It’s important to stay aware of the tides on visits to the estuary. Then you won’t be surprised when access to the boat ramp you launched from ebbs away, or when the path to the sandbar you hiked out on floods. Good timing also makes for better exploration. Hunting around the rocky shore at low tide, for example, allows you to find more kinds of intertidal creatures.
Incoming and outgoing tides drive currents, making them particularly fast and unpredictable in certain waterways of the estuary. The timing of high and low tides vary considerably from one end of the estuary to the other.
Two Forces
Twice each day seawater floods Great Bay Estuary, and then ebbs away. The same forces keeping the earth and the moon in their daily orbit around each other drive this primeval tidal rhythm.
A gravitational force pulls the earth and moon together, and a centrifugal force pulls them apart. Our ocean is continually drawn in the direction of the moon’s gravitational force. It actually causes a bulge of water on the side of the planet facing the moon. When this water is intercepted by one of the continents, its shoreline experiences high tide. The ocean eventually ebbs to the low tide mark as the moon moves along on its orbit.
If the moon orbits the earth only once a day, why are there two tidal cycles? The centrifugal force of the earth’s rotation around the moon creates a second bulge of water on the opposite side of the planet from the moon. We get the second high tide of the day when the moon is over the other side of the world.
But the estuary’s rivers and bays do not flood or drain simultaneously. It takes time for tidal currents to make their 15-mile trip up and down the estuary, flowing around all the topographical features along the way. This is why you have to factor in a certain amount of lag time, depending where you are in the estuary, when you look at a chart of Portland, NH tides. For example, Dover Point is about 1.5 hours behind Portsmouth.
Here the tide is first low in Portsmouth at 2:03 a.m.
| Low a.m. |
High |
Low p.m. |
High p.m. |
|
| Portsmouth | 2:03 | 8:19 | 2:15 | 8:41 |
| Dover Point | 3:13 | 9:30 | 3:25 | 9:52 |
| Salmon Falls River | 3:38 | 9:32 | 3:50 | 9:54 |
| Squamscott River RR Bridge |
4:27 | 10:16 | 4:39 | 10:38 |
Did you notice that you add a little less time for low tides? This is because ebb tide currents are faster than flood tide currents.
Also, each day the timing of the tides is 50 minutes later than the day before, because it takes the moon 24 hours and 50 minutes to complete its daily rotation around the earth. Since there are four cycles a day (two high, two low), each cycle will last six hours and 12.5 minutes apart. [4 x 12.5 = 50 minutes]
Spring/Neap Tides
Twice a month (around the new and full moons) the sun comes into line with the earth and the moon, creating additional gravitational pull to the tides. This causes high tides to be at a maximum high, and the low tides to be at a maximum low. These are called “spring tides,” referring to the added force driving the water. At alternate times during the month the sun and moon are at right angles to each other, and the sun’s influence minimizes the moon’s gravitational pull. This causes minimal high tides and minimal low tides, also called “neap tides.”
Currents
The Piscataqua River has one of the powerful currents on the Eastern Seaboard. They are especially so while tides are coming and going, and ease off during transitions, a period called slack tide. In spring, rain storms swell river flows. Those pulses of water funnel down rivers into the estuary driving yet stronger currents.
Boaters should watch for dangerous cross currents and changes in direction of currents. Areas to be especially cautious of include: the cove next to the Coast Guard station by the mouth of the Piscataqua; the channels at Dover Point; and Furber Straight leading into Great Bay.
Other water bodies in the estuary have their own challenges. Portsmouth harbor is a deepwater commercial port with lots of boat traffic. Great Bay, by comparison, is a backwater with much less traffic. Yet its shallow waters and expansive surface can make for very wavy conditions. The ebbing tide leaves more than half of Great Bay exposed as mudflats.