You're listening to Shorewave from NPR.

Here's a fun fact.

Two thirds of the world's population lives alongside a river.

That includes people in Washington, D.C., where I live.

We're here at the Potomac River.

The southern boundary of D.C. is cradled by two rivers, the Anacostia and the Potomac.

Millions of people live in its watershed, and I can just see the top of the water glinting

in the light of the Kennedy Center.

And obviously there's a lot of traffic along the edges of the river, foot traffic, car traffic.

Life as we know it wouldn't exist without rivers.

And Lawrence Smith, professor of Environmental Studies at Earth Sciences at Brown University,

considers them part and parcel to human civilization.

Rivers are everywhere, and their origins are ancient.

Around four billion years ago, if not earlier, rain began falling out of the sky.

And water began to pool into lakes and seep into the ground.

And water flowed over land into riverlets and streams and rivers to newly filling seas.

This water evaporated into the primordial atmosphere, creating water vapor, condensing into clouds,

and raining down again.

Behold the early hydrologic cycle.

These rivers were born and began eroding the earth's very early thickening continental crusts.

These rivers are concentrators of mass and energy among us, powering the places where humans have settled for hundreds of years.

And in the United States, there's two rivers in particular that have been pushed to the brink.

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