Rivers, lakes and other freshwater habitats are home to all sorts of fish, including bream and trout.
Fast-flowing streams are preferred by fish such as trout and grayling. Slow-flowing rivers and lakes are home to tench, rudd and carp.
Some fish feed on floating plant matter, while others take insects from the surface of the water.
Common bream and barbel hunt on the riverbed, eating insect larvae, worms and molluscs.
Perch and pike are predators of lakes and slow-flowing rivers.
Pike are the sharks of the river – deadly hunters that lurk among weeds waiting for fish, or even rats and birds. Pike can weigh up to 30 kg.
Mammals of rivers and lakes include voles, water rats and otters.
Birds of rivers and lakes include birds that dive for fish (such as kingfishers), small wading birds (such as redshanks, avocets and curlews), large wading birds (such as herons, storks and flamingos), and waterfowl (such as ducks, swans and geese).
Insects include dragonflies and water boatmen.
Amphibians include frogs and newts.
Some aquatic (water) plants are rooted in the mud and have their leaves above the surface like water lilies.
Some water plants grow underwater but for their flowers, like water milfoils and some plantains. They may have bladders or air pockets to help keep the stem upright.
Tiny plants called algae grow in river.
Water hyacinths grow so quickly that they may red, green or brown films on rivers, double in number every 10 days.
Water hyacinths are purple American water flowers. They grow very quickly and can sometimes clog up slow-running streams.
Giant water lilies have huge leaves with the edges upturned like a shallow pan to keep them afloat.
The leaves of the royal or Amazon lily can be 2 m across.
Papyrus is a grass-like water plant that grows in the Nile river. Stems were rolled flat by the Ancient Egyptians to write on. The word ‘paper’ comes from papyrus.
Many grass-like plants grow in water, including reeds, mace, flag and rushes such as bulrushes and cattails.
Mangroves, bald cypresses, cotton gum and other ‘hydrophytic’ trees are adapted to living in water.
Rivers carve out valleys as they wear away their channels.
High up in the mountains, much of a river’s energy goes into carving into the river-bed. The valleys there are deep, with steep sides.
Down towards the sea, more of a river’s erosive energy goes into wearing away its banks. It carves out a broader valley as it winds back and forth.
Large meanders normally develop only when a river is crossing broad plains in its lower reaches.
Incised meanders are meanders carved into deep valleys. The meanders formed when the river was flowing across a low plain. The plain was lifted up and the river cut down into it, keeping its meanders.
The Grand Canyon is made of incised meanders. They were created as the Colorado River cut into the Colorado Plateau after it was uplifted 17 million years ago.
The shape of a river valley depends partly on the structure of the rocks over which it is flowing.
Many large valleys with misfit rivers were carved out melt waters.
The world’s rivers wear the entire land surface down every 1000 years.
Rivers are filled with water from rainfall running directly off the land, from melting snow or ice or from a spring bubbling out water that is soaked into the ground.
High up in mountains near their source (start), rivers are usually small. They tumble over rocks through narrow valleys which they carved out over thousands of years.
All the rivers in a certain area, called a catchment area, flow down to join each other, like branches on a tree. The branches are called tributaries. The bigger the river, the more tributaries it is likely to have.
As rivers flow downhill, they are joined by tributaries and grow bigger. They often flow in smooth channels made not of big rocks but of fine debris washed down from higher up. River valleys are wider and gentler lower down, and the river may wind across the valley floor.
In its lower reaches a river is often wide and deep. It winds back and forth in meanders (see river channels) across floodplains made of silt from higher up.
Rivers flow fast over rapids in their upper reaches. On average, they flow as fast in the lower reaches where the channel is smoother because there is much less turbulence.
Rivers wear away their banks and beds, mainly by battering them with bits of gravel and sand and by the sheer force of the moving water.
Every river carries sediment, which consists of large stones rolled along the river-bed, sand bounced along the bed and fine silt that floats in the water.
The discharge of a river is the amount of water flowing past a particular point each second.
Rivers that flow only after heavy rainstorms are ‘intermittent’. Rivers that flow all year round are ‘perennial’ — they are kept going between rains by water flowing from underground.