Pawtuckaway Lake Evaluation – Spring 2016 Update

Prepared by the NH Department of Environmental Services (NHDES)

Over the last year, NHDES continued its evaluation of Pawtuckaway Lake’s phosphorus, aquatic plant surveys and lake outflow volumes and phosphorus concentrations. Semi-annual meetings were held in spring and fall. A meeting was held September 17, 2015 in Nottingham where the summer relief pulses and the public hearings about the draft 2015 Report of the Instream Flow Pilot Program were discussed.

Lake Drawdown – On October 13, 2015 the NHDES Dam Bureau started the fall drawdown by opening the Drowns gate and removing a stoplog from Dolloff dam, which was subsequently replaced. Most of the flow from Pawtuckaway during the last two years’ drawdown had gone through Drowns. Initially, the target release ratio was 70:30 (Drowns: Dolloff), but later the ratio drifted to a 90:10 split in trying to maximize phosphorus export. However, this has resulted in long periods when the Pawtuckaway River received no flow other than leakage. This is much less than is needed to support that river’s ecosystem.  A more balanced release will occur next time while still preferentially releasing higher phosphorus concentrations at Drowns. From October 30 to November 1, NHDES rerouted all outflow through Dolloff Dam when Fish & Game Department requested that alewives needed egress from the lake over Dolloff Dam. NHDES and NH Fish & Game are conferring on how to operate the drawdown to support the alewives’ migration out of the lake.

2015 Phosphorus values – Phosphorus export during the fall of 2015 was significantly less than in 2014. From October 10 to December 3, 2015, 211 pounds of phosphorus were released from Pawtuckaway Lake during the early stages of the lake drawdown, compared to 1066 pounds in 2014 as a result of lower phosphorus concentrations and less water entering and leaving the lake. The volume released from the lake in 2015 was 37 percent of 2014’s volume, and the peak phosphorus concentration at Drowns Dam outlet in 2015 was 40.2 ug/L compared to 2014’s concentration. NHDES is looking into the reasons for lower phosphorus concentrations in the lake.

Wiswall Dam water use – Under the UNH/Durham Water System’s Instream Flow Program water management plan, UDWS must stop withdrawing water from the Lamprey River whenever flows at the USGS gage fall below 16 cfs.  UNH has developed a website at http://energy.sr.unh.edu/water/ which shows UDWS’s water withdrawals from Wiswall Reservoir and the reservoir levels.   The lake level drawdown for 2016/2016 target will be -5 feet (minus 5 feet).

Click here to read the full report (PDF):  2016-plia-article-from-des

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