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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. |
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