Meltwater is caused by the surface of the glacier melting due to sunlight and high temperatures, particularly during long polar days in the summer. However, these days are becoming hotter and are happening more frequently due to climate change. Meltwater causes streams on the surface of glaciers, which can quickly turn into rivers and channels. The surface meltwater can even penetrate below and through the glacier. These subglacial meltwater channels and waterfalls can be complicated and extensive with the longest subglacial channels recorded to be up to 100 km long.