A source is the place where a river begins. A river’s source, or headwaters, can come from different points, including glaciers, underground springs, or lakes.

Many rivers start in mountains or hills. When rain falls on steep slopes, the water runs across the surface and collects into rills, or small streams. Melting snow and ice also add water to streams. These tributaries then join together to form a river. In this case, the source is the tributary that is farthest from the river’s end.

Sometimes, a river’s source is a melting glacier. For example, the Rhône Glacier in the Swiss Alps is the source of the Rhône River. Springs also feed rivers. Springs pop up in places where groundwater rises to the surface. A river’s source can be a lake that has outflowing streams. The Mississippi River’s primary source is Lake Itasca, a glacial lake in Minnesota.

Finding a river’s source can be tricky. A large river’s watershed can have many springs, lakes, and tributaries that drain into the main river. The source of the Amazon River remains contested. Expeditions in the 20th century and early 2000s pointed to the Nevado Mismi, a mountain in the Peruvian Andes, as the Amazon’s most distant source. But in 2014, a team of explorers suggested that the Mantaro River in Peru was the true source of the Amazon River.

Does it actually matter where the source of a river lies? To environmental experts, the answer is yes. Because pollution at a source can harm the health of a river downstream, it is important to know where a river gets its water. In the past, National Geographic Explorers have worked to understand river systems, starting from the source. In 2011, Explorer Shannon L. Switzer Swanson trekked 145 kilometers (90 miles) from the headwaters of California’s San Dieguito River to where it empties into the ocean. Starting in 2015, a team from National Geographic began exploring the sources of the Okavango River in southern Africa. Understanding the sources better will make it easier to protect them and, by extension, the entire Okavango River Basin.