Water is clear because it is composed of hydrogen and oxygen, two elements that are invisible in their natural state; when combined, they remain colorless. When water has color, it is due to the addition of other particles, such as dirt or algae. The color of water is also affected by the portion of the light spectrum that it absorbs.
Water absorbs the red end of the visible light spectrum, resulting in a blue tint. However, it can be difficult to see this tint when viewing completely pure water, so it is likely to appear clear. The type and amount of sediment that an ecosystem has also affects water's color. If a fast-moving river has a clay bottom, the suspended clay sediment gives the water a reddish-brown color.