Enlarge / In this aerial view, barges, stranded by low water, sit at the Port of Rosedale along the Mississippi River on October 20, 2022, in Rosedale, Mississippi. (credit: Scott Olson / Getty Images )

Rivers are critical corridors that connect cities and ecosystems alike. When drought develops, water levels fall, making river navigation harder and more expensive.

In 2022, water levels in some of the world’s largest rivers, including the Rhine in Europe and the Yangtze in China , fell to historically low levels. The Mississippi River fell so low in Memphis, Tennessee, in mid-October that barges were unable to float , requiring dredging and special water releases from upstream reservoirs to keep channels navigable .

Conditions on the lower Mississippi may be easing somewhat, thanks to early winter rains . But as Earth scientists at the University of Memphis , we see this year’s dramatic plunge in water levels as a preview of a climate-altered future.

Enlarge / A smoggy sunrise in Krakow, Poland earlier this summer. (credit: NurPhoto via Getty Images )

On the morning news, you see the weather forecast is for high heat, and there is an “ excessive heat watch ” for later in the week. You were hoping the weather would cool down, but yet another heat wave is threatening human health and increasing the chance of wildfires. On top of these warm days and nights, air quality data has been showing unhealthy levels of pollution.

Sound familiar? This scenario is increasingly the new normal in many parts of the world.

High heat and air pollution are each problematic for human health, particularly for vulnerable populations such as older adults. But what happens when they hit at the same time?

Enlarge / The Dolmen of Guadalperal completely visible in July 2019 due to a low water level in the Valdecañas reservoir. (credit: Pleonr /CC BY-SA 4.0 )

Last week we told you about the flurry of recent coverage resurfacing 2018 news stories about the re-emergence of so-called " hunger stones " due to extreme drought conditions in Europe. We also noted that Europe is once again in the midst of a historically severe drought. Now an ancient site known as the "Spanish Stonehenge"—submerged underwater by a reservoir for decades—has been fully exposed for the second time since 2019 due to low water levels in the reservoir.

The site is also known as the Dolmen of Guadalperal , a circular grouping of 150 large vertical granite stones (called orthostats) dating back to between 2000 and 3000 BCE. However, Roman artifacts recovered at the site—a coin, ceramic fragments, and a grinding stone—suggest it might have been used even earlier. A team led by German archaeologist Hugo Obermaier discovered the monument in 1926 near a town called Peraleda de la Mata.

Among the recovered artifacts were 11 axes, flint knives, ceramics, and a copper punch. A nearby settlement likely housed the people who built the monument, given the presence of houses, charcoal and ash stains, pottery, and stones to hone axes. Obermaier restored some of the granite stones to their rightful places and made reproductions of the engravings, which were published in 1960.

Enlarge / A hunger stone in the Elbe River in Děčín, Czech Republic. The oldest readable carving is from 1616, with older carvings (1417 and 1473) having been wiped out by anchoring ships over the years. (credit: Dr. Bernd Gross/CC BY-SA 3.0 )

Stories have been circling around the Internet this past week about the re-emergence in certain Czech and German rivers of so-called " hunger stones "—rocks embedded in rivers during droughts to mark the water level and warn future generations of the likely famine and hardship to come whenever the stones became visible again. The coverage has been fueled largely by an August 11 tweet noting one stone in particular, inscribed with a dire warning: "If you see me, weep."

Hunger stones ( hungerstein ) are very much a real thing with a long and fascinating history. And Europe is in the midst of a historically severe drought —severe enough that water levels may indeed be sufficiently low for the stones to re-emerge once more. But that August 11 tweet and the related coverage are actually rehashing a series of news stories from 2018, when the re-emergence of the hunger stones in the midst of that year's extreme drought in Europe made headlines.

It's hardly an egregious case of misinformation, but it does provide an illustrative example of why including context is so important in the digital age—even in a relatively simple tweet enthusing about newly acquired knowledge.