Submicrometer variations in the median diameter of siliciclastic mud in Orca Basin, northern Gulf of Mexico, sensitively monitor Laurentide ice sheet runoff to the gulf during early stages of the last deglaciation. Grain-size data document a prominent pulse of very fine-grained sediment to the northern Gulf from 12.6 to 12.0 ka, corresponding with the well-known sea-surface meltwater spike in δ18O records. Meltwater megafloods at this time eroded and transported large volumes of clay to the Gulf via the Mississippi River system. The nonquartzose megaflood sediments appear to have been derived from clay-rich midcontinental terrace deposits rather than from glacial outwash supplied by the Laurentide ice margin. Sediment grain-size data, in combination with other terrigenous proxies (clay minerals and reworked calcareous nannofossils), indicate substantial variability in deglacial meltwater discharge and associated sediment load of meltwater runoff. This variability reflects changes in continental plumbing consistent with terrestrial records of North American deglaciation.