Oregon Drift

Nearly the lowest tide of the year just off North Bend's new boardwalk. Photo by Lou Sennick

Clamming, crabbing, fishing, beachcombing or boating –  much of what we do around here requires knowledge of the tides. But deciphering the tide table is just the beginning of understanding them.

Every day on the West Coast, we get a higher high tide, a lower low tide, a lower high tide, and a higher low tide. The two high tides are a little more than 12 hours apart, so the time of the tides gets later every day.

If you spend a week clamming on Coos Bay, and you want to clam at lower low tide, you’ll have to plan every day differently. For example, if lower low tide is at noon on Monday, it’ll be at about 1:08 p.m. on Tuesday. Wednesday it’ll be at 2:15.

Clammers also should be aware that in the summer, the lowest low tide occurs in the morning, whereas in winter it occurs in the afternoon or evening.

On the bay, the range between the high and low tides –  the amplitude –  is six to 10 feet. The amplitude varies during the month. At the new and full moons, the amplitude is greatest, because the sun and the moon are lined up. Particularly high and low tides are seen about twice a year, when the new or full moon coincides with perigee, the time that the moon approaches the earth most closely in its orbit.

This photo shows high tide just off North Bend's new boardwalk. Photo by Lou Sennick

On the Oregon Coast, the tide sweeps from south to north. Tide tables list the times of high and low tides for reference points along the coast and provide correction factors for other places. For example, if your tide table says ‘Humboldt Bay,” it will have correction factors for Charleston, Empire, and Coos Bay.

The tide takes only 9 minutes to get from Humboldt Bay in Northern California to the mouth of Coos Bay. But from there, the tide takes 7 minutes to get to Charleston, another 50 minutes to Empire, and another hour to reach all the way to the Coos Bay waterfront. At Shinglehouse Slough near Millington, the delay is about two hours; at Green Acres Boat Ramp, about three.

This delay is caused by the shape of Coos Bay, explained Mike Graybill, director of South Slough National Estuarine Research Reserve.

‘The bay is 10,000 acres, and 15 miles long, and there’s only a small hole at Charleston for the water to come in, so it’s like filling a swimming pool with a garden hose,” he said. ‘Also, our bay is really shallow, like a ribbon of water, so there’s a lot of friction as water moves into the bay.”

When the incoming tide meets the outflowing river, water actually piles up in the lower part of the bay until the tide can overcome the pressure of the fresh water.

Conversely, crossing the bar at Charleston is particularly dangerous on a receding tide, Graybill said. ‘The water rushing out stands up the ocean waves and makes them unstable.”

In our shallow estuary, fresh and salt water mix thoroughly, unlike the deep estuary of the Umpqua River near Reedsport.

‘In the Umpqua, you can see cattails, a freshwater plant, as far up as Dean Creek,” Graybill said. ‘The fresh water floats on the salt water, and the incoming salt water pushes the fresh water back up the river without mixing with it.”

The interaction of currents in the estuary can be seen in the form of ‘tidelines,” which are long strings of floating foam and debris that develop when two currents rub against each other as a result of wind action or the flow of streams. Birds and fish (and, therefore, fishermen) are drawn to these lines because food is abundant there.

To help you plan activities around the tides, tide table booklets are available at hardware and tackle shops. You can also see tide charts at

tidesandcurrents.noaa.gov/tide_predictions.shtml.

If you’ll be further up in the bay than the Coos Bay waterfront, the map Oregon’s Coos Bay Estuary Water Trails provides tide corrections for all boat launch points on Coos Bay and South Slough. You can get a copy at South Slough Reserve’s interpretive center, or download it at www.coostrails.com.

By Gail Elber