The reactor was water-cooled, with cooling water pumped from the Hanford Reach of the Columbia River through the aluminum tubes around the uranium slugs at the rate of 75,000 US gal (280,000 L) per minute. The water was discharged into settling basins, then returned to the river after allowing time for the decay of radioactive materials, the settling out of particulate matter, and for the water to cool so it could be returned to the Columbia River. Water returning to the river was prohibited from raising the temperature of the river by more than 11 °F.