Rivers form the principal part of the water systems of Azerbaijan. There are 8,359 rivers of various lengths within Azerbaijan. Of them 8,188 rivers are less than 25 kilometers in length. Only 24 rivers are over 100 kilometers long. Kur, Araz, Qanix (in Alazan), Qabirri (Iori), Samur, Terter, Turyan, Agstafa, Hekeri, Vilesh and others are the largest rivers that flow through the country.
The country's rivers are divided into three groups:
1) The Kur basin rivers (Qanix, Qabirri, Turyan, Agstafa, Shekir, Terter, Khachin, etc.);
2) The Araz basin rivers (Arpachay, Nakhchivan, Okhchu, Hekeri, Kondelenchay, etc.);
3) Rivers, flowing directly into the Caspian Sea (Samur, Gudyal, Velvele, Vilesh, Lenkeran, etc.).
Kur and Araz run through the Kur-Araz lowland. The rivers that directly flow into the Caspian Sea, originate mainly from the north-eastern slope of the Major Caucasus and Talysh mountains and run along the Samur-Devechi and Lenkeran lowlands.
Azerbaijan river systems are changing and evolving under the influence of various physiographic factors: climate, landscape, geological structure, soil and vegetation. The density of the river network increases, then gradually decreases later with higher altitudes. Except for the Talysh region (1.6-2.2 km/sq.km), the river system density is the highest (1-2km/sq. km) at 1,000-2,500 kilometers, while in the area of the Talysh mountains it peaks at 1.6-2.2 km/sq.km at 500-1,000 km. The average density of the river system of Azerbaijan is 0.39 km/sq.km. The density is even lower than 0.05 km/sq.km in the plains.
The Kur River basin area (86,000 sq. km) up to the junction with the Araz River is smaller than the Araz water basin (101,937 sq.km). The river is still called Kur on the junction because the water level of the Kur is twice as high as that of the Araz River.
Like in all other countries, rivers have different feeding sources in Azerbaijan. Most rivers are fed by snow, rainfalls and ground waters. Snow is the predominant feeding source for the rivers of the Major Caucasus, while ground waters contribute the most to water supply of rivers in the Minor Caucasus. The Kur and Araz rivers pass Azerbaijan in their lower and middle courses.
The Kur river is the largest river of Azerbaijan. It stretches for 1,515 kilometers and covers an area of 188 thousand sq. km. The Kur originates from the Hel River in Turkey, passes through Azerbaijan and flows into the Caspian Sea in south-eastern part of the country. The Araz River covers an area of 86 thousand sq. km until its junction with the Kur River. It originates from the Bingol mountains in Turkey at the altitude of 3300 meters. On the whole, the Araz River forms Azerbaijan's border with Turkey and Iran. It passes through Azerbaijan in its lower 80 kilometers and joins the Kur River near Sabirabad. These two rivers belong to the group of rivers, flowing at full under the influence of snow and rainfalls in spring and rainfalls in autumn.
Weather produces the greatest impact on the river flow in Azerbaijan. Intensive rise in temperature causes melting of snow at heights of over 1500. The melting of snow further intensifies after heavy rainfalls of April and May. Snow melts more intensively in the high altitudes (over 2500-3000 meters) from early April through May until June. The melting process influences river flow even in summer time. Thus, melted snow water, absorbed by soil, emerges on the surface and raises water level in rivers. Low river basins (except for those of the Talysh region) are less influenced by the precipitation in spring and summer periods. Winter and autumn rainfalls account for the most part of precipitations in the Talysh region. Rivers are less full of water in summer in Azerbaijan. Heavy rainfalls that may from time to time occur in July and August, lead to floods, causing agricultural damages. Severe floods have been registered in the rivers of southwestern slopes of Major Caucasus Zengezur part. Rivers of the Major and Minor Caucasus mainly flow in hot seasons, while rivers of the Talysh regions flow in colder seasons of year. Rivers, flowing in hot seasons account for most part of all rivers (60-80%).
Such seasonal flows are difficult for industrial use. On the whole, rivers of the Azerbaijan Republic are divided into two groups, according to their water regime: 1) rivers of full-flowing regime; 2) rivers of flood regime. Flood rivers are the Lenkoran rivers and episodic rivers of Gobustan. Other rivers are included into the first group of rivers.
Complex topography and other natural factors cause a non-standard flow across the country. The flow increases with altitudes and reaches its top at a certain height (2800, on the north-eastern slope of the Major Caucasus, 2000-2200-on its southern slope and 2200-2400 on the Minor Caucasus). The flow starts to decline from above the indicated height. Due to the orographic specifications of the Talysh mountains, the flow is inconsistent with the average height. It decreases with the increase of altitude in the Talysh mountains, while in Peshteser and Burovar mountains it rises with the altitude.
The full-flowing rivers of the Azerbaijan Republic mainly flow on the southern slope of the Minor Caucasus. The average flow of such rivers exceeds 45 l-cm. The flow falls to 5 l-cm till the Alazan-Ayrichay lowland. The flow module of rivers of the north-eastern slope of the Major Caucasus 18 l-cm. The increase of flow with the increase of altitude is relatively uniform in this part of the Major Caucasus. The intensive increase in the module of flow is registered on the area between the Yah mountain chains and the Major Caucasus mountains. (upper Qusar, Qudyal and other rivers.). The Average annual module of flow is from swings hesitates from 10 to 20 l-cm.
The flow of rivers, originating in the slopes of the Yah mountains, differs from that of the rivers, flowing from the Major Caucasus. The flow increases intensively and reaches from 6 to 18 l-cm at a height of 1000-2000 meters, due to high level of precipitation. The flow gradually decreases till the Caspian Sea shore down to 0.5 l-cm. the flow decreases beginning from the north-west of till south east of the seaside lowland and reaches zero level on the Apsheron peninsula. Compared with the Major Caucasus, the flow in the Minor Caucasus is more complicated, due to its orographic complexity and differing location of mountain chains. The highest flow has been registered in the rivers flowing from the slopes of Gamish and Qapidjic mountains (over 28 l-cm).