The lake system into which the Tarim River and Shule River empty is the last remnant of the historical post-glacial Tarim Lake, which once covered more than in the Tarim Basin. Lop Nur is hydrologically endorheic — it is landbound and there is no outlet. The lake measured in 1928, but has dried up due to construction of dams which blocked the flow of water feeding into the lake system, and only small seasonal lakes and marshes may form. The dried-up Lop Nur Basin is covered with a salt crust ranging from in thickness.