Wednesday, April 25, 2012

Spring snowmelt management: a frog year?

As the spring unfolds and late rains continue to fall, it's clear this is not the dry year it could have been.  However, the rains came too late to make up for the mostly precipitation-free winter.  The good news is this provides another opportunity to test the snowmelt management strategies being developed for O'Shaughnessy Dam to benefit the Poopenaut Valley wetlands and the rest of the Tuolumne River ecosystem.  This year, operational tests will focus on managing available snowmelt spill to fill the Poopenaut Valley pond while also providing good conditions for foothill yellow-legged frogs.  Dry years provide the best opportunity to benefit the small foothill yellow-legged frog population present upstream of Early Intake.  In wetter years the upper mainstem Tuolumne features higher and more variable snowmelt flows that can last into the summer.  Wet year high flows can delay egg-laying or scour egg clutches, and are challenging for egg and tadpole development, as they likely were even under natural conditions.  Dry year snowmelt spill flows are designed to peak early and provide more stable flow conditions for egg laying and tadpole development.  Filling the Poopenaut pond will provide good conditions for benthic macroinvertebrates, chorus frogs, garter snake, bats, and other wildlife that depend on the pond ecosystem over the remainder of spring and into early summer.

Planned snowmelt management hydrograph.
Flow releases from O'Shaughnessy Dam will remain at current levels through Friday, April 27 to accomodate biological monitoring work, but will increase on Saturday to 200 cfs and possibly slightly higher thru Monday.  Flows will ramp up on Tuesday, with a peak on Wednesday (May 2) of 4100 cfs.  The peak will be timed for mid-day Wednesday to allow the highest flow to pass boaters in the Lumsden reach went they are off the water in the evening and allow for observation at O’Shaughnessy Dam and the Poopenaut Valley overlook during daylight.  The current plan is to ramp down to 250 cfs by Thursday afternoon; however flows may stay higher if needed to manage snowmelt inflows.