Why is water different colors?

When we think of a lake or river, we picture clear, blue water. But water color can range from red to brown to green to gray. The color you see is the result of material in the water that reflects back different wavelengths of the light spectrum. This material can be either dissolved or suspended. Dissolved material may make water look clear and blue or clear and brown. Suspended particles in the water intercept light and reflect back color to our eyes, making water look muddy brown, cloudy green, or gray.

Blue water

Clear, blue water does not contain a lot of the particles or dissolved, colored material that intercepts and reflects light. (Think of a clear blue sky versus a hazy, smoggy sky--the hazy air has more particles in it). As a result, the longer wavelengths of light toward the blue part of the light spectrum penetrate into the water, and the blue is reflected back to our eyes. The darker the blue, the deeper the water. Shallow areas appear lighter blue or greenish blue.

Clear water

Why do some lakes and rivers have no color? Clear water has less dissolved and suspended material. Mountain streams that start as snowmelt or runoff are often clear, because they run over bare rock without sediment or vegetation. Seepage lakes in sand and gravel settings may also look clear, and shallow water is clear because there is not enough depth for the long, blue wavelengths of light to travel and be reflected back.

Crystal clear Roaring Brook drains the rocky slopes of Katahdin.

Clear brown or reddish water

Sometimes water is clear but brown like tea or root beer. The color is the result of dissolved organic material from the breakdown of plants and animals. The material leaches into slow moving streams and lakes from surrounding forests, bogs and wetlands, and stains the water brown or reddish brown.

This stream flows out of a cedar swamp (Appleton Bog) in midcoast Maine.

Muddy or murky brown water

Suspended material causes water to look murky or cloudy (this is sometimes referred to as turbidity). Eroding soil can make water muddy brown in color. Strong winds and waves may stir up sediment from a lake bottom, and water near shore may look cloudy as a result. During spring snowmelt, rivers may appear brown as heavy rains and snowmelt send a pulse of sediment, grit, and dust into streams.

Gray or cloudy water

Runoff from urban areas can make water look gray, as in this stream that drains an area that has a lot of commercial and industrial development (parking lots, buildings, roads).

Algae and other microscopic organisms have colored pigments. When they grow in large numbers ("bloom"), they can color certain areas or entire lakes and streams. Blooms of an organism called Euglena may appear red. A bloom of diatoms, a kind of algae, can look brown. (See the section on Algae for more about blooms.)