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The cruel summer heat-wave that continues to scorch agricultural crops across much of the United States and which is prompting comparisons with the severe droughts of the 1930s and 1950s is also leading to record-breaking water temperatures in rivers and streams, including the Mississippi, as well as fast-falling navigation levels.

While in the northern reaches of the Mississippi, near Moline in Illinois, the temperature touched 90 degrees last week – warmer than the Gulf of Mexico around the Florida Keys – towards the river’s southern reaches the US Army Corps of Engineers is dredging around the clock to try to keep barges from grounding as water levels dive.

For scientists the impact of a long, hot summer that has plunged more than two-thirds of the country into drought conditions – sometimes extreme – has been particularly striking in the Great Lakes. According to the Great Lakes Environmental Research Laboratory, all are experiencing unusual spikes in water temperature this year. It is especially the case for Lake Superior, the northernmost, the deepest, and therefore the coolest.

“It’s pretty safe to say that what we’re seeing here is the warmest that we’ve seen in Lake Superior in a century,” said Jay Austin, a professor at the University of Minnesota at Duluth. The average temperature recorded for the lake last week was 68F (20C). That compares with 56F (13C) at this time last year.