The classification of European rivers comes from the fish fauna found in them. Changes in taxonomic composition relate to physical and chemical changes that occur longitudinally.

Trout zone

This zone has a characteristic steep gradient, fast flowing water and cool temperature. The fast flow rate causes turbulence which keeps the water well oxygenated. Fish species found in this zone usually lay adhesive

Adhesive

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 eggs that can stick to the substrate

Substrate (marine biology)

Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....

, this is to help prevent eggs being carried down stream by the water flow.

Characteristic fish species are:

• Brown trout
  Brown trout
  The brown trout and the sea trout are fish of the same species....
  
   (Salmo trutta)
• Atlantic salmon
  Atlantic salmon
  The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....
  
   (Salmo salar)
• Bullhead
  European bullhead
  The bullhead is a freshwater fish that is widely distributed in Europe, mainly in rivers. It is a member of the Cottidae family, a type of sculpin...
  
   (Cottus gobio)
• Stone loach
  Stone Loach
  The Stone loach, Barbatula barbatula, is a species of ray-finned fish in the Balitoridae family.It is found in Austria, Belarus, Belgium, Bulgaria, China, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Ireland, Italy, Japan, North Korea, South Korea, Latvia, Liechtenstein,...
  
   (Barbatula barbatula)

Grayling zone

Similar in physical characteristics to the Trout zone, although the temperature is usually slightly higher. Fish species in this zone also lay adhesive eggs.

Characteristic fish species include all of the above species, with the addition of;

• Grayling
  Grayling (species)
  The grayling is a species of freshwater fish in the salmon family of order Salmoniformes. It is the type species of its genus. Native to the Palearctic ecozone, the grayling is widespread throughout northern Europe, from the United Kingdom and France to the Ural Mountains in Russia...
  
   (Thymallus thymallus)
• Minnow
  Eurasian minnow
  The Common minnow is a species offreshwater fish. It is a member of the carpfamily oforder Cypriniformes, and is the type species ofgenus Phoxinus. It is ubiquitous throughout much of Eurasia,...
  
   (Phoxinus phoxinus)
• Chub
  European chub
  The European chub , sometimes called the round chub, fat chub, chevin, pollard or simply "the" chub, is a freshwater fish of the family Cyprinidae...
  
   (Leuciscus cephalus)
• Dace
  Common dace
  The common dace , also known as the dace or the Eurasian dace, is a fresh- or brackish-water fish belonging to the family Cyprinidae. It is an inhabitant of the rivers and streams of Europe north of the Alps as well as in Asia. It is most abundant in France and Germany, and has also spread to...
  
   (Leuciscus leuciscus)

Barbel zone

This zone is essentially lowland, but retains some characteristics of upland rivers. It has a gentle gradient with a moderate water flow and temperature. It also has a good oxygen content and a mixed substrate of silt and gravel in which plants can take root. Most of the fish species found in this zone lay their eggs in the vegetation on the river bed, this provides them with good protection and allows the eggs a good supply of oxygen given off from photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 in the plants.

Characteristic fish species include all of the species from the previous zones with addition of;

• Barbel
  Barbel (fish species)
  Barbels are group of small carp-like freshwater fish, almost all of the genus Barbus. They are usually found in gravel and rocky-bottomed slow-flowing waters with high dissolved oxygen content. A typical adult barbel will range from 25 to 100 cm in length and weigh anywhere between 200 g...
  
   (Barbus barbus)
• Roach (Rutilus rutilus)
• Rudd
  Rudd
  The common rudd Scardinius erythropthalmus is a bentho-pelagic freshwater fish, widely spread in Europe and middle Asia, around the basins of the North, Baltic, Black, Caspian and Aral seas.-Artificially introduced:...
  
   (Scardinius erythrophthalmus)
• European perch
  European perch
  The European perch, Perca fluviatilis, is a predatory species of perch found in Europe and Asia. In some areas it is known as the redfin perch or English perch, and it is often known simply as perch. The species is a popular quarry for anglers and has been widely introduced beyond its native area,...
  
   (Perca fluviatilis)
• Pike
  Northern Pike
  The northern pike , is a species of carnivorous fish of the genus Esox...
  
   (Esox lucius)
• Eel
  European eel
  The European eel, Anguilla anguilla, is a species of eel, a snake-like, catadromous fish. They can reach in exceptional cases a length of 1½ m, but are normally much smaller, about 60–80 cm, and rarely more than 1 m....
  
   (Anguilla anguilla)

Bream zone

The true lowland zone, has a very gentle gradient and slow flowing water, there is usually good oxygen content but the temperature is much more variable than in the other zones. This zone has a silty substrate and is often turbid. Fish species found in this zone lay adhesive eggs in the weeds. Most upland fish species can not survive in this zone.

Characteristic fish species include only a few species from the Barble zone (Roach, Rudd, Perch and Pike), with the addition of:

• Bream (Abramis brama)
• Tench
  Tench
  The tench or doctor fish is a freshwater and brackish water fish of the cyprinid family found throughout Eurasia from Western Europe including the British Isles east into Asia as far as the Ob and Yenisei Rivers. It is also found in Lake Baikal...
  
   (Tinca tinca)
• Carp
  Common carp
  The Common carp is a widespread freshwater fish of eutrophic waters in lakes and large rivers in Europe and Asia. The wild populations are considered vulnerable to extinction, but the species has also been domesticated and introduced into environments worldwide, and is often considered an invasive...
  
  (Cyprinis carpio)