In the span of three weeks, spring crept over the Siberian landscape surrounding the northern half of the Lena River. Many of the rivers in Earth’s temperate zones run high in the spring when melting snow and spring rain flood river basins. On the Lena River, however, spring flooding is almost inevitable for another reason: ice. Like other north-flowing rivers, the upper reaches of the Lena melt before their downstream counterparts. Because the northern mouth of the river remains frozen while the southern body of the river flows freely, water naturally builds behind the ice, forming a temporary reservoir that drains as the ice dwindles.