Localized rain storms and occasional heavy precipitation, as well as a few National Weather Service notices of flash flooding in the Hudson River and Black River watersheds, has had the tendency to leave the casual observer believing that both river systems have received “plenty” of water throughout the summer. Those situations have also made it difficult for the same casual observers to understand why the Great Sacandaga Lake and Stillwater Reservoir elevations have been lower than normal since early June.
In the simplest of terms, reservoir operation is centered on the quantity of water entering the reservoir (inflow) and quantity of water leaving the reservoir (release). The elevation of the reservoir varies depending upon whether the inflow to the reservoir is greater or less than the release of water from the reservoir. Release of water is controlled, inflow is not.
When the amount of water entering a reservoir is greater than the amount leaving the reservoir, the elevation of the reservoir increases. Conversely, when the amount of water entering is less than the amount exiting, the elevation decreases. In the first scenario – when the inflow exceeds the release – water is stored in the reservoir, in the second scenario it is not.
The release of water from a reservoir is typically a known and determined quantity and, generally, based on fixed demands downstream of the reservoir such as waste assimilation, environmental concerns, and industrial needs, to name a few.
Despite periodic rainfall in June, July, and August, river flow in the Hudson and Black River watershed continued to remain significantly below average. Hudson River monthly average flow measured only 47% of historic average in June and 47% and 48% in July and August, respectively.
Black River monthly average flow (without augmentation from Stillwater Reservoir) measured approximately 49%, 57%, and 71% of historic average in June, July, and August, respectively.
This lower than average natural river flow produced greater than normal demand for the release of water from the reservoirs.
The amount of water entering a reservoir, or inflow, is dependent upon the amount of precipitation that falls on the reservoir watershed and how much of that precipitation runs into the reservoir.
During the three month period June through August the Great Sacandaga Lake and Stillwater reservoir received significantly less inflow than historically enters the reservoirs.
|Percent of Average Reservoir Inflow – June, July, August 2018|
|Great Sacandaga Lake||35%||24%||65%|
The product of greater than normal demand for the release of water and significantly less inflow than historically enters the reservoirs sustained a condition similar to the second scenario detailed above and, for the three month period, resulted in inflow which was less than the release causing the elevation of the reservoirs to decrease.