Grade 4

Tidal Power—It's Out of This World

Behold the power of tides. Twice a day, the ocean's surface rises and, about six hours later, falls. When it floods, or rises, it is high tide. When it ebbs, or retreats, it is low tide.

Earth's rotation and the Moon's gravitational pull cause tides. Everything on Earth—land, water, and air—is pulled toward the Moon. The pull is strongest on the side of Earth closest to the Moon. Thus, as Earth rotates, the ocean facing the Moon is tugged forward for high tide. It is also high tide on the side farthest from the Moon. This happens when the Moon gently pulls the center of Earth away from the opposite ocean. Low tides occur in places not lined up with the Moon.

The Sun's gravity pulls Earth, too. Twice a year, the Sun, the Moon, and Earth are in a straight line, and especially high tides result. These spring tides occur because the Sun and Moon's gravity tug on Earth together. Weaker, or neap, tides happen when the Sun, the Moon, and Earth form an L-shape. Now the gravitational pull of the Sun and the Moon are tugging in different directions.

The difference in height between high and low tide is the tidal range. Ocean depth and the shape of the coast determine the range. Canada's Bay of Fundy has the world's highest tidal range. During spring tides, the range is 50 feet! The bay's funnel shape and the upward slope of its floor cause these giant tides. As water flows into the narrowing bay, it cannot spread out. Instead, it must rise.

In the St. John River, Fundy's tides produce a phenomenon called a tidal bore. At low tide, the river flows calmly into the bay. When the tide rises, the river is slammed backward in a single wave or tidal bore. The bore goes back up the river. At the famous Reversing Falls, the bore climbs the waterfall and gushes down the other side. When the tide lowers, the river reverses again, flowing over the waterfall and out into the bay.

Tidal bores occur in other rivers, too. The bore on England's Severn River is famous among surfers. During spring tides, daring surfers ride the bore as far upriver as possible. The surfing world record was set on the Severn in 1996. One surfer rode the bore for almost six miles!

More important, the movement of tides can provide fuel. The flow of the tides powers turbines, or engines that produce electricity. Electricity generated by tides does not send harmful gases into the atmosphere. Unlike coal or oil, tidal energy is renewable. As long as the tide keeps turning, this clean, awesome energy source will never run out!


  1. What causes the oceans to rise and fall?
  2. Why are some high tides higher than other high tides?
    Explain your answer in a sentence or two.
  3. Why are tides a good source of energy?