Obviously healthy water for a river is not the same as clean water for people to drink. What makes river water healthy? Obiously, a heatlhy river usually has an abundance of LIFE in it. Life takes many forms in the river: from microscopic (which means they are too small to see without a microscrope) plants and animals to huge alligators! So what does a healthy river give these creatures?

One of the most important things it gives them is oxygen. Most living creatures need oxygen to live. We get ours from the air, while fish get it from the water. Oxygen levels are extremely important to the welfare of a body of water. There are many factors which influence the level of oxygen, but they can all be put in two groups. One group is called sinks, and one is called sources.

A sink? what is a sink? Well, think about the sink you have in your bathroom. What is it there for? It captures the water that comes out of the sink, right? Well, sinks are anything that capture (uses) a substance. So things like fish and anything else that uses oxygen is considered a "sink". The total amount of oxygen that is used by organisms (living or dead) is called the biological oxygen demand (BOD). This is an extremely important concept, because if the oxygen level in the river drops below the BOD, fish and other creatures will start to die.

A source is anything that adds oxygen to the water. Oxygen naturally will move into the water if the oxygen level in the water is lower than in the air. This happens through a process called diffusion. The total effect of having oxygen move into the water from the air through diffusion is called reaeration. Water dropping down into a river from a waterfall will also help increase the oxygen content, and finally there is the all important effect of photosynthesis.