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


All rivers flow to the ocean

By Marisa Repasch

From land to sea: how rivers create unique pathways for water and life. Except for the driest deserts, every place on Earth has a connection to the ocean through a river. Even the highest mountain in the world, Mt. Everest, is connected to the sea by flowing water. Because of their connectivity, rivers gave rise to human civilization. So how do these natural waterways form?
One basic principle of physics is that water flows downhill. When it rains, gravity pulls water down toward the core of the Earth. Earth’s surface is underlain by bedrock, through which water cannot flow. Instead, water usually flows over rock, taking the path of least resistance as it travels down slope toward the deepest parts of Earth’s surface – ocean basins.

The amount of water that is collected by a single stream depends on the size of its watershed. A watershed is the area of land upstream of a river where a drop of water that falls onto the ground always flows downhill into that river. For thousands of years, water has flowed through watersheds, down these paths of least resistance, to the ocean.

During intense storms, river water carries a lot of energy, enough to pick up sand and gravel, and to move rocks along the river bed. Over time, the downward force of moving water and rocks can carve into bedrock, creating valleys and canyons. It is the basic principle of gravity that allows rivers to constantly flow from the mountains to the sea, as long as there is a continuous source of water.

Types of Streams

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:

How are Rivers Formed

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How connected are the rivers of the world?

Photo of map: Connectivity of the world’s rivers (Grill et al., 2019). Blue colors show free-flowing rivers not impacted by human-built infrastructure. Red colors show rivers that are disconnected by dams and other infrastructure. Green colors highlight rivers with good connectivity, but small parts of these rivers are affected by human development.

Humans have also impacted the connectivity of rivers around the world, primarily by building dams. Only 37% of rivers longer than 1000 kilometers (621 miles) are free-flowing over their entire length (Grill et al., 2019).

Most of these free-flowing rivers are located in remote parts of the world that are not greatly developed by people, such as the Arctic and the Amazon rainforest. In more densely-populated landscapes, most free-flowing rivers are located upstream from very large, disconnected rivers.

This means that, even though 97% of rivers shorter than 100 kilometers (62 miles) are free-flowing, they are isolated from other free-flowing rivers. This means that native species in headwater regions are confined to a small area, ultimately decreasing the biodiversity of the whole river ecosystem.

For rivers that connect directly to the ocean, only 23% are free-flowing (Grill et al., 2019). As people built dams on these oceanic rivers, the anadromous fish species that relied on their connectivity to the ocean were suddenly unable to swim upstream to their spawning grounds. This effect continues to reduce fish populations, even in the remote Amazon River system, where proposed dams threaten the great biodiversity of its river ecosystems (Anderson et al., 2018).

River System Diagram

Dams and fish in the Amazon River Basin (Anderson et al., 2018). Red circles show existing dams. Yellow circles show proposed future dams. Different colors represent smaller watersheds within the greater Amazon River watershed. The fish symbols show the number of fish species counted in each smaller watershed.

What can people do to help maintain connectivity and improve river health?

River connectivity helps maintain the biodiversity of river ecosystems, and humans are certainly a part of these ecosystems. We rely heavily on rivers for our water supply, food, and recreation, but everything we do with our land and wastewater affects rivers and oceans just downstream from us.

There are a number of things we can do to help maintain and improve the health of our nearby river ecosystems. To promote clean runoff into rivers, we can pick up litter from our yards, roads, and sidewalks, and we can properly dispose of chemicals like weed killer, fertilizer, gasoline, and household cleaning products. To help maintain river connectivity, we can publicly oppose the construction of new dams that might disconnect fish and animal species from their natural habitats.

Because rivers are so important for our well-being, we should do everything we can to protect their well-being. Love cliff jumping into rivers in Oregon? Read more:

References cited
Anderson, E. P., Jenkins, C. N., Heilpern, S., Maldonado-Ocampo, J. A., Carvajal-Vallejos, F. M., Encalada, A. C., … Tedesco, P. A. (2018). Fragmentation of Andes-to-Amazon connectivity by hydropower dams. Science Advances, 4(1).
Grill, G., Lehner, B., Thieme, M., Geenen, B., Tickner, D., Antonelli, F., … Zarfl, C. (2019). Mapping the world’s free-flowing rivers. Nature, 569(7755), 215–221.

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