William; 1974: [Knik Arm Tidal Bore Report OPR-469-RA-74
Upper Cook Inlet. Alaska.]{.underline}

Lynch. D.K.; October 1982: [Tidal Bore: Scientific
American.]{.underline} V. 242, No. 4, pp. 146-156.

NOAA, National Ocean Service, [2008 Tide Tables, West Coast of North and
South America Including the Hawaiian Islands]{.underline}

Woods, Von S.: November-December, 1981: [Bore Tides in Upper Cook Inlet:
Mariners]{.underline}

[Weather Log]{.underline} V. 25, No. 6, pp. 383-385.