‘What are we willing to sacrifice?’ A journey down America’s most endangered river

The serene beauty of the Colorado River belies an ecosystem that is forever changed. I met the scientists fighting for its future

Our small metal boat glides through the current as it snakes between steep sandstone cliffs. Tucked between America's two largest reservoirs, the stretch of the Colorado River that lies north-east of the Grand Canyon is cool and calm. As the water glimmers in the early morning light, it is easy to forget this serene waterway has been named the most endangered in the US.

But as we round a bend, a shadow envelops the boat and Glen Canyon Dam comes into view - 4.9m cubic yards of concrete ominously arching 710ft into a cloudless sky. This is as close as we can get," says Ted Kennedy, a biologist and one of my guides for the day, as he cuts the engine.