General Operating Constraints:
We are constrained to release a minimum of 250 cfs throughout the summer months to
fulfill the following obligations and limiting restraints:
1. Hydroelectric plants downstream of the lake have permanent deeded rights to
an outflow or 250 cfs in the river. These historical water rights were
established in the 19th century but are still valid today.
2. The State maintains water user contracts to other hydroelectric power
producers downstream. These contracts are based on the presumption the State
will continue with the practice of maintaining the 250 cfs release rate.
Investment in the plants and long term operational expenditures in the plants
are based on this implicit understanding.
3. The NH Fish and Game Department has requested a minimum runoff in the
Winnipesaukee River based on a runoff rate per square mile. The runoff rate
they have requested is 0.5 cubic feet per second per square mile. The
Winnipesaukee basin at the outlet from Silver Lake is 458 square miles, so
the rate they would like to see would be 229 cfs, close to what we are bound
to supply the hydropower companies.
4. Silver Lake does not have a dam at its outlet, so that the discharge capacity
of the natural channel downstream from the lake controls the lake level. At
below 250 cfs flow through the system, Silver Lake’s water level drops very
low, below that which is usable for many of the residents.
5. Any significant reduction in summertime flow releases at Lakeport would
affect flow in the Merrimack River and reduce the capacity of the river to
assimilate pollutants. The 250 cfs release rate from Lake Winnipesaukee
augments low flows downstream during drought conditions. Records from the
Merrimack River gauging stations, which reflect the historical releases at
Lakeport Dam, are used in NHDES permit limit calculations. From an NPDES
permitting perspective, it is important flow release at Lakeport Dam because
permits issued to wastewater treatment facilities and industries that discharge
to the Merrimack River include effluent limitations that are based on 7Q10 low
flow conditions in the Merrimack River. New Hampshire wastewater treatment
facilities that discharge to the Merrimack River include the Winnipesaukee
River Basin Program in Franklin, the Merrimack County Complex in Boscawen,
and wastewater facilities in Penacook, Concord, Allenstown, Hooksett,
Manchester, Derry, Merrimack and Nashua.
List of constraints limiting the ability to maintain a higher lake level:
1. When the ground is saturated after multiple storm events or in the spring,
an inch of rainfall can bring up Lake Winnipesaukee by over 5 inches. Due
to this it is imprudent to allow the lake level to rise above full lake level
for long periods.
2. Because the water level in the lake drops so significantly over the summer
season as outlined above, many residents’ docks are built at around the
average summer water level, not at full lake. During the early summer when
the water level is high this can be problematic as wave action will surge and
pop boards off of docks as the waves pass.
List of constraints limiting the ability to maintain a lower lake level:
1. The deeded release rate listed above along with significant summer
evaporation results in a drop of the lake level by approximately 15 inches
throughout the summer months. The evaporation rate alone removes water at a
rate approximately equal to the 250 cfs release rate at the dam. Every 250
cfs drops the lake level by 0.01’ per day, so with evaporation and no rainfall
the lake can be expected to drop 0.02’ or about a quarter inch per day.
Historically there is a lack of offsetting runoff in the lake summer months
so that the only source of water available to maintain the lake level is water
stored from the spring runoff.
In anticipation of this drop throughout the summer, a starting summer target
elevation is set for Lake Winnipesaukee of 504.32’ to allow for enough water
to be able to keep the lake as close to usable level as possible at the end
of the summer and into the early fall.
2. Early in the summer there are years Lake Winnipesaukee begins the summer
season high. Hydropower producers have limited ability to pass large volumes
of water, the maximum rate being 750 cfs at one facility. In order to utilize
this source of energy efforts are made to limit the discharge rate from the
lake to match this value. Other sites can use up to 1100 cfs, and this is also
used as a target discharge rate when the lake is too high and efforts are made
to lower it by releasing water.
3. High release rates of water can cause shoreline damage, and many complaints
arise even during flooding conditions when it is imperative to drop the lake
levels by releasing more water. This is particularly true in the springtime,
when ice sheets moved by rapidly moving water can damage structures along the
shore. An effort is made to weigh the offsetting sources of damage, high water
vs. rapidly moving water against the shoreline corresponding to high flow