Causes of the 2017 high water event

What were the causes of the 2017 high water event?

The primary cause was extremely wet weather in April and May across the entire Lake Ontario and St. Lawrence River basin that followed wet weather from January-March. Secondary, related causes were the ensuing record high inflows from the Ottawa River in April and May, well above-average inflows from Lake Erie throughout 2017, and unusual ice-formation on the St. Lawrence River that required temporary reductions of Lake Ontario outflows on several occasions during January-March.

Did precipitation set records?

Yes, many locations in the Lake Ontario basin received record precipitation during the five-month period of January through May 2017, when peak water levels occurred, including the cities of Toronto, ON and Rochester, NY. Combined with high inflows to Lake Ontario from Lake Erie via the Niagara River, the total water supplies to Lake Ontario set a record in May, and April brought the second highest water supplies on record. Precipitation over the Ottawa River basin, which flows into the St. Lawrence River at Montreal, also set records during the spring and for the year of 2017 as a whole.

Did Plan 2014 cause the high water levels?

No, the record high water levels in 2017 resulted from extreme, and at times unprecedented weather conditions, including extremely high, natural water supplies and highly unusual winter weather conditions. These conditions largely dictated how outflows had to be managed during the first five months of 2017. First, from January through late March, wet weather and unusual temperature fluctuations required that Lake Ontario outflows be almost continuously adjusted to manage highly variable ice conditions in the St. Lawrence River to prevent ice jams that could have severely restricted flows and resulted in immediate localized flooding. Then, from April through May, during this period of record inflows to Lake Ontario and record Ottawa River flows, Lake Ontario outflows were again almost continuously adjusted in order to balance high water impacts upstream and downstream. The outflows during the first five months of 2017 were all made according to the rules of Plan 2014, but these rules– namely, the “I” (ice) limit and “F” (flood) limit – were established on the basis of how the Board had operated during similar conditions in the past when it deviated from Plan 1958-D to achieve the same result.

At the end of April, water levels exceeded the Criterion H14 high triggers, giving the Board authority to deviate from the rules of Plan 2014. Starting on May 24, as flooding conditions subsided downstream, outflows were increased above those prescribed by Plan 2014, up to the maximum possible without stopping commercial navigation on the St. Lawrence Seaway. During this time, outflows exceeded the highest flows ever previously released on a sustained basis, and these unprecedented outflows were maintained from mid-June into August. Outflows were reduced subsequently as Lake Ontario levels declined, once more in order to prevent closure of the St. Lawrence Seaway. Nonetheless, the outflow remained above the rules of Plan 2014 until the start of September.

In summary, the Board managed outflows during the unusual and extreme weather conditions from January through late May according to Plan 2014 rules that were based on Board operations under the previous regulation plan. From late May through August, the Board did not follow the rules of Plan 2014, and instead the Board decided to release higher outflows in order to provide relief to Lake Ontario shoreline property owners.  Starting in September, the Board returned to Plan 2014, which generally prescribed maximum possible flows while maintaining commercial navigation operations in the St. Lawrence Seaway throughout the remainder of 2017.  

Did Plan 2014 hold water back on Lake Ontario?

The factors that constrained outflows in 2017 are uncontrolled, natural factors that would have been the same under any regulation plan: highly variable ice conditions in the St. Lawrence River, upstream and downstream flooding and navigation safety. Despite these constraints, total releases were well above average in 2017 and outflows were increased to record-setting rates as downstream flooding subsided from late May to mid-August.

Could dams on the Ottawa River have been operated to reduce flows to the St. Lawrence River?

The IJC has no authority over the dams in the Ottawa River; however, the dams were operated to reduce high flows into the St. Lawrence River. In response to the extreme flooding, every measure was taken to reduce discharges from upstream reservoirs. The combined flow reduction, due to storage in the northern reservoirs, amounted to approximately 2,800 cubic meters per second in reduced discharge to the St. Lawrence River at the peak on May 8, 2017. During flood events, the safety and security of riparian residents and the integrity of water retention structures take priority over hydropower production.

While flow reductions in 2017 were significant, there are limitations to using the dams to reduce flows on the Ottawa River, particularly under the conditions experienced during 2017. Storage capacity in the Ottawa River basin is small compared to the total volume of the annual spring freshet, the surge that occurs in the spring when rains combine with snow melt. Total runoff from the 2017 spring freshet was three times the total storage volume of the reservoirs in the basin. In addition, approximately 60 percent of the drainage area of the Ottawa River basin is uncontrolled and has no significant storage capacity. The majority of the extreme rainfall received in late April and early May in 2017 was centered over this uncontrolled portion of the basin, which is also at the downstream end of the system, closest to the St. Lawrence River. The physical geography of this area does not allow further development of flood reservoirs – in fact, this was clearly illustrated in 2017 by the extensive and severe flooding that occurred along this stretch of the lower Ottawa River during the record flows in early May. Flow conditions were already above normal due to April precipitation that was double normal values combined with late snow melt runoff.