these humans and the essential water cycle...,

thescientist | A massive fish die-off in the Des Moines River this summer wasn’t just local news. An initial body count of 40,000 shovelnose sturgeon attracted the attention of the national media—and raised concern among conservationists around the world.

Indeed, the Des Moines die-off was not an isolated event. Fish biologists in Nebraska dealt with the deaths of thousands of dead sturgeon, catfish, and carp, and officials in Illinois reported dead large- and small-mouth bass and channel catfish. The cause isn’t a mystery. At least 62 percent of the contiguous United States experienced moderate to exceptional drought this summer. Flows of major Midwestern rivers hit record lows. In some places, such as Illinois’ Aux Sable Creek, the largest habitat for the state-endangered greater redhorse fish, the water dried up completely. For those streams, rivers, and lakes that remained, temperatures were abnormally warm, like tepid bathwater. Warm water can be lethal outright to some fish species. It also fosters diseases that weaken or kill others. Furthermore, warm water holds less oxygen than cool water, so an overheated stream can literally suffocate its inhabitants.

This ongoing dilemma is bigger than just one or two exceptionally warm summers, and it is more complex than the effects of a warming climate. To an ecologist, it is clear that human modifications of the landscape play as large a role in these piscine disasters as rising global temperatures.

Broken connections

For millennia, the hydrologic cycle has simply worked, with water circulating through the air and ground in ways that support all life on Earth. Water evaporates from oceans and lakes, falling back to the earth as rain and snow. This same water percolates through the surface of the earth into underground aquifers. It is cooled and filtered in the process, gradually re-emerging in seeps and springs that flow into the planet’s streams and rivers, and back into its oceans and lakes.

In times of heat and drought, slow incursion of cool ground water counteract the effects of rising air temperature on our lakes and streams, sustaining surface water temperatures favorable to a diversity of aquatic life. But in the past decade, something seems to have happened to this natural temperature regulation, resulting in fish die-offs that are increasing in magnitude and frequency.

It seems that the very plumbing of our nation’s rivers, lakes, and streams is broken. The natural hydrologic connectivity of landscapes has been ruptured, blocked, and re-routed in innumerable ways—by dams, roads, buildings, agriculture, and more.