Kyshtym disaster

The Soviets, not knowing the danger posed by radiation, disposed of radioactive waste very carelessly.

An excerpt from the article 42 facts about Kyshtym disaster

Initially, the liquid waste, which included isotopes of Cesium (Cs-137) and Strontium (Sr-90), was poured into the Techa River. This practice was also used by the Americans - they poured waste from the Hanford nuclear facility in Washington state into the Columbia River, but not in the same quantities as the Soviets.

The difference between the two rivers fundamentally affected the danger - the current of the Columbia River allowed the isotopes to travel faster toward the Pacific Ocean and spread evenly. The much weaker current of the Techa River caused radioactive isotopes to settle on its banks and in the river sediment.