Tides work in a river as waves propagating inland from the ocean, influenced by the river's unique conditions.
Tides, primarily driven by the gravitational pull of the Moon and Sun on Earth's oceans, don't stop abruptly at the coastline. Instead, they extend into connected waterways like rivers.
The Ocean's Influence
The tide in a river consists of waves which propagate into it from the ocean. These are essentially tidal waves that travel up the river channel. The strength and distance these waves travel upstream depend heavily on factors like the size and shape of the river estuary and the volume of freshwater flowing downstream.
Factors Distorting River Tides
As the tidal wave moves upstream, it interacts with the river environment. It is distorted by friction and by the discharge of fresh water.
- Friction: The riverbed and banks create friction that slows the tidal wave and reduces its energy. This effect is more pronounced in shallower or narrower parts of the river.
- Freshwater Discharge: The continuous flow of freshwater downstream opposes the incoming tidal flow. This current modifies the speed and shape of the tidal wave. A strong freshwater discharge can significantly limit how far upstream the tide can penetrate.
Characteristics of River Tides Upstream
The interaction of the oceanic tidal wave with the river's specific conditions leads to observable changes in the tidal pattern as it progresses upstream.
- Changing Tidal Shape: Unlike the relatively symmetrical rise and fall of ocean tides, river tides often become asymmetrical.
- Duration Changes: Observations show that, as it progresses upstream, the time interval between low water (LW) and high water (HW) shortens so that the duration of ebb increases steadily. This means the flood tide (water rising) happens more quickly, while the ebb tide (water falling) takes longer. This phenomenon is often called "tidal asymmetry" and can create stronger flood currents than ebb currents.
- Amplitude Reduction: Generally, the tidal range (the difference between high and low water) decreases as you move further upstream due to friction and the opposing freshwater flow. However, in certain estuary shapes, the range might initially increase before decreasing.
- Tidal Bore: In some rivers with specific channel shapes and large tidal ranges, the leading edge of the incoming tide can form a wave known as a tidal bore. This is a dramatic example of the tidal wave being distorted as it moves into a shallowing and narrowing channel.
In essence, river tides are a modified version of ocean tides, shaped by the physical characteristics of the river channel and the dynamics of freshwater flow.
