From land to sea: how rivers create unique pathways for water and life

Where does river water come from?

In most places, like the continental United States, rain is the most important source of water to rivers. In many places, rain drops can flow through soil and sand and continue to flow underground, as groundwater, to the river at the valley bottom. If raindrops hit a solid surface, like bedrock or a paved road, then rain will turn into runoff, and flow on top of the ground through small streams to the valley bottom.

In colder places, like the Arctic or tall mountain ranges, most river water comes from melting snow and glaciers. For example, melting snow and ice on Mt. Everest flows downhill to one of the largest rivers in the world – the Ganges River- which eventually drains into the Indian Ocean.

In some dry landscapes, most of a river’s water might come during just one season. Rivers that have their headwaters in the Rocky Mountains rely on water from snowpack that melts from spring into early summer. Even rivers as large as the Colorado River, which flows to the Pacific Ocean, receives most of its water during this snowmelt period. Life in these seasonally dry environments depends almost entirely on the water delivered by rivers.

Rivers do more than carry water
Because of their connection between land and sea, rivers have evolved into special ecosystems with incredible biodiversity. Several animal species rely on rivers to sustain their life cycle. Some freshwater fish migrate hundreds of kilometers in just one year to find food and to breed. Anadromous fish, who spend most of their lives in saltwater, migrate upstream along rivers to spawn in freshwater streams and lakes.

The most well-known fish to do this is the salmon. In the ocean, there is plenty of food for the fish, but there is little protection from predators. The coarse gravel at the beds of mountain rivers provide protection for fish eggs and newborn fish until they are old enough to defend themselves. Without the connectivity between the ocean and mountain streams, many fish species would not survive.

Rivers also nourish plant and animal life by delivering minerals, organic matter, and nutrients from upstream to downstream. Animal life typically flourishes around river mouths and deltas because they feed on the nutrients delivered by the rivers. In some cases, human activities, like construction on river banks, farming, and industrial pollution, have changed the natural balance of sediment and nutrients in rivers.

Rain storms that occur over fertilized farmland create runoff with high levels of nitrogen and phosphorus. This runoff flows into rivers, and then to the oceans. Sometimes this causes algae to overgrow or ‘bloom’ in coastal waters, which consumes all oxygen, killing animal life in these areas.

Additionally, humans have introduced a new kind of sediment to rivers – plastic. Plastic waste that doesn’t end up in our garbage cans and recycling bins is often picked up by runoff and ends up in rivers.

Plastic doesn’t dissolve or biodegrade, so it travels with river water and sediment to the ocean. On its journey to the ocean, plastic breaks down into tiny pieces called microplastic, which can be toxic to marine animals. It is important to be aware of the connectivity of rivers, and the effects that human activities can have on those special ecosystems. Read about Oregon wild rivers: