The glaciers truly look like rivers of ice from the air. The dark lines are moraine, or dirt, that they pick up. As a glacier moves down a mountain, it acts like sandpaper on the edges and scrapes dirt away. When two glaciers join or when a tributary glacier enters the main glacier, there will be a "line" of moraine down the glacier. Thus, each line of dirt represent a tributary back up the glacier.
The plane with us and our pilot