Many people don't realize that glaciers are not solid ice. There is a great quantity of water forming different structures around glaciers.
Glacial water comes from a variety of sources.
First there is surface melting. Next, there is the meltwater produced by geothermal heat. Since glaciers can, and do, occur near such heat sources as volcanoes and hot springs, some of the heat produced by them goes directly into the glacier. This melts the glacier bottom just above the ground. Third, there is precipitation that falls on the glaciers.