The beginning of a river is known as its source or headwaters. Rivers often originate as underground springs that surface or as marshes that take in melted snow. A river's headwaters can also consist of many smaller streams, called tributaries, joining together to create a larger body of flowing water.
The ecological health of the area around a river's headwaters is crucial to the overall health of a river. Substances that enter the river at the headwaters can be dispersed down the entire length of the river, potentially influencing ecosystems at the mouth of the river, as well as affect the tributaries.