The Marmara Sea (area 11,350 km(2); volume 3378 km(3); central basins >1100 m deep) straddles the North Anatolian Transform Fault separating the Eurasian and Aegean-Anatolian tectonic plates. Along with the shallow straits of Dardanelles and Bosphorus (depths similar to 63 m and similar to 40 m, respectively), the Marmara Sea forms the only marine connection between the Black Sea and the eastern Mediterranean. During Pleistocene glacial stages, the modern straits were subaerial valleys and the modern Marmara basin was occupied by the landlocked Propontis Lake. Previous researchers attributed major portions of a widely distributed uppermost Pleistocene-Holocene mud blanket (locally >10-25 m thick; volume 43-47 km(3)) to transport of suspended load through one or both of the straits, as either the Aegean Sea (at similar to 13.8 cal ka) or the Neoeuxine Lake (today's Black Sea, at similar to 11.1 cal ka) began to spill into the Marmara basin. To test these suggestions, the thicknesses and volume of the mud blanket were determined from >5000 line- km of airgun, sparker and boomer profiles and > 100 cores, and compared with the contemporary supply from local rivers to decide, by difference, if the straits might have had a significant role. Volume calculations for the detrital supply from rivers rely on (1) decades of dailywater- and sediment-discharge data from gauging stations, acquired before 20th century damconstruction and, independently, (2) the BQART model which uses a variety of hydrological, geomorphic, geological and climate data. These calculations demonstrate that >85-90% of the detritus in the offshore mud blanket was supplied by steep rivers (Kocasu River and its tributaries) and mountainous streams draining the highlands of the southern Marmara region. Geochemistry of the <38 mu m fraction supports this source. Any input through the Dardanelles has been sporadic and limited to perhaps similar to 5 Gt of suspended load (equivalent to similar to 5.2 km(3) of porous mud when deposited) because of changing directions and rates of flow since the Last Glacial Maximum. Resedimentation through mass wasting and transgressive shoreface erosion appear to be minor compared with river supply. The isolated nature of the Marmara basin and its supply from mostly a single watershed afford an opportunity to verify the reliability of this type of hindcast analysis, based upon sediment-discharge data and catchment models - analysis which cannot be completed with a comparable level of certainty along open marine coastlines elsewhere. (C) 2021 Elsevier B.V. All rights reserved.