The Pregolya or Pregola is a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in the Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n Kaliningrad Oblast

Kaliningrad Oblast

Kaliningrad Oblast is a federal subject of Russia situated on the Baltic coast. It has a population of The oblast forms the westernmost part of the Russian Federation, but it has no land connection to the rest of Russia. Since its creation it has been an exclave of the Russian SFSR and then the...

 exclave.

It starts as a confluence

Confluence

Confluence, in geography, describes the meeting of two or more bodies of water.Confluence may also refer to:* Confluence , a property of term rewriting systems...

 of the Instruch and the Angrapa

Angrapa

Angrapa is a river in northeastern Poland and Russia . It originates in Masuria and flows into the Pregolya, of which it is a main tributary.The Angrapa has a length of 172 km and a basin area of 3,639 km²...

 and drains into the Baltic Sea

Baltic Sea

The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

 through Vistula Lagoon

Vistula Lagoon

The Vistula Lagoon is a fresh water lagoon on the Baltic Sea separated from Gdańsk Bay by the Vistula Spit. It is sometimes known as the Vistula Bay or Vistula Gulf. The modern German name, Frisches Haff, is derived from an earlier form, Friesisches Haff. Both this term and the earlier Polish...

. Its length under the name of Pregolya is 123 km, 292 km including the Angrapa

Angrapa

Angrapa is a river in northeastern Poland and Russia . It originates in Masuria and flows into the Pregolya, of which it is a main tributary.The Angrapa has a length of 172 km and a basin area of 3,639 km²...

. The basin

Drainage basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

 has an area of 15,500 km². The average flow is 90 m³/s.

Euler's Seven Bridges of Königsberg

Seven Bridges of Königsberg

The Seven Bridges of Königsberg is a historically notable problem in mathematics. Its negative resolution by Leonhard Euler in 1735 laid the foundations of graph theory and prefigured the idea of topology....

 problem was based on the rivers' bridges in Königsberg

Königsberg

Königsberg was the capital of East Prussia from the Late Middle Ages until 1945 as well as the northernmost and easternmost German city with 286,666 inhabitants . Due to the multicultural society in and around the city, there are several local names for it...

, now Kaliningrad

Kaliningrad

Kaliningrad is a seaport and the administrative center of Kaliningrad Oblast, the Russian exclave between Poland and Lithuania on the Baltic Sea...

.

Cities and towns

• Chernyakhovsk
  Chernyakhovsk
  Chernyakhovsk is a town and the administrative center of Chernyakhovsky District of Kaliningrad Oblast, Russia, located at the confluence of the Instruch and the Angrapa Rivers, forming the Pregolya...
  
  
• Znamensk
  Znamensk, Kaliningrad Oblast
  Znamensk is a settlement in Kaliningrad Oblast, Russia. It is located on the right bank of the Pregolya River at its confluence with the Lava River some 50 km east of Kaliningrad...
  
  
• Gvardeysk
  Gvardeysk
  Gvardeysk is a town and the administrative center of Gvardeysky District of Kaliningrad Oblast, Russia, located on the right bank of the Pregolya River east of Kaliningrad. Population: -History:...
  
  
• Kaliningrad
  Kaliningrad
  Kaliningrad is a seaport and the administrative center of Kaliningrad Oblast, the Russian exclave between Poland and Lithuania on the Baltic Sea...
  
  

Tributaries

• Pissa
• Lava/Łyna
  Lyna
  The Łyna , or Lava , is a river in northern Poland's Warmian-Masurian Voivodeship as well as in Russia's Kaliningrad Oblast.The Łyna is a tributary of the Pregolya River, and has a total length of 264 km and a basin area of 7,126 km²...
  
  
• Angrapa
  Angrapa
  Angrapa is a river in northeastern Poland and Russia . It originates in Masuria and flows into the Pregolya, of which it is a main tributary.The Angrapa has a length of 172 km and a basin area of 3,639 km²...
  
  
• Instruch