Rivers and streams in Iceland come in varying shapes and sizes, and they do not share a single origin. These surface running waters are of three different types: runoff from impermeable bedrock, groundwater from loose or porous strata, and meltwater from snowdrifts or glaciers.
An area of land in which water flows into a single river or other common outlet is called a drainage basin or catchment area. The boundary between these catchment areas is known as the water divide. The largest catchment areas in Iceland are those of the rivers Jökulsá á Fjöllum (7,750 km2) and Þjórsá (7,530 km2). Þjórsá is Iceland’s longest river, 230 km long, but Jökulsá á Fjöllum is a close second at 206 km. Although the natural catchment area of Jökulsá á Fjöllum is larger, the average discharge is much less. Under natural conditions, the average discharge at the mouth of the Þjórsá river is 383 m3/sec. The average discharge of the Ölfusá river measures an even higher 440 m3/sec. For Jökulsá á Fjöllum, this figure is 212 m3/sec. The reason for this disparity is that the catchment areas of the Þjórsá and Ölfusá rivers are in South Iceland, while Jökulsá á Fjöllum is in Northeast Iceland, to the north of the highland glaciers. This area receives much less precipitation.
Surface running waters in Iceland can be classified into three categories on the basis of their origin and their geomorphological settings:
- Direct-runoff rivers are found in areas where the bedrock is fairly impermeable and altered or in regions of West, North, and East Iceland where the basalt bedrock is from the Miocene-Pliocene Succession (the Blágrýti bedrock). These rivers typically have no clear source. They originate as streamlets in depressions and valleys, growing gradually larger as they flow down the river’s channel. The discharge of direct-runoff rivers depends greatly on the local climate, and flooding is common. Rains cause these rivers to swell rapidly, but they dwindle in droughts and freezing weather. The water temperature in direct-runoff rivers is closely tied to the air temperature. These rivers are warm in summer but cold in winter. When air temperatures dip below freezing, they quickly become choked with ice, thus often swelling before they eventually freeze over. During cold weather, the level of direct-runoff rivers thus remains low, and flow can cease altogether. During thaws, they grow rapidly again, shaking off the ice and bursting downstream. Hummocky ice cover or ice jams often form in them, causing them to overflow their banks. Fluvial sediment can sometimes colour the swelling river a dark rust-brown.
- Spring-fed rivers and streams have a clear source, namely springs in lava fields, hyaloclastite areas, and bedrock from the Pliocene-Pleistocene Succession (the Grágrýti bedrock). These springs are often powerful, and spring-fed rivers sometimes reach their full size near their source. The discharge and temperature of spring-fed rivers remains constant year-round. Fluctuations in air temperature and precipitation thus have little effect on spring-fed rivers. Flooding is rare, occurring mainly during winter or spring thaws, when the ground remains frozen and thus prevents local precipitation and meltwater from soaking into the earth. Spring-fed rivers never freeze at their source, but they can freeze over downstream in very severe frost conditions.
- Glacier-fed rivers emerge from under glaciers. The water flowing in these rivers is melted glacier ice. The volume of glacier-fed rivers is highly dependent on air temperature: they are vastly larger in the summer than in the winter. They reach their greatest size during the summer rains, when both rainwater and glacial meltwater cause them to swell. Glacier-fed rivers rise rapidly in warm, sunny weather, and there are regular daily fluctuations in discharge. At the glacial margin, discharge peaks between noon and three o’clock in the afternoon but is lowest in the morning at sunrise. The water temperature at the glacial margin is around 0°C, but farther downstream the water can reach a temperature of 15–20°C on a calm, sunny day in summer. Hummocky ice cover forms rapidly on glacier-fed rivers in winter conditions; they ordinarily freeze over in the first frosts. Glacier-fed rivers are a murky light grey in colour due to the fluvial sediment in them, consisting of detritus, mud, and clay that glaciers have scraped and scoured from their glacier beds. In frosty weather, the water in glacier-fed rivers can become almost a clear blue.
Most larger rivers in Iceland are a combination of more than one of these types. For example, Jökulsá á Fjöllum is a glacier-fed river that also contains large quantities of spring water, which drains from Ódáðahraun and other lava fields in the catchment area. The same is true of Eyjafjarðará, which is classified as a direct-runoff river even though a considerable volume of spring water also flows into Eyjafjarðará. This spring water drains from young bedrock in the Central Highlands south of the fjord of Eyjafjörður and the Eyjafjarðardalur valley. Another river in North Iceland, Hörgá, is a direct-runoff river that is also fed by springs and myriad small glaciers in the mountains of the Tröllaskagi peninsula.