Many assessments of glacial sediment yield rely solely on measurements of fine-grained suspended sediment. We show that suspended sediment contributes <20% of the total clastic sediment discharge into Iceberg Lake, an Alaskan proglacial lake. Drainage of this lake in 1999 exposed outcrops of varved lacustrine sediment that record suspended sediment deposition. Since a Little Ice Age highstand two centuries ago, lake level has dropped in four abrupt spillway-controlled events that are recorded stratigraphically as transient increases in the basinwide deposition of unusually fine grained sediments. Varve counting constrains ages of these events and, hence, of each abandoned shoreline and associated delta. Volumes of deltaic and lake-floor sediments constrain specific yields from four multidecadal intervals between A.D. 1825 and 1999. Total yield ranges from 3.8 to 4.9 × 103 t·km−2·yr−1, and most of the sediment (81%–86%) is sand and gravel now in the deltas. A range of evidence suggests that sediment yield faithfully records upstream sediment production in this small (66 km2) and heavily glacierized basin, implying an effective subglacial erosion rate of 1.6 mm/yr. These data show that estimates of total yield based upon suspended sediment alone are subject to large errors and suggest that quarrying accounts for a large fraction of the glacial erosion in this system.