Default text size Medium text size Large text size
skip navigation
New Hampshire Department of Environmental Services
PUBLIC GOVERNMENT BUSINESS A to Z LIST

Operations Information
List of Specific Station Operations

Current Watershed Operations Information - Winnipesaukee

Station: Silver Lake at Lochmere

This station is located on Silver Lake, just below the canal and hydroelectric station
associated with the Lochmere Dam in Belmont. The following parameters are currently
being measured at this station:

Silver Lake Stage (Lake Elevation)
Precipitation
Air Temperature

Silver Lake Operating Information

Water Level and Flow Constraints:
The combination of the natural outlet of Silver Lake and the flat marshy expanse at the
junction of the Tioga and Winnipesaukee Rivers acts to slow the movement of runoff
through Silver Lake.  When flows into this part of the watershed increase, either as a
result of increased discharge at Lakeport/Lochmere Dams or uncontrolled runoff from the
Tioga River basin, the level of Silver Lake must increase to pass this additional flow.
By correlating lake level data at Silver Lake with flows at the Winnipesaukee River gage
in Tilton, there seems to be evidence to support that the stage-discharge relationship
of the natural channel at the downstream end of Silver Lake may vary due to the
periodic build-up of river sediment and/or blockage along the outlet channel.  During
low flow periods slow moving water allows sediment to settle, while springtime flows or
larger floods may have the opposite effect of removing accumulated sediment.  This
constantly changing nature of the streambed in the area of the natural outlet of Silver
Lake causes the natural outlet's ability to discharge water to fluctuate. 

Routine operations of upstream dams result in flows between 250 and 1,000 cfs at the
river gage in Tilton, and the corresponding levels of the lake are 463.80 and 465.76,
respectively.    When the flows are reduced significantly, as can be the case during the
annual 2-week shutdown of Lakeport Dam, the level of the lake can get as low as 462.81
when flows are down to 40 cfs.  When flows at the Tilton gage reach 4,300 cfs, Silver
Lake reaches a level of 470.36.   History has shown that when the lake level gets to
about 468, which corresponds to a flow of approximately 2,000 cfs at the Tilton gage,
low-lying properties begin to experience some form of low to moderate flooding.

Summary of Operation Under Normal Conditions:
The elevation of Silver Lake is a function of the amount of water entering and leaving
the lake, as the lake has a natural outlet with no man-made influences to affect
discharge capacity.  Since the powerhouse at the Lochmere Dam discharges directly into
Silver Lake, the releases at the dam have a significant effect on the operating level
under normal meteorological conditions. The 32 square mile Tioga River watershed enters
near the downstream end of Silver Lake.  The lake’s outlet control appears to be located
in the area of the NH Route 140 bridge crossing.    Comparing the level of the lake to
the flows measured at the flow-measuring gage in Tilton shows that for flows between 250
cfs and 1,000 cfs, which are those considered normal and reasonably expected on a
routine basis, the level fluctuates over a range of approximately 1.9 feet.  The level
of Silver Lake can and does fluctuate on a frequent basis.  Most shorefront residents
are well aware of this and, when these fluctuations are within these limits, use of
shorefront property and recreational pursuits are essentially unaffected.  

Summary of Operation Under Flood Conditions: 
Since there is no ability to pass additional flows at the outlet of the lake beyond the
capacity of the natural channel, the only variable that can be controlled, which during
floods may be limited, is the discharge into the lake from Lake Winnisquam (Lochmere
Dam).  This discharge, in turn, is usually dictated by conditions at Lakeport Dam.  In
addition, the Tioga River drainage area alone can produce a significant amount of
runoff.  Under conditions that require additional discharges from Lochmere Dam, the
Tioga River is generally also approaching peak flows, which can contribute to elevating
peak lake levels and the corresponding flooding around Silver Lake.  Operational
decisions related to managing the level of Silver Lake involve both Lakeport and
Lochmere dams.  In cases when storage room exists in either Winnipesaukee or Winnisquam
lakes, use of this storage may result in a significant reduction of the peak level of
Silver Lake during an event.  Lake Winnisquam, however, has over 50 square miles of
additional contributing drainage area the runoff from which can quickly fill any
available flood storage.  In addition, the peak inflows to Winnisquam and Silver lakes
from each of their respective local drainage areas arrive at approximately the same
time, necessitating releases at Lochmere Dam at times when the level of Silver Lake is
already high.
  
Drawdown

There is no annual drawdown of Silver Lake.  However, when flows at Lochmere Dam are
reduced to approximately 50 cfs, as they are every second year when Lake Winnisquam does
not experience a temporary 2-week drawdown, the level of Silver Lake is reduced to about
1.2 feet below it's normal summer operating level corresponding to a 250 cfs release 
from Lochmere Dam. 

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

copyright 2008. State of New Hampshire