With new pumping operation in place, all eyes on clarity
August 2, 2016
On a still, clear morning in Grand Lake, the surface of the water looks so motionless, like one single 600-acre pane of glass, that it's hard to imagine it could be in a constant state of flow. And yet, mountain streams and the channel from shadow mountain feed water into the lake on all sides while, simultaneously, a large sub-surface intake sucks water out of the lake and down the 13 miles of the Alva B. Adams Tunnel toward farms and houses on the east slope.
That imperceptible flow has, over time, taken plumes of sediment and vegetation from the shallow waters of Shadow Mountain Reservoir into the depths of Grand Lake.
In late June, representatives from the Bureau of Reclamation presented a plan to the Three Lakes Watershed Association outlining a new pumping schedule for the Adams tunnel intended to improve the clarity issues in Grand Lake. The tunnel will now draw an almost constant 250 cubic-feet-per-second, less than half of its 550 cfs maximum and roughly equivalent to the flow of a small stream.
The new "low and steady" operation was set to go into effect on July 1, but was delayed to July 8, according to Bureau of Reclamation public affairs officer Tyler Johnson. It's too early to conclude much from the clarity data gathered since the shift, but a brief analysis of the clarity data published by Northern Water from June 8 to July 27 shows a decrease in average clarity after the shift in pumping systems. A comparison to the same period in 2015 also shows a decrease in average clarity. The new pumping operation will stay in place through September 11, and the overall effect of the process may take years to unfold.
Clarity readings in the lake have been recorded since 1941, when the measuring tool, a ten-inch white disk called a Secchi disk, could be seen nine meters below the surface, or about 27 feet. More recent Secchi disk readings have hovered between three and four meters, but have reached as deep as 21 feet and as shallow as three feet in the last five years in certain monitoring locations. For comparison, Lake Tahoe in California averaged a Secchi disk reading of 73 feet in 2015.
In June, the Grand Lake Clarity Adaptive Management Group, including representatives from Grand County and Northern Water, set a clarity standard for the lake of 3.8 meters, and that it not go below 2.5 meters.
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Since the pumping schedule change, average Secchi disk depth on Grand Lake has been observed at 3.39 meters.