Winter/Spring Runoff Is Earlier, More Protracted, and Increasing in Volume in the Laurentian Great Lakes Basin

Winter/spring runoff has changed in streams worldwide due to climate change, particularly in temperate areas where winter/spring streamflow depends on snowmelt. Such changes potentially affect receiving waters through altered nutrient loading and mixing patterns. The Laurentian Great Lakes are an important freshwater resource and have experienced a myriad of impacts due to climate change. We analyzed 70 years of stream gauge data in the Great Lakes Basin to test for changes in timing, duration, and amount of winter/spring runoff during the period 1950–2019. We found strong evidence for earlier runoff in each of the Great Lakes except Lake Erie, protracted winter/spring runoff throughout the Great Lakes Basin, and a higher runoff depth during the winter‐spring period over time for all watersheds except Lake Superior. Lake Ontario had the greatest change in the date by which 50% of the Jan–May runoff had been discharged (6 days earlier from 1950 to 2019). For winter/spring runoff duration, the most extreme change was observed in Lake Erie (increase of 19 days), and for runoff depth, the greatest change was in the Lake Huron Basin (increase of 3.3 cm). Results were similar for natural and impacted streams. Our results demonstrate dramatic changes in runoff patterns over the last seven decades in the Great Lakes Basin concomitant with previously published changes in precipitation and snowpack. Shifts toward earlier, more protracted, and more voluminous runoff likely change nutrient loading and mixing patterns that influence primary producers, particularly in the nearshore areas of the Great Lakes. Plain Language Summary Climate change has altered the timing of when snowmelt happens in the spring and how long it takes for the snowpack to melt. Such changes in snowmelt are caused by warming temperatures, and potentially, changes in winter and spring precipitation to include a greater proportion of rain. One way to measure the effects of melting and precipitation is to examine the amount and timing of stream flow, which includes water from snowmelt and precipitation. We examined stream flow data from the Great Lakes Basin since 1950 and found that the timing of streamflow in the winter and spring has become earlier and is stretched out over a longer period of time. This means that we are shifting away from “spring floods” toward smaller, more spread out melting events that now begin earlier during the winter. The amount of stream flow during the winter and spring