A sequence of aerial photographs of Mendel Glacier, a debris-covered glacier in the central Sierra Nevada, California, suggests that at least one outburst flood originated from a subglacial reservoir beneath a prominent mid-glacier surface depression. By 1962, a ring of crevasses 250 m in diameter was centered within the depression, implying recent ice collapse. In 1972, the crevasses had almost disappeared, and by 1987, a lake of similar area had replaced the crevasse ring. No lake was present in 1995/6. Theoretical examination of ice overburden and hydrostatic pressure at the glacier bed indicate that a subglacial water reservoir may have formed beneath the surface depression prior to 1962. According to theory, as water accumulates within the reservoir, the hydrostatic pressure increases until it is equal to the ice overburden pressure down-glacier from the reservoir, at which point an outburst flood initiates. Cesium-137 measurements from the Mendel Glacier outwash plain indicate the post-bomb-testing deposition of approximately 20 cm of sand, lending support to the flood hypothesis. The supraglacial lake is ephemeral, and likely reflects relatively high inflow rates or low outflow rates. This sequence of events is possible because the debris cover reduces local ablation rates, forming a depression up-glacier of the debriscover, which in turn forces water to flow into a subglacial reservoir. As long as the debris content is relatively low (<13% by weight), the reservoir water will eventually float the glacier and initiate an outburst flood. Debris-covered glaciers should be recognized as potential sources of outburst floods in alpine regions.