Lake Powell, an iconic centerpiece of the Colorado River Basin and the second-largest reservoir in the United States, is facing an escalating crisis as decades of sediment buildup threaten its future.

Just as water moves through a river, rivers themselves move across the landscape. They carve valleys and canyons, create floodplains and deltas, and transport sediment from the uplands to the ocean.

Picture an hourglass lying on its side, the top resting in the Mississippi River to capture the sands of time floating downstream from the Midwest. The sand flows through the neck of the hourglass and ...