CALVING (TIDEWATER) GLACIERS: Early European explorers were much perplexed by the loud, booming noises they heard in the vicinity of glaciers. Hearing thunderous booms in Columbia Bay, the Spanish explorer Salvador Fidalgo postulated a volcano at this location. Capt. George Vancouver recounts in his journal the final solution to the mystery: "Whilst at dinner in this situation [College Fiord] they [Whidbey's exploratory party] frequently heard a very loud rumpling noise, not unlike loud, but distant thunder; similar sounds had often been heard when the party was in the neighborhood of large bodies of ice, but they had not been able to trace the cause. They now found the noise to originate from immense ponderous fragments of ice, breaking off from the higher parts of the main body, and falling from a very considerable height, which in one instance produced so violent a shock, that it was sensibly felt by the whole party, although the ground on which they were was at least 2 leagues (6 miles) from the spot where the fall of ice had taken place." (Vancouver 1798, pp. 183-184).
It is interesting to note that Vancouver was apparently unfamiliar with the word "glacier" although LaPerouse had used it earlier on his maps. Neither Cook nor Vancouver show any glaciers on their maps of Prince William Sound.
As time passed, the early explorers seeking the northwest passage were followed by men of science who came to map and study the sound. Mendenhall, Gilbert and Gannett, Grant and Higgins, and Tarr and Martin photographed the glaciers. And, as they collected information, a second puzzle emerged. Some of these scientists noticed that some tidewater glaciers were retreating while others flowing from the same icefield were advancing. They reasoned that if climate alone determined whether glaciers were retreating or advancing, then all the glaciers in a particular area ought to advance and retreat together. The behavior of tidewater glaciers led glaciologists to suspect that terrain, altitude, firn limit, and a glacier's accumulation area ratio are also significant factors. But even these did not provide a complete explanation of the seemingly erratic behavior of tidewater glaciers flowing from the same icefield.
Then, in the 1970s, Austin Post (Us Geological Survey Project Office of Glaciology in Tacoma, Washington, under the direction of Dr. Mark Meier) developed a new theory taking into account the effect of salt water on a glacier's terminus. According to Post, the depth of a tidewater glacier at its terminus and the condition of its accumulation zone together may determine whether a glacier is retreating, stable, or advancing more than individual climatic changes. Glaciers ending in shallow water or on moraines may still be retreating (Nellie Juan and Shoup) or can be slowly advancing (Harriman, Harvard, and Meares). Before a tidewater glacier can shift from retreating to advancing, it must first reach a stable position where its accumulation and ablation zones are in balance. The glacier then begins to build a terminal moraine through eroding its valley walls and bedrock cradle and depositing the rocks at its terminus. As this moraine grows, it protects more and more of the glacier's ice front from the rapid melting effect of the warm salt water. Gradually, loess and less ice melts each year in the ablation zone, while ice from the accumulation area continues to push seaward. Slowly, at times no more than a few inches a year, ice descending from the accumulation area pushes the terminal moraine forward. Eventually, the advancing glacier reaches an extended position of equilibrium between accumulation and melting. Like Columbia, it may rest here nearly stable for many centuries.
Tidewater glaciers remain stable until the climate changes raising the firn limit, the glacier advances so far that it over-extends its "ice-budget," or the glacier pushes its protecting terminal moraine over a submarine canyon. In all cases, the ablation area increases, so the accumulation area ratio lowers. With increased melting, calving reaches a critical point.; As the terminus thins, the glacier's slope increases-further accelerating its rate of flow. More and more ice is drawn down from the icefield's reservoir, augmenting the thinning process there. When the glacier's snout thins sufficiently, it retreats off the supporting moraine. Drastic retreat begins.
Tidewater glaciers retreat relatively fast in depths of more than 240 ft. (80m.), because relatively more of its terminus is exposed to the water which serves as an inexhaustible source of heat. When the channel narrows or turns, the glacier retreats more slowly, probably because less of its terminus is exposed, and discharge lessens, thus reducing the iceberg-calving rate. Retreat remains slow until the terminus reaches another broad or straight part of its channel whereupon the glacier will again retreat drastically until it reaches a stable position in shallow water. Once the glacier stabilizes, it slowly begins to build a new moraine. As the moraine grows, it provides the glacier's terminus with increased protection from warm water. The ablation rate slows. The glacier is ready once again to begin its slow push down the fiord. The cycle of slow advance and drastic retreat takes place over centuries for valley glaciers with a low angle of slope, such as Yale and Harvard, and over decades for smaller, glaciers descending precipitous slopes, such as Bryn Mawr and Wellesley.
Some tidewater glaciers in the sound are now in their stable retracted positions. Others are advancing from retreated positions. In the mid-'70s Austin Post pointed out that Columbia Glacier held the unique position of being the only Prince William Sound tidewater glacier to be in a maximum extended position (Post 1975). After several years of study and modeling of iceberg-calving dynamics, the USGS predicted that rapid retreat would begin in 1982 (Meier 1980).
(Chapter 3, pp. 19-21).