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New Hampshire Department of Environmental Services

Operations Information
List of Specific Station Operations

Current Watershed Operations Information - Winnipesaukee

Station: Opechee Bay at Laconia

No meteorological data is collected at this site.  There are 2 staff gages, one at the
Avery Dam and another on a pier just upstream of the Messer Street Bridge, that are
read manually 2 or 3 times per week.  The staff gage upstream of the Messer Street
Bridge more accurately represents the level of Opechee Lake.

Opechee Lake Operating Information:

Water Level and Flow Constraints:
Under normal operating conditions at Lakeport Dam, flows into Opechee Lake are well within
the range of the two turbines and automated crest gates at Avery Dam (outlet from Opechee).
At flows of between 250 and 1,000 cfs, the level of Opechee ranges between 492.7 and 493.2,
respectively.  However, at flows above about 500 cfs, the localized drawdown between the
Opechee Lake gage and the gage at the Avery Dam becomes more pronounced.  These two staff
gages are separated by approximately 0.6 miles of relatively narrow river channel with two
roadway bridges and one railroad crossing.  When viewed during drawdown conditions, this
section of river exhibits numerous riffle sections.

During the June 1984 flood event, the difference in elevation between the main body of
Opechee and the dam peaked at 2.3 feet (494.3 vs. 492) at an estimated flow of
approximately 3,000 cfs.  Similarly, in June 1998 this difference was approximately 1.8
feet (494.5 vs. 492.7) at a flow of nearly 2,600 cfs.  The structural configuration of the
dam was modified some time after 1984 during the installation of the hydro works, so
discharge capacities at the dam differed between the two flood events.  

Summary of Operation Under Normal Conditions:
The level of Opechee Lake is maintained at a relatively constant level throughout the year
through a combination of hydroelectric energy generation and a system of automated leaf
gates along the crest of the Avery Dam.  For inflows up to a maximum of 700 cfs, the
turbines at Avery Dam self-adjust to pass exactly what is entering the lake, and the net
effect is a level reservoir.  For flows higher than 700 cfs, the turbines will be at
maximum capacity and the leaf gates at Avery Dam will automatically lower to pass surplus
flows and maintain a relatively constant reservoir level.

Summary of Operation Under Flood Conditions:
For flows up to approximately 1,500 to 2,000 cfs, the two turbines are kept on line while
the automatic leaf gates adjust in response to pool levels, waste gates are opened and
stoplogs are removed.  Beyond 2,000 cfs, the turbines are raised from their pits providing
additional capacity, which allows the crest gates to rebound upward in response to the
additional discharge capacity through the vacated turbine pits.  As inflows continue to
climb, the leaf gates will again lower automatically to keep pace.  Should inflows
increase to the point that the leaf gates have fully lowered, there are two 3.25 high by
25 wide flashboard sections that must be manually tripped.  None of the high water events
to date have required tripping either of the flashboard sections, and it appears that the
channel upstream of the dam controls discharges.     

Opechee Lake is drawn down approximately 5 feet every other Columbus Day (even numbered
years) for a period of approximately two weeks, but it has occasionally ebbed lower.
Except for the obvious impacts to access and recreation during the bi-annual two-week
drawdown, low water issues are rarely encountered.  Because of its automated hydroelectric
machinery, lake levels can be managed to avoid low water conditions.

Related Information

NH Department of Environmental Services | 29 Hazen Drive | PO Box 95 | Concord, NH 03302-0095
(603) 271-3503 | TDD Access: Relay NH 1-800-735-2964 | Hours: M-F, 8am-4pm

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