Why is Crystal Dam spilling this year?
When Crystal Dam spilled earlier this month, it was to meet a 2009 court-adjudicated peak flow rate for the Black Canyon of the Gunnison National Park.
That peak flow is a balancing act determined by a Byzantine formula based on the May 1 forecast for runoff inflow into Blue Mesa Reservoir.
Which means, depending on future snowfall, you may never see Crystal spill again.
Or it might happen every year.
“That’s right, we never know until we get the May 1 forecast,” said Eric Knight, a hydrologist for the Bureau of Reclamation in Grand Junction.
This year, Crystal began spilling on June 4 as runoff waters crept over the 323-foot high dam’s spillway (elevation 6,755 feet).
Flows peaked on June 7 and 8 at just over 7,400 cubic feet per second, enabling the Bureau to meet the 24-hour peak target flow of 6,793 cfs.
The release included 2,100 cfs through the power plant, another 1,000 cfs through each of two bypass tubes and another 3,300 cfs over the spillway.
One cfs is equal to 449 gallons per minute, which means at the peak more than 3.3 million gallons per minute were flowing over and through dam.
The exact peak flows are determined by the amount of runoff available for Blue Mesa Reservoir, the uppermost of the three Aspinall Unit dams.
Dam-controlled flows can be regulated closely but the Cimarron River, which flows into Crystal Reservoir just below Morrow Point Dam, has long been the wild card.
Until this year, when a streamflow gauge was installed near the mouth of the Cimarron, there was no way to knowing how much water the Cimarron was carrying, which made the exact spill amount and date less certain.
Hydrologists with the Bureau’s Grand Junction office, wanting to pinpoint the timing and height of the spill, spoke daily with runoff forecasters at the Colorado Basin River Forecast Center in Salt Lake City.