Young chinook salmon held in live boxes were drifted through Hanford reactor discharges in the central Columbia River, with the objective of evaluating the affect of heated effluents on fish survival. No significant mortalities of fish were recorded from 12 drifts during the spring (March--April) when base river temperatures were low, ranging from 4 to 7.6 C. Some mortalities occurred, however, in 3 of 24 drifts conducted in late summer (August--September) when base river temperatures extended from 17 to 19.6 C. None of the mortalities resulted from passage through midstream discharge plumes, which were characterized by relatively moderate temperature increase and short duration of exposure to maximum temperatures. Mortalities occurred only when fish were drifted along the south bank of the river through areas receiving heated water via intragravel seepage from shoreline retention basins and experimental subsurface disposal of reactor effluents. The shoreline areas were characterized by impeded river flow, which restricted horizontal and vertical mixing and prolonged the exposure of drifted fish to excessively warm temperatures. The extent of thermal shock sufficient to cause death in the field is primarily an integrated function of temperature increase and exposure duration, as has been demonstrated in laboratory experiments. The numerous possible …