International Lake Ontario - St. Lawrence River Board

2017 High Water Levels - Questions and Answers

Causes of this 2017 high water event

Operations in 2017

Print Version

             ---------------------------------------------------------------------------------------------------------------------

Causes of this 2017 high water event


What were the causes of high water this year?

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.

Back to top

Did precipitation set a record this year?

Yes, many locations in the Lake Ontario basin received record precipitation during the five-month period of January through May, 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 empties into the St. Lawrence River at Montreal, also set records this spring.

Back to top

Did Plan 2014 cause this year’s high water levels?

No, the record high water levels this year 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 based on how the Board had operated during similar conditions in the past when it in fact 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. In fact, outflows also 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, 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 September, 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. As a result, Lake Ontario outflows and water levels under the conditions experienced this year would have been nearly identical under the previous regulation plan.

Back to top

Did Plan 2014 hold water back on Lake Ontario?

No, Plan 2014 has not constrained Lake Ontario outflows. The factors that have constrained outflows this year 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.

Back to top

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 this year were significant, there are limitations to using the dams to reduce flows on the Ottawa River, particularly under the conditions experienced this year. 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 this year’s 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 this year 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.

Back to top


Operations in 2017


Why didn’t the ILOSLRB release more water from Lake Ontario this spring?

As there was flooding above and below the dam in spring 2017, Lake Ontario outflows were set to balance upstream and downstream flooding impacts.

Back to top

Why didn’t the ILOSLRB release more water in 2016 when downstream flooding was not an issue?

The Board saw no reason to release more water since Lake Ontario was below its long-term average from May through December and because the basin experienced severe drought during late summer and fall. The capability to predict whether conditions would be wet or dry in the following year does not exist.

For example, Lake Ontario was at the same level at the end of March 2017 as it was at the end of March 2016. An extreme drought followed in 2016 while extremely high rainfall followed in 2017. Neither scenario could have been predicted, and neither was.

Back to top

Would the Board have had more flexibility to release water if Plan 1958DD had been in place in 2017?

No. While the Board would have had greater authority to deviate and release flows other than those prescribed by the plan, this greater authority could not have been exercised during the extreme weather conditions in 2017, and therefore it would have made little or no difference. For example, while the Board had the authority to deviate from Plan 2014 flows when Lake Ontario reached its high water trigger level on April 28, the Board chose to follow Plan 2014 until May 24 because the need to balance upstream and downstream flooding was best addressed by the rules in the plan. The outflow decisions the Board made once it began to deviate from the Plan 2014 flow would more than likely have been the same outflow decisions it would have made under the Plan 1958DD because the Board would have been faced with the same considerations.

Back to top

Why were flows reduced for navigation?

The Board’s priority this year has been to reduce the impacts from high water upstream and downstream. However, in setting the outflow, the Board must consider the degree of relief that can be provided as well as the consequences to all interests. Higher flows than those set by the Board would have increased currents in the international section of the St. Lawrence River to an extent that would have effectively forced the stoppage of commercial navigation. This would have impacted people’s lives and the economy throughout the Great Lakes region by disrupting the transport of raw materials and finished products, without providing a great deal of relief on Lake Ontario.

Starting on May 24, as flooding conditions subsided downstream, outflows were increased above the flows that would otherwise have been prescribed by Plan 2014. In fact, outflows also exceeded the highest flows ever previously released on a sustained basis, and these unprecedented outflows were maintained from mid-June into August. To cope with these sustained, record high flows, the Seaway entities imposed speed limits, no passing restrictions and other mitigation measures.

The gradual decline of Lake Ontario through the summer months caused the velocity in the St. Lawrence River to gradually increase. This presented challenges for navigation, and eventually, maintaining record-high flows was no longer safe. As a result, starting in August, flows had to be gradually reduced in order to ensure safe conditions so ship transit could continue. Nonetheless, outflows have been set at or near the maximum possible rate consistent with safe navigation since the end of May in order to lower Lake Ontario levels as quickly and safely as possible.

Back to top

Why did the Board not set flows at a rate that would have resulted in temporary navigation stoppages, such as those that occurred in 1993?

The interruptions of Seaway operations in 1993 were a short-term, experimental measure and removed just over an inch of water from Lake Ontario. This year Lake Ontario outflows comparable to, or higher than those released in 1993 on a weekly basis, have been sustained over an extended period. This resulted in record releases and a greater rate of lowering of Lake Ontario than that which was achieved in 1993, and with fewer impacts on other stakeholders. In the four months following the peak water level (June through September), Lake Ontario dropped a record 93 centimeters (36.6 inches); the next largest decline during this period was 86 centimeters (33.9 inches) in 1993.

Back to top

Why were flows reduced in October for boat haul out?

The extremely high outflows, especially after Lake Ontario reached lower levels in August, reduced the level of Lake St. Lawrence just above the dam at Cornwall and Massena. Levels there were lower than they had been since 1987, leaving many boats grounded, and many marinas and boaters were unable to remove their vessels prior to winter. Reducing the outflow for one weekend in the fall (October 7-8 this year) raised the level of Lake St. Lawrence sufficiently to assist with boat haul out without any significant impact on the level of Lake Ontario. In any case, the effect on Lake Ontario was subsequently removed by releasing slightly higher flows over the following weeks.

Back to top

What actions can be taken to lower water levels and prevent a flood in 2018?

As we near the end of 2017, Plan 2014 continues to release very high outflows to draw down the level of Lake Ontario and reduce the risk of flooding next year, but whether or not a flood occurs next spring will depend on weather conditions and water supplies, not the regulation plan. While the Board and the plan are doing all that can be done, no flow management plan can eliminate the risk of flooding.

Back to top

Why not draw Lake Ontario down each fall so that there is sufficient storage to prevent flooding in the spring?

The physical capacity simply does not exist to prevent flooding in years when water supplies upstream and downstream are as extreme as those experienced in 2017. Previous IJC studies have shown that it is not possible to prevent all flooding on Lake Ontario, even if this was the only objective of the regulation plan. Furthermore, the impacts that can occur to other interests, such as navigation and recreational boating, during low water years must be considered. If there is a drought the following spring, as often occurs, drawing down Lake Ontario as much as possible each fall would result in significant economic impacts to other interests that would far exceed the reduction in economic damages to Lake Ontario shoreline property.

Back to top

How did outflows under Plan 2014 during fall 2017 differ from those that would have occurred under Plan 1958DD?

Both Plan 2014 and Plan 1958DD would have prescribed about the same high outflows in the fall of 2017. Lake Ontario levels were influenced more by the natural water supplies than by the regulation plan.

Back to top