Former Park Service regional director John Cook said Tuesday more experiments and "hard science" are needed to either validate or invalidate the expected results of this week's release.

Bureau of Reclamation commissioner Robert Johnson and National Park Service director Mary Bomar called for continued experimenting toward balancing the need for water storage, power delivery and resource conservation.

"Our descendants 500 years from now should be able to enjoy the same awesome vista that you and I are enjoying here today — and the same wildlife and the same recreational opportunities that we are enjoying today," Bomar said.

Hundreds of archaeological sites along the river may also benefit from the extra sand that shows up on river banks downstream. Then, through a more natural process generated mostly by wind, the goal is to see the new sand act as a barrier against the elements to protect those irreplaceable sites.

Nature in the form of spring runoff, monsoonal rains and occasional flooding used to take care of all of that, so much so that most of the archaeological sites were not even visible in the 1960s. But by 1966 the 710-foot tall dam (about 583 feet of it is visible) was complete and water began filling in most of Glen Canyon and surrounding areas. Lake Powell formed, becoming a magnet for boaters and fisherman as well as a source of fresh water in the desert Southwest and hydroelectric power for thousands across several states.

U.S. Geological Survey director Mark Myers said Wednesday the dam now traps about 90 percent of sediment generated by tributaries and runoff upstream. This latest experiment is timed to coincide with sediment from tributaries below the dam currently feeding into the Colorado.

What's left of Glen Canyon below the dam is stunning, characterized by tall red-rock cliffs brushed by desert varnish and punctuated in places by natural amphitheaters where portions of cliff face have fallen away. Along the way there is evidence of human activity dating back thousands of years, glimpses into their lives left behind in petroglyphs, pottery shards and pieces of burned wood.

But south of the dam, over the years, the ecosystem and riparian habitat changed. The waters between the dam and Lee's Ferry, 15 miles downstream, turned clear with blue-green tones in certain light. Some of that green comes from the buildup of algae and rooted aquatic plants. Without the dam the river's color, in a more natural state, would be more brown or milky with sediment. Below Lee's Ferry right now those contrasting colors can be seen mixing where the Paria River and its major source of sediment feeds into the Colorado.