Measurements of 234Th/238U disequilibria were made on an approximately bimonthly basis over the course of a year in nearshore (10--40 m deep) and offshore (160 m deep) surface waters of the southern basin of Lake Michigan (18,100 km2). The mean activity of 238U in Lake Michigan measured 230 ¿ 20 dpm m-3, approximately 1 order of magnitude lower than what is typically found in marine systems. Measured median activities of dissolved (<0.45 ¿m) and excess particle-bound 234Th were 8.0 and 60.6 dpm m-3, respectively. Using a simple one-dimensional model, median residence times for dissolved and particle-bound 234Th were ~1 and ~14 days, respectively. 234Th-based particle settling velocities had a median value of 0.4 m d-1 (range: ~0.0--1.4 m d-1), and instantaneous 234Th-based estimates of the net vertical mass flux had a median value of 0.4 g m-2 d-1 (range: ~0.0--5.2 g m-2 d-1). Average particle settling velocities were generally constant over time and increased only slightly in the shallowest (10 m) sampling stations. Calculated mass fluxes showed a strong correlation with temporal and spatial changes in the concentration of total suspended matter, which, in the nearshore area, closely followed seasonal variations in wind-induced wave height. Using a simple two-box model, the 234Th-based cross-margin mass export rate for the entire southern basin of Lake Michigan was equal to 1.35 ¿ 109 kg yr-1, which is in excellent agreement with a 210Pb - 137Cs based, basin-wide mass sedimentation rate of 1.28 ¿ 109 kg yr-1.