CHECK LAKE LEVELS
Tracking Lake Levels
The Commission oversees the lake’s water level by tracking the lake’s surface elevation and insuring that discharges occur in conformance with the “rule curve.” The rule curve sets a target lake level for each day of the year. The targets are based on state law and on historic operating guidelines designed to maintain the lake at optimum levels for navigation and recreation.
The Rogers Rock Gage is a continuous monitoring device operated by the United States Geological Survey (USGS). The Commission records and reports the level each business day about 9:00 am. This becomes the daily lake level used and reported by the Commission. 316.06' is 0 datum on the RRG.
The lake’s target elevation for summertime is 3.5' on the Rogers Rock Gage (RRG). This converts to an elevation of 319.56' MSL (mean sea level). A daily lake level reading higher than the rule curve means that water is discharged from the lake. A lake level lower than the target level requires that the discharge gates are closed.
What affects the lake level?
Precipitation is the source of the lake. Snow and rain falling on the 44 +/- square mile surface of the lake contribute water directly. Precipitation on the 233 +/- square mile watershed results in surface water runoff or groundwater flows. The amount of runoff from the watershed is affected by several factors. The condition of the watershed’s soil (frozen, unfrozen, dry to saturated) the level of intermittent lakes, ponds and wetlands in the watershed as well as the state of trees and vegetation (active or dormant) all affect the amount of runoff.
Lake George is a natural lake whose surface elevation has been raised slightly (perhaps 2-3 feet on average) by a dam constructed at the outlet in Ticonderoga along the La Chute River. The additional elevation of lake level created by the dam can be regulated to some extent by the dam’s discharge structures. By using the discharge capacity to draw down the lake to accommodate spring runoff and by limiting discharges at other times, people are able to keep the lake’s level, for the most part, within an annual range of about 12 to 16 inches. However, natural forces sometimes exceed human designs. Excessive precipitation and runoff can cause the lake to rise occasionally to undesirable levels. The limited discharge capacity of the outlet means that when the lake is too high it is slow to return to normal. Persistent dry weather and evaporation can cause lower than desired lake levels, as well.
How is the summertime target level set?
The Summertime (June 1, to September 15) target elevation for the lake of 3.5 on the Rogers Rock Gage (RRG) or 319.56 above mean sea level is established by Section 33 of the New York Navigation Law. Under this law, the outlet facilities must be operated in a way that, with due allowances for natural fluctuations, will maintain the lake at 3.5 (RRG). This level is considered an optimum level for recreation and navigation. Excess water is discharged and this water may be routed through a hydroelectric generation facility and used for power generation.
Why is the lake level too low on occasion?
Lake George is a natural lake that is usually balanced within a very narrow regime of 12 to16 inches annually, and five to six inches in the summer. Precipitation is the only source of the lake. Runoff, snow melt and groundwater are all aspects of precipitation. During dry periods, especially late Summer and Fall, evaporation from the surface of the lake can exceed the incoming water and the lake level falls. During such conditions the discharge gates are closed and the lake water may not be used for power generation. A lake level of 3.0 RRG, 6 inches below the target, is enough of a change from normal to generate a spate of inquiries to the Commission office. Compare this to neighboring lakes such a Sacandaga Reservoir and Lake Champlain that may range 8 to 9 feet.
Why does the lake get too high at times?
The capacity to release water from the lake is small in relationship to the volume of heavy runoff. The capacity has been likened to draining a bathtub through a pinhole. When water entering the lake exceeds the maximum rate that can be discharged, the lake rises.
Each winter the lake is drawn down in anticipation of the spring snow melt. Snow surveys help to estimate the amount of water that is resident in the basin’s snow pack and refine “winter pool” target elevations. Under ideal conditions the spring thaw is gradual, re-charging groundwater and tributaries and allowing the lake to slowly replenish a foot or more of elevation to the target level of 4.0 (RRG) around April 1. However, nature can be anything but predictable. A combination of rain and melting snow can produce an inflow of water that exceeds the storage capacity created by draw down. Even when inflow conditions return to normal, it is possible only to lower the lake level at a rate of about an inch per day.
What has caused the high lake levels during June in recent years?
High water conditions may also be experienced in late Spring such as in 2005 and 2006. In these years very heavy rainfalls saturated the ground and created large runoff events. With a land drainage basin area to a lake surface area ratio about 4:1. Two inches of rain runoff can raise the lake 8 inches. A lake level already near its Springtime crest has little room to accept a series of large storms. In 2005 and 2006 lake levels rose to above normal levels despite maximum discharge occurring. Again levels of no more than 6 inches above normal were cause for problems on a lake normally maintained in a narrow range.
Why not adjust the rule curve to lower the lake in anticipation of heavy late spring rains?
The late Spring targets, which have served well for years, reserve a couple of inches of elevation as a hedge. Without this reserve, lower than desired Summer lake levels may occur more often. The Commission is monitoring weather trends. Climatic changes causing sustained increases in precipitation would be cause to reconsider the rule curve targets.