All water bodies on Earth, whether they are freshwater or seawater, will have a specific color based on factors such as purity, depth, and composition of the bottom. These factors affect how light is absorbed and reflected, which in turn affects how we perceive the color of the water.
If you have ever visited the Caribbean, you may still remember the crystal-clear blue waters surrounding the islands. However, if you are from Maine, you might associate the stormy ocean with a dark grey color. Similarly, if you live near a river in India or a large lake in Wyoming, your perception of water color may be different. While many people think of water as blue, there is actually a wide range of colors that water can appear to our eyes.
String Lake, Wyoming (Photo Credit: Paul Brady Photography/Shutterstock)
So, why does the view of a tropical beach differ from that of a riverbank in the Congo? What causes the water around a coral reef to have a different color than the water in the middle of the ocean? What factors contribute to this surprisingly large range of colors?
Water And Light
While seawater and freshwater differ in terms of salinity, density, freezing point, and tonicity, these factors do not affect how water interacts with light. Before we explore the variation of colors in water bodies around the world, let’s have a quick reminder of the nature of light and its interaction with the natural world.
When we perceive a “color,” we are actually seeing the wavelength of light that is not absorbed by the observed object. Natural light contains all the colors of the visible spectrum, but most of the wavelengths are absorbed, and only one color is reflected back to our eyes. For example, when sunlight passes through our atmosphere, the short blue wavelengths of light are scattered more easily, giving the sky its signature blue color, as the other colors are absorbed more than reflected.
This may come as a surprise, but water is naturally colorless. However, when natural light interacts with a large quantity of water, the water molecules absorb most of the red, orange, green, and yellow wavelengths, while reflecting back the blue wavelengths. This gives the water a blue hue. Therefore, in response to the original question of this article, seawater is not inherently darker or lighter than freshwater; the factors explained below will determine the perceived shade of water.
Factors Affecting Water Color
When we consider the wide range of water colors found around the world, from black and red volcanic lakes to bright-green rivers and royal-blue stretches of ocean, there are several factors that can explain the variation. These include water depth, weather conditions, the composition of the water body floor, and the purity of the water.
Weather
Parents often tell their children that water is blue because it reflects the blue color of the sky. While this is partially true, it is not the only reason why water appears blue. The color of the sky is reflected on the surface of the water, which contributes to its blueness. However, the color of the water may change under different weather conditions, such as cloudy skies, regardless of sunlight.
Beautiful lake mimicking the color of the sky (Photo Credit : Evgeniia Ozerkina/Shutterstock)
Water Depth
When observing shallow water, we not only see the reflection of the sky and the natural blue color of water, but also the reflection of light from the ground. The light reflected off the sand brightens the perceived color of the water. In places like the Greek isles or shallow Caribbean shores, the light-colored material under the water makes it appear lighter blue. However, when we look down from the middle of the ocean, the water appears deep blue because the light is fully absorbed and only the blue light is reflected, without any influence from the sea floor. Therefore, deep waters often have a richer and deeper shade of blue compared to shallow waters.
Sea Floor Composition
The depth of the sea floor or river bottom is not the only factor that affects the color of the water; the composition of the surface also plays a role. The colors and qualities of sand in different parts of the world, which are formed by local geological formations over millions of years, impact how light is absorbed and reflected in shallow waters. This results in a wide range of potential colors. For example, the color of light reflected in seaweed forests will be different from that reflected in coral reefs.
Water Purity
The purity of the water is a crucial factor in determining its color. Estuaries, where freshwater mixes with seawater, often exhibit different colors due to variations in solute and particulate concentration. The minerals dissolved in water heavily influence its color, leading to the presence of red rivers with high iron content or black volcanic lakes with oxygen or sulfur caused by underwater upwellings of steam and gas. The color of rivers can vary from deep blue and grey to bright green or pink, depending on the mineral deposits they contain.
Apart from the minerals found in bodies of water, such as rivers, lakes, and oceans, the color of water can be greatly influenced by the presence of algae. Depending on the species of algae, the water can range from dark green to red or yellow. Pollution is also a contributing factor to the color of water, whether it is caused by man-made pollutants, soil runoff, or damage from storms. Chemicals and waste materials can alter the color of rivers and lakes, such as the Cuyahoga River in Ohio, which has experienced multiple incidents of catching fire due to its high pollution levels. Major storms or floods can also result in rivers running brown or black as debris and soil are swept into the water.
To conclude, when comparing the color of seawater and freshwater, there is no inherent relationship or superiority between the two. Oceans can be dark and rivers can be bright, just as seas can be turquoise and lakes can be black. The factors that should be considered instead are the depth of the water, the composition of the water body, the presence of impurities, and even the color of the sky at the time of observation. The remarkable diversity of the world’s waters can be better understood when standing by the Yellow River in China, contemplating a swim in one of Australia’s pink lakes, or snorkeling in the crystal-blue waters of Grand Cayman.