It depends on the river.
It could be pollution or a natural occurrence.
Aquatic
Aquatic producers use material from land, like tree leaves.
Water, sediment and aquatic life.
fish, turtle and heron
Of itself, the water in a river is just water, but it supports a wide variety of aquatic life.
The Rappahannock River is home to a diverse range of aquatic life, including various species of fish such as bass, catfish, and shad. The river also supports populations of freshwater mussels, crabs, and amphibians, along with a variety of aquatic plants. Additionally, migratory species like striped bass and American shad can be found during their spawning runs. The river's ecosystem is vital for both wildlife and the surrounding communities.
Water, mud and marine life
The green water in the river can indicate the presence of algae or other plant life, which can affect the ecosystem by providing oxygen and food for aquatic organisms. It can also be a sign of pollution or nutrient runoff, which can harm the water quality and aquatic life. Monitoring and managing the green water is important for maintaining a healthy river ecosystem.
Enough of it can totally destroy a river by killing-off all forms of life in it and ensuring that the river cannot support life for a considerable time to come.
Aquatic life style. River horse, again a large-sized amphibious mammal. as do Elephants. they make love in the waters.
They form habitats for aquatic animals, mainly invertebrates but also a few species of fish and salamander.
Until about 30 years ago, the River Thames in London was virtually an open sewer so there was hardly any aquatic life in the river. Since then, the river has been cleaned up and there many species of fish in the river now.