Guide to bore tide sightings

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Photo – NOAA Central Library


by Marianne Molchan-Douthit

July, 1985


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

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 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.

*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.

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 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.

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:

Bartsch-Winkler, Susan; Schmoll, H.R.; 1984: Guide to Late Pleistocene and Holocene Deposits of Turnagain Arm: Alaska Geological Society Field Guide. pp. 17, 63-64.
Carter III, Samuel; 1966: Kingdom of the Tides, pp. 71-75. Hawthorn Books Inc.
Jeffers, K. William; 1974: Knik Arm Tidal Bore Report OPR-469-RA-74 Upper Cook Inlet. Alaska.
Lynch. D.K.; October 1982: Tidal Bore: Scientific American. 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
Woods, Von S.: November-December, 1981: Bore Tides in Upper Cook Inlet: Mariners

Weather Log V. 25, No. 6, pp. 383-385.

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