Water flow experiment begins at Grand Canyon

Previous experiments in 1996, 2004 and 2008 were one-time fact-finding missions instead of fundamental shifts in river management.

"This (Obama) administration can be patted on the back and thanked for doing what we've been trying to do, seriously, for 15 years," Lash added.

The previous experiments yielded mixed results, partly because a return to up-and-down flows timed partly to regional summer hydropower needs wiped out many of the new beaches and sandbars.

Advocates hope the effects will be longer lasting if these floods come more regularly and if a longer-term Interior Department planning effort leads to steadier flows through the summers.

But critics say there's little environmental benefit and that it comes at a cost.

In comments submitted to the Interior Department before the decision to go forward with regular flushes, the Colorado River Energy Distributors Association, a group of non-profit energy utilities, noted that previous springtime flood experiments helped boost the population of non-native trout that feed on the endangered humpback chub.