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Possibility Of Flooding, Sunday March 6th 2011
As of noon, Friday 3/4/11, the water level of the Schoharie Reservoir was at 1,115’ above sea level which means that the reservoir is 15’ below the Gilboa Dam spillway crest el. of 1130’ and 9.5’ below the 220’x5.5’ notch elevation of 1124.5. Expressed in terms of the water volume of the Schoharie Reservoir when filled to full capacity, el. 1130’, the reservoir was 75% full at 8 a.m., 3/4/11. Due to colder than average temperatures and above average snow fall this winter, the snowpack in the 314 sq. mile watershed upstream of the Gilboa Dam is both largely intact and high in water content. As of 3/4/11 the SWE (snow water equivalent) of the snowpack is 3.58”.
Temperatures well above freezing are forecast for Sat., 3/5 and Sun. 3/6 which will lead to substantial thawing and run off. Coupled with warm temperatures is the threat of potentially heavy rainfall starting Sat. night and extending into Sun. Thus, there exists a very real threat of flooding Sun. afternoon and evening.
Because of this winter’s extended periods of cold weather, a very extensive build up of ice exists in the Mohawk River system and its tributaries including the Schoharie Creek. Any significant rise in stream flow and water levels will probably lead to a breaking up of river and reservoir ice and possible ice jams.
Cold weather occurring prior to the first snowfall has led to a general deep-freezing of an already saturated soil within the drainage basin. This will both enhance and expedite run off of snowmelt water into the Schoharie Reservoir and Creek. If the present forecast “holds true”, the reservoir will likely fill to notch el. 1124.5’ and possibly reach the full spillway crest el. of 1130’. As the notch has a capability of discharging around 8600 cfs before it is finally “topped at el. 1130’” it serves to attenuate or slow down the “filling and spilling” of a full Schoharie Reservoir across the 1324’ long expanse of its spillway.
Several factors are critical in determining the impact of a late winter/early spring flood. Some are: 1. The duration and quantity of rainfall, 2. Atmospheric temperatures, 3. Snow water equivalent, 4. Soil conditions and 5. in the case of the Schoharie/Mohawk watershed downstream of the Gilboa Dam, the presence or absence of a ”void”/storage place for rain and melt water in the Schoharie Reservoir.
As 6 of the top 10 floods on the Schoharie Creek have been snowmelt/rain induced, it is prudent to be aware of the very real threat of flooding in the next 5 weeks. For the latest information of possible flooding this weekend use the DCC, Inc. website links (highlighted in blue) on the Home Page.