Freshwater
Rivers and lakes — the planet’s most precious water.
Rivers, streams, lakes and ponds: a tiny share of Earth’s water holding a surprising variety of life.
Overview
Freshwater ecosystems include flowing water — springs, streams and rivers — and still water such as lakes and ponds. Fast rivers are rich in oxygen; deep lakes form layers of warm and cold water. Because they are isolated from each other, many lakes and rivers have their own unique species.
Climate
From tropical rivers to ice-covered lakes; temperature and flow change with the seasons.
Where it is found
Every continent, from mountain springs to great river deltas.
Vegetation
Algae, water plants, reeds along the banks and riverside forests.
Living conditions
- Flow: Moving water carries oxygen, sediment and food downstream.
- Salinity: Very little dissolved salt, unlike seawater.
- Connectivity: Dams and barriers can cut rivers into isolated pieces.
Key facts
- Share of Earth’s water: About 2.5% is fresh water (USGS)
- Salinity: Less than about 0.5 g of salt per kg of water
- Seasonality: Flows rise and fall with rain and snowmelt
How the ecosystem works
- Upstream to downstream: Conditions change gradually from cold springs to warm, slow rivers.
- Isolation: Separate lakes and rivers evolve their own species.
- Land–water link: Riverside forests feed rivers with leaves and insects.
Food web
- Producers
- Algae, Water plants
- Grazers
- Insect larvae, Snails, Small fish
- Predators
- Axolotl, Bald eagle
- Decomposers
- Bacteria, Fungi
Biodiversity
Fresh water covers a small fraction of the planet but supports a large variety of fish, amphibians, insects and plants — many of them found in a single river or lake.
Threats
- Dams & barriers — Dams block migration and change natural river flows.
- Pollution — Runoff and waste water reduce oxygen and poison wildlife.
- Over-extraction — Taking too much water for farms and cities dries rivers.
- Invasive species — Introduced fish and plants outcompete native species.
Conservation
Every community depends on clean fresh water. Protecting rivers and lakes protects drinking water, food and some of the most threatened species on the planet.
- Keeping rivers free-flowing where possible.
- Reducing pollution from farms and cities.
- Protecting riverside vegetation.