What Makes Rivers Different Colors?

Today’s post is by guest author Taylor Cox. Taylor was the Fall/Winter 2014 Communications Intern. She is majoring in Communication, Law, Economics, and Government (CLEG) with a minor in Environmental Science at American University.

Have you ever wondered what makes rivers different colors? We are taught growing up that water is blue, but some rivers are black, brown, emerald blue, green, or even blue-green. So what gives these rivers their colors?  In the next few blog posts, we’ll explore the five different major colors of rivers, and what makes them that color.

Wight-Brook-ME--Jym-St-PierreWight Brook, ME | Jym St Pierre

Water’s Natural Color

First we need to understand what color is. Natural light is actually made up of a number of different colors associated with different wave lengths within the light spectrum. Think of the multiple colors of light when refracted by a prism. The color we associate with anything is the range of light waves that is reflected back to our eyes by the molecules that comprise the entity. We don’t see the colors of light that are absorbed. When it comes to pure water, blue light is reflected while other colors of light, especially reds, are absorbed.

Apparent Color vs True Color

When looking at the color of water, it is important to note the difference between the apparent color and true color of a body of water. The apparent color is the color of the water when looking at it without removing any suspended and dissolved particles. True color is the color of the water after the suspended particles have been removed.  Suspended particles are things such as algae, sediments, or small particles of a mineral. Dissolved particles are things such as tannins (a yellowish-brown organic acid that is found in plant tissues), or particles of iron and manganese from rocks or soil. Water that is blue has a very low amount of dissolved particles in it.

Bison in Yellowstone River, MO | Josh Robbins - NPSBison in Yellowstone River, MO | Josh Robbins – NPS

Factors of Color

Factors such as minerals, soil runoff and sediment, and even algae can cause water to vary from its natural color of blue.

The most common cause for water to change color is minerals. When a rock is weathered down over time, the minerals from the rock are dissolved and small pieces are released into the water causing different colors. Iron, manganese, and calcium carbonate from limestone all common minerals that can cause water to range in color from red and orange to green and blue.

Sediment and soil runoff can also change water’s color – sometimes as a temporary color change after storms and sometimes permanently if the river constantly carries lots of sediment. Erosion from river banks brings soil into the river, changing the color. After heavy storms, many rivers run brown from all the runoff flowing into the river. Clay can cause rivers to be murky whiteuddy brown, or yellow.

Elwha-River-in-Olympic-National-Park--Lance-McCoyElwha River in Olympic National Park | Lance McCoy

Algal blooms are natural occurring overgrowths of algae caused by sunlight, slow water, or nutrients. Pollution runoff from humans can also increase nutrients in the water and cause an algal bloom. Algae affect not only the health of a river but also the color. The color caused by algae can vary from a dark green to almost a reddish color. Algae consume nutrients from the water along with dissolved oxygen causing negative effects on the ecosystem of the river. Once the algae begin decaying it releases methane gas causing foul odors.

Come join us for specific blogs on each of these color families. We’ll explore specific rivers from each color and share lots of photos. The next post will dive into why some rivers are “black water” rivers.

What river color most stands out from your river trips or river photos that you’ve seen?

8 Responses to “What Makes Rivers Different Colors?”

    J. Aldstadt

    Good question! The amount of light that is absorbed is proportional to its depth (or as a scientist would say, its “pathlength”*). Once a pathlength of ~10 feet or more is present, the absorption of red light over that distance is high enough to give the water a blue color. The reason the red light is absorbed in the first place is that it causes the O–H bonds in water to vibrate at that energy (so that particular energy – i.e., color – is lost).

    *This is described quantitatively by the Beer-Lambert-Bougher Law (more commonly just called “Beer’s Law”) where absorbance is equal to a constant time concentration. The constant has two factors: (a) “molar absorptivity” which is proportional to the probability that a given atom or molecule will absorb light of a particular energy (i.e., of a particular color in the visible part of the spectrum) and (b) the pathlength of the light through the material.


Way back in 1978, I brought a pretty good set of artist’s colored pencils on a trip to the Rogue River in Southern Oregon and tried to capture all the various shades of green I saw in the water. I couldn’t do it justice. I’ve been fascinated by river colors ever since. Thank you for this blog post.

Mike ciccone

I am amazed when I see that the monongahala river is green in West Virginia and brown by the time it gets to Pittsburgh.

Karen Parlette

I was amazed by the brilliant emerald green of the swollen, rushing rivers in southern Oregon when I visited in 1992. I’ve looked for that spot again and been unable to find it. Perhaps it was just there for me when I needed it. I’ve always thought that the green was a result of the water reflecting the dense tree cover surrounding it. I hate to think of it coming from algae that was making it sick. At the time, to me, it was an image of primal freshness and cleanness that washed me through and through.