Geothermal rivers

A cold surface river meets hot groundwater and the two mix along a stretch of the channel. Temperature varies with distance downstream, with rainfall and with the season, so the comfortable stretch moves. These are among the most bathable features in Iceland because the mixing is self-regulating: walk upstream to get hotter, downstream to cool off.

They are also the features most damaged by traffic. Bank erosion from thousands of footsteps is visible at the well-known ones, which is one reason many landowners are cautious about new attention.

Natural pools and hot pots

A natural pool is a basin where geothermal water collects with little or no construction. A hot pot (heitur pottur) usually implies some human work — a stone rim, a pipe, a drain plug. Many of the pools travellers picture as wild are in fact century-old farm hot pots that someone still maintains.

Maintenance matters for your safety and for access. A pool with an owner has a drain, a cleaning cycle and often a rule about numbers. A truly unmanaged pool has none of those, and its water can sit warm and stagnant for weeks.

Features you never enter

Some geothermal features look inviting in photographs and will injure you badly.

• Spouting springs and geysers — the water is at or near boiling and eruptions are irregular.
• Fumaroles and steam vents — superheated steam causes deep burns at a distance.
• Mud pools — boiling, acidic, and the surrounding crust is often unstable.
• Any pool inside an active high-temperature field without a marked path.

Village pools and geothermal lagoons

Almost every Icelandic settlement has a municipal swimming pool heated geothermally, and they are the backbone of local social life. Commercial lagoons are a separate category again: constructed, ticketed and often fed by an industrial borehole rather than a surface spring.

Neither is a wild hot spring, but both belong in a survey of geothermal bathing because they are how most people in Iceland actually bathe.