With higher-than-expected Kootenay River inflows to the Columbia River forcing BC Hydro to minimize downstreat flood damage by limiting discharge from the Hugh Keenleyside Dam, the level of the Arrow Lakes Reservoir is expected to hit its normal full pool level tonight (July 4).
Jennifer Walker-Larsen, BC Hydro’s Revelstoke-based stakeholder engagement advisor, said the utility is expecting the Arrow Lakes Reservoir to reach its normal full pool level of 440.1 metres (1,444 feet) tonight nd to continue rising as much as 0.6 metres (2 feet) above normal full pool over the next several weeks depending on weather conditions.
This year recorded inflows to the reservoir from February to July 3 are the fourth-highest when compared to inflows recorded in other years since 1970. Without the operation of upstream Columbia River Treaty dams, the peak flow in the Columbia River at Castlegar/Trail would have been approximately double the current flow and within 5% of the historic maximum flows seen in the major pre-dam flood years of 1948 and 1961.
“As specified by our water licence, BC Hydro has approval from the provincial Comptroller of Water Rights to raise Arrow Lakes Reservoir up to 0.6 metres (2 feet) above normal full pool to a maximum level of 440.7 metres (1,446 feet) as required for flood control until July 31, 2012,” Walker-Larsen said.
“BC Hydro will minimize the duration of the unusually high water level to the extent possible and plans to increase discharge flows from Hugh Keenleyside dam as soon as the Kootenay River flows subside, expected as early as this weekend depending on weather conditions. BC Hydro is currently operating Columbia and Kootenay basin facilities in coordination with the United States to minimize flood impacts.”
Although uncommon in recent years, BC Hydro regularly operated Arrow Lakes Reservoir above 440.1 metres (1,444 feet) during the 1970s and 1980s, a typical operation under high water conditions. The last time was 1997 and 1991 when the reservoir level went slightly above 440.1 metres (1,444 feet). The last time the reservoir level went to 440.7 metres (1,446 feet) was in 1990.
Record rainfalls are contributing to the situation. Walker-Larsen said BC Hydro’s hydrology staff compiled the following interesting rain statistics:
- Revelstoke airport recorded 146mm in June 2012. The previous June record was 116mm in June 1971. This means June 2012 exceeded the previous record by 30mm (26%). The average June monthly precipitation at Revelstoke is 68mm; this means this month (146mm) has seen over twice its normal precipitation.
- Castlegar airport recorded 221mm in June 2012. The previous June record was 118mm in June 2005. This means June 2012 shattered the previous record by 103mm (87%) – by over 4 inches!! The average June monthly precipitation at Castlegar is 66mm; this means this month (221mm) has seen over three times its normal precipitation. If that’s not impressive enough, Castlegar also recorded its record wettest month on record in June 2012 (regardless of the month). June 2012 eclipsed December 1996 (195mm) by 26mm (one inch)! (“This touches a personal note for me as I grew up in the West Kootenays and remember how heavy the snow was that December,” Walker-Larsen said.)
- The wettest storms in June 2012 occurred June 4-6, 16-17, 22-23, 26, and 30, with generally unsettled weather in between these storm days.
- Indexed precipitation for all of BC Hydro’s basins in the Columbia/Kootenay system were near or above record monthly values in June 2012. These indexed values take into account multiple weather stations in or near each basin. The Arrow Basin broke its record June precipitation by the largest amount.
Without the operation of upstream Treaty dams, the peak flow in the Columbia River at Castlegar/Trail would have been approximately double the current flow and 1% below the historic maximum flows seen in the major pre-dam flood years of 1961 and 4% above the 1948 peak flows.
BC Hydro also said it will continue to release water over the Revelstoke Dam spillway.
Beginning in May 2012, it began release water over the spillway in response to high water conditions, low domestic demand and electricity market conditions and the need to provide a minimum fish flow from the dam under the Columbia River Water Use Plan. A spill is most likely to occur at low demand periods overnight between 11 pm and 6 am and on weekends.
“A spill from the Revelstoke Dam is a typical operation for hydroelectric facilities and does not present any risk to dam, employee or public safety,” Hydro said in a statement, adding that it routinely tests the Revelstoke dam spillway as part of a regular maintenance program.
BC Hydro will continue to maintain spill volumes below normal maximum facility discharge levels. The largest volume of water spilled so far has been 680 m3/s (24,000 cfs) on Sunday June 17, 2012.
Typically the Revelstoke Generating Station discharges 2,125 m3/s (75,000 cfs) at maximum generation from the five generating units during peak demand periods.