Only sea water appears blue in color, why this is not happening in river water?

The reason is not salt water. Large masses of water seem to be blue, both oceans and swimming pools.

Water absorbs most of the red wavelength. Most materials have their color from their surface level valence electron's absorption and re-emission processes. In water, it is not like that, because most of the color is caused by vibration of the molecules. Because of the photons get inelastically scattered (instead of just being absorbed), part of their energies get transformed into the vibrational energies of the molecules.

Now when a photon interacts with an atom, three things can happen:

  1. elastic scattering, Rayleigh scattering, when the photon keeps its energy and changes the angle

  2. inelastic scattering, the photon gives part of its energy to the atom and changes the angle

  3. absorption, the photon gives all its energy to the atom, and the absorbing electron moves to a higher energy level.

In the case of large masses of water, three things cause the color:

  1. elastic scattering, Rayleigh scattering, on the surface of the water, sunlight is reflected and appears to be blue from certain angles

  2. inelastic scattering, part of the photons' energies transfers into the energies of the vibrational energies of the molecules. This means, that because of Hydrogen bonding, the red color wavelength photons will be shifted to blue color wavelengths.

  3. water can absorb and re-emit light too, but because of the high ratio of refraction and reflection in water, the ratio of photons absorbed is low.

This gives a body of water when looking through it a blue color. But then comes the question, why is a body of water when looking at it (not through) blue? Light scattering by suspended matter is required for the color in this case, and the blue light needs to return to the surface to be visible. Such suspended matter can also shift the scattered light to green color, often seen in rivers. The answer to the question of why rivers appear green instead of blue is because of those three factors have different effects on different types of masses of water.

Pools appear usually more colorless, but still blue, and not green. This is because the effects come in this order of importance:

  1. reflection of the bottom of pool is dominant

  2. reflected light from the surface (blue) is less dominant, and vibrational shifts are less dominant than the color of the bottom

  3. reflected light from suspended matter (green) is less dominant

  4. angle of observation less dominant

Oceans appear bluer because:

  1. reflected light from the surface is dominant and vibrational shifts

  2. angle of observation (and depth) is important because oceans look blue from far away, when deep enough (so that the bottom is not reflecting)

  3. reflected light from suspended matter is not so dominant because oceans tend to be clear

Rivers, shallow lakes:

  1. Reflection from suspended matter (green) is important because shallow lakes and rivers tend to have more sand and dust then oceans and pools

  2. Reflection of the bottom is important because they are shallow, and the bottom is not blue (more gray like sand)

  3. angle of reflection is important because lakes from far away and certain angles appear bluer when the bottom is not visible and surface reflection becomes dominant

  4. Vibrational shift become less dominant because the suspended matter is more dominant and the water is not deep enough

The color of a body of water is usually due primarily to light reflected from the sky and surroundings. Of course the ocean usually doesn't have a lot around it except the sky, but in a place where a lot of light from vegetation reflects off the water, the ocean can appear green.

This is a fresh-water lake; you can see reflections of white sky, blue sky, green trees, and so on.

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The colors in the water, especially those in reflections from the sky, tend to be darker and different because the colors in the sky are often polarized. When light glances off a smooth surface, the surface reflects light with horizontal polarization more effectively than light with vertical polarization (see Brewster's Angle), so some skylight can end up darkened - or brightened - relative to other colors depending on the angle of polarization and the angle of incidence.