The longest source for Hubbard Glacier originates 122 kilometres from its snout and is located at about 61°00′N 140°09′W / 61.000°N 140.150°W / 61.000; -140.150, approximately 8 kilometres west of Mount Walsh with an elevation around 11,000 feet. A shorter tributary glacier begins at the easternmost summit on the Mount Logan ridge at about 18,300 feet at about 60°35′0″N 140°22′40″W / 60.58333°N 140.37778°W / 60.58333; -140.37778.
Before it reaches the sea, Hubbard is joined by the Valerie Glacier to the west, which, through forward surges of its own ice, has contributed to the advance of the ice flow that experts believe will eventually dam the Russell Fjord from Disenchantment Bay waters.
The Hubbard Glacier ice margin has continued to advance for about a century. In May 1986, the Hubbard Glacier surged forward, blocking the outlet of Russell Fjord and creating "Russell Lake". All that summer, the new lake filled with runoff; its water level rose 25 metres, and the decrease in salinity threatened its sea life.
Around midnight on October 8, the dam began to give way. In the next 24 hours, an estimated 5.3 cubic kilometres of water gushed through the gap, and the fjord was reconnected to the ocean at its previous level. This was the second largest glacial lake outburst flood in recorded history, and had the equivalent flow of about 35 Niagara Falls.
In spring 2002, the glacier again approached Bert Point. It pushed a terminal moraine ahead of its face and closed the opening again in July. On August 14, the terminal moraine was washed away after rains had raised the water level behind the dam it formed to 18 m above sea level. The fjord could become dammed again, and perhaps permanently. If this happens, the fjord could overflow its southern banks and drain through the Situk River instead, threatening trout habitat and a local airport.
It takes about 400 years for ice to traverse the length of the glacier, meaning that the ice at the foot of the glacier is about 400 years old. The glacier routinely calves off icebergs the size of a ten-story building. Where the glacier meets the bay, most of the ice is below the waterline, and newly calved icebergs can shoot up quite dramatically, so that ships must keep their distance from the edge of the glacier in Disenchantment Bay.