A river or water flow is a natural stream of water that flows continuously through a channel on the earth's surface . It has a certain flow , is rarely constant throughout the year , and flows into the sea , a lake or another river, in which case it is called a tributary . The final part of a river is its mouth . Sometimes they end up in desert areas where their waters are lost through infiltration and evaporation (see allochthonous river ).

Rivers are part of the hydrological cycle . Water generally accumulates in a stream of precipitation through a basin of runoff surface and other sources as recharging groundwater , springs and release of stored water in natural ice and snow layers

Some freshwater fish are:

Anguilla : they are born in the Sargasso Sea . The larvae migrate to the European and North African rivers, where they reach three years of age (elvers). In the rivers they will spend between 4 and 10 years, at which time they begin their reproductive migration to the place where they were born. Adults die after reproducing ;

Catfish , in Central and South America ;

Bream : lives in smooth running waters;

Caribbean or piranha , in the rivers of South America;

Cachama , in South America;

Tent ;

Escardinio : lives in calm waters and very full of vegetation;

Sturgeon , a large Eurasian fish, with whose roe caviar is made ;

Goby ;

Ródeo ;

Rutile : it adapts to muddy waters although it prefers clear ones;

Salmon : born in mountain rivers with fast-flowing waters and stony bottoms. At the age of three they migrate to the sea and return to the river to reproduce;

Shaker , gimnoto or electric eel (South America);

River trout : lives in clear and cold waters, rich in oxygen ;

Barbel .