We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many large iceberg-calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving-related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre-existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re-advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenland's larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance.