Newly created sandbars crucial to wildlife in the Grand Canyon rapidly eroded in the last four months, some shrinking back to the size they were before a costly manmade flood.

Water was released from the Glen Canyon Dam on the Arizona-Utah border in March to mimic natural flooding. Scientists had expected erosion following the flood but they hadn't expected so much so fast.

Grand Canyon Monitoring and Research Center chief John Hammill says the rapid erosion happened because extra water was later released from the dam into the Colorado River.

When Lake Powell above the dam has extra water, there's a requirement that some must be released so it can flow to Lake Mead.

That ensures that Colorado River states have equal amounts of available water.