On an unseasonably warm day last month, I stood atop a concrete valve that inhales millions of gallons of river water and spreads it through the faucets, showers and lawn sprinklers of some 600,000 people around Des Moines, Iowa. Thanks partly to climate-related weather changes, that water is now at risk. Just around a bend from where I was standing, the sandy river bottom was exposed, due to a second consecutive year of drought; below me, at the lip of the intake, a film of gray-black foam and a hint of algae undulated near the water’s surface.This unplanned-for combination — extreme weather and surging pollutants — is imperiling water supplies well beyond Iowa. The troubles here reveal a long-term and insidious threat to many cities: a quality crisis that emerges as water ebbs and heats up, boosting the concentration of agricultural waste and micro-organisms.Across the U.S., communities that once took clean, safe water for granted are having their expectations upended by climate change. Without meaningful efforts to address the problem, more American towns and cities will face crises just like this one.*****In 1871, the year the Des Moines Water Works was founded, few people thought that the city might run short of water. But local officials did worry about typhoid and other water-borne illnesses. Their solution was to bury a pipe riddled with holes in the sand and gravel beside the Raccoon River (one of two that sustain the city, along with the Des Moines). As water seeped through the soil and into the pipe, it was naturally filtered of impurities. Over the years, the pipe was extended for miles.Today, infiltration, as this process is called, is the preferred means of obtaining water for the city, and typically accounts for millions of gallons each day. However, it’s not sufficient for this growing city’s demand, which must also be met by river water fed by streams and farm drainage upstream. That water needs to be treated for bacteria, algae, sediments and minerals. As much as possible, the utility seeks to maximize the amount of infiltrated water that it uses, and meets added customer demand with a mix of river and well water. Late on a weekday morning, Ted Corrigan, the chief executive officer and general manager at the Des Moines Water Works, walked me through a 70-year-old section of the Fleur Drive Water Treatment Plant. Gallery windows looked out on 16 pools that were slowly screening sediments from the water.“When this was built, nobody was too worried about nitrates,” Corrigan told me. Nitrates are chemicals found in common fertilizers. They’re crucial to animal and plant growth, but when they’re concentrated too heavily in water, they can create serious health risks. By one estimate, more than 12,000 U.S. cancer cases each year may be caused by nitrate exposure from drinking water.In the 1940s, American farmers used about 2 million tons of chemical fertilizers per year; by 2015, they were using 22 million tons. Such chemicals boost farm productivity, but they don’t simply disappear after growing season. Instead, they’re flushed into streams, rivers and drinking-water systems. One recent study of four Midwestern states found that 86% of the communities with high levels of nitrates in their drinking water were in counties where at least 70% of the cropland is fertilized. In Iowa, elevated nitrate levels were found in the tap water of 236 towns and cities between 2003 and 2017.Corrigan led me into a large room dominated by eight, 15-foot-tall tanks. Each is capable of filtering about a million gallons of nitrates a day. When the facility was built, in 1992, it was the largest of its kind in the world, and highly effective. When total nitrate levels from Des Moines’s water sources — rivers, reservoirs and wells — surge above federal standards, the utility turns it on (at a current cost of $10,000 per day). Some years, it’ll run for weeks; others, it won’t run at all.For the past three years, the nitrate-removal system hasn’t run at all. “This was built for the old normal,” Corrigan said. Thanks to the drought, there’s been little runoff and hence few nitrates in the water. The problem is that farmers don’t stop applying fertilizer or running large breeding operations because of drought. So the nitrates build up. When they’re eventually washed into rivers and reservoirs, they could overwhelm the system and leave the city unable to meet demand.“The worst-case scenario for us is the drought ends with a wet spring and a huge flush of nitrates.”Unfortunately, the worst case is becoming more likely. As the climate heats up, scientists project that Iowa’s summer droughts will become more severe, while springtime will only become wetter.*****Even when the nitrates aren’t flowing, their impact is felt in unsettling ways. For years, scientists have tracked outbreaks of blue-green algae that produce microcystins, a class of toxins, in freshwater sources around the U.S. Among the factors contributing to these blooms are rising temperatures, standing or slow-moving waters, and fertilizer runoff. Sure enough, in the mid-2010s, Des Moines started to detect microcystins in local water. Last year, levels rose so high that officials declared the Des Moines River “essentially unusable” and switched to other sources, including the (very low) Raccoon River.Such outbreaks are becoming more common, especially on the warming Great Lakes. Between 2010 and 2020, 85 locations in 22 states spent more than $1 billion managing toxic algae blooms. In several cases, local officials have warned residents to avoid tap water. Algae-related poisonings are still relatively rare, but annual reported incidents have been rising steadily for decades.Droughts will make matters worse. “If the Raccoon River continues to be as low as it is, and the quality of the Des Moines River is bad, that’ll be a very challenging time,” Corrigan said. “It could be a situation where we have water but it’s not suitable for consumption.”At first, such a scenario would lead to prohibitions on lawn sprinkling, car washing and other non-essential uses, while the city relied on reserves. But what about a longer-term drought, like the one that’s affecting the desert Southwest? “We hope this isn’t an ongoing pattern but you just don’t know,” Corrigan told me. “We’re reliant on the rivers, and if they both dry up there isn’t another plan. There isn’t.”*****In what is perhaps an ominous sign, officials in Des Moines are planning new alluvial groundwater wells to help avoid drawing directly from the city’s rivers. The irony of a river town taking such a step isn’t lost on Corrigan. “It’s unusual for a community who uses direct water to look for another source when there’s ample water in the river,” he admitted. “It’s a quality issue for us.”So long as farmers leak nutrients into waterways such issues will persist, in Iowa and elsewhere. Most small rural communities and private well owners can’t afford to build water-treatment plants devoted to eliminating nitrates. But even if they could, they’d have no way of knowing what will be sufficient in a long-term scrambled by climate change.A better approach is to halt nutrient pollution at the source. That, too, is expensive, but the necessary techniques are well known. Restoring wetlands and planting vegetation buffers between streams and fertilized fields has achieved dramatic reductions in nitrate pollution from farms. Planting year-round cover, such as perennials, can have a similar effect. As the threats to drinking water rise, state and federal authorities should consider subsidies or mandates for such nitrate-reduction strategies, especially in regions where pollution levels are highest.Climate change won’t shut off the taps any time soon. But clean water is going to cost everyone more, from the farmers who grow food to the city-dwellers who buy it. The longer policy makers wait to face this fact, the hotter the water will become.