Causes of the 2019 High Water Event

Question 1. What are the causes of high water levels in 2019?

Answer:

Too much water entering Lake Ontario from a flooded Lake Erie, and nowhere for it to go but into a flooded St. Lawrence River.

Explanation:

These two primary factors, each stemming from above-normal precipitation, caused Lake Ontario levels to rise rapidly in the spring of 2019, eventually exceeding their record-peak of 2017, and resulting in wide-spread flooding, erosion and other high-water impacts.

The 2019 high water event was basin-wide, caused primarily by wet weather conditions across the entire Great Lakes – St. Lawrence River system. These conditions have persisted for several years and increased in intensity more recently, notably in 2017, and again during the several months of fall and winter leading up to the spring of 2019.  These persistently and at times exceptionally wet conditions culminated in record or near-record high water levels and flows across the entire Great Lakes – St. Lawrence River basin.

On Lake Ontario and the St. Lawrence River specifically, after the record-wet conditions of 2017 followed by a more average spring-summer 2018, generally wet weather started again in late-fall 2018 and persisted through the spring of 2019.  Similar wet conditions upstream on the upper Great Lakes resulted in record-high water levels and flooding there, and culminated in record-high inflows from Lake Erie into Lake Ontario by spring.  Downstream of Lake Ontario, a late, heavy snowmelt coupled with an extremely wet spring resulted in record-high flows from the Ottawa River into the lower St. Lawrence River and rapidly rising water levels downstream.  This required a significant reduction in Lake Ontario outflows and further contributed to rapidly rising water levels and eventual flooding throughout the Lake Ontario – St. Lawrence River system.

The high water levels were not caused by regulation of outflows or by Plan 2014.  Water levels of both Lake Ontario and the lower St. Lawrence River would have been higher in both 2017 and 2019 had the St. Lawrence Seaway and Moses-Saunders Dam never been constructed, and the higher levels would have lasted for a longer duration.

Question 2. How do high water levels on the other Great Lakes affect Lake Ontario and the St. Lawrence River?

Answer:

Since water flowing from Lake Erie through the Niagara River tends to be the largest component of the total inflow to Lake Ontario, when water levels of the upper Great Lakes (including Lake Erie) are high, the flow of water into Lake Ontario also tends to be high.  This raises water levels of Lake Ontario and increases the amount of water that must be released through the St. Lawrence River.

Explanation:

The Great Lakes and St. Lawrence River form an interconnected system of water bodies and river channels (Figure 3).  With record-high water levels reached or exceeded across the basin in 2019, this represents an exceptionally high volume of water flowing through the system, the vast majority of which makes its way into Lake Ontario and then out through the St. Lawrence River.

Starting at Lake Superior, water passes through the St. Marys River into Lake Michigan-Huron (hydraulically considered one lake due to their wide, deep connection at the Straits of Mackinac).  Outflows through the St. Marys River are regulated by the IJC’s International Lake Superior Board of Control (ILSBC). Similar to Lake Ontario and the St. Lawrence River, regulation of St Marys River outflows must take into consideration water levels and impacts both upstream and downstream on Lake Superior and Lake Michigan-Huron.  From Lake Michigan-Huron, water flows uncontrolled through the St. Clair River, into Lake St. Clair and then downstream through the Detroit River, which empties into Lake Erie.  Water from Lake Erie flows uncontrolled* through the Niagara River and Welland Canal into Lake Ontario, before eventually making its way into the St. Lawrence River on its way to the Atlantic Ocean. 

Water levels fluctuate naturally on the Great Lakes due primarily to weather driven variations in water supplies, with periods of high and low water levels occurring throughout recorded history (Figure 4).  Water supplies include water flowing in from the lake upstream and from precipitation (rain/snow) that falls directly on the lake and its surrounding watershed, minus the water that leaves the lake through evaporation.  Water also flows out of each lake, through its outlet channel, and into the downstream water body.  Over longer periods lasting more than several years, outflows tend to equal inflows; otherwise, lake levels would continually rise or fall.  However, over shorter periods of days, weeks and months, differences in the amount of water entering and leaving the lakes cause their levels to rise or fall, and can result in periods of high or low levels that may persist for up to several years. 

After nearly 15 years of below-average water levels on Lake Superior and Lake Michigan-Huron, water levels of the upper Great Lakes started rising in 2013, and have been well above-average for several years.  Then in 2019, following several months of wet weather, water levels on Lake Superior, Lake St. Clair and Lake Erie each exceeded their seasonal record-highs in early-May, prior to record-highs being exceeded on Lake Ontario in late-May.  Lake Michigan-Huron was at or near record-highs since the end of May, but did not exceed them in 2019.

Whether regulated or unregulated, high Great Lakes water levels also result in high outflows.  With all of the Great Lakes above or near record-highs, this represents an unprecedented volume of water in the Great Lakes system, and other than water lost to evaporation, it all eventually makes its way into Lake Ontario and out the St. Lawrence River.

Question 3.    Did the outflow from Lake Erie through the Niagara River set a record in 2019 and how did this contribute to Lake Ontario’s record-high water levels?

Answer: 

Yes, record-high outflows from Lake Erie, into Lake Ontario, occurred in the spring and were a primary cause of the record-high levels on Lake Ontario in 2019, even more so than in 2017.

Explanation: 

Lake Ontario receives the majority of its water from Lake Erie - about 85 percent on average, with most of it entering from the Niagara River and a much smaller amount entering from the Welland Canal.  The total flow out of Lake Erie is completely uncontrolled. 

Leading into 2019, water levels of the upper Great Lakes, including Lake Erie, had been above average for several years.  This was followed by generally persistent, widespread, above-average precipitation during the winter and spring of 2019, resulting in rapidly rising water levels, with the upper Great Lakes reaching levels not seen since the previous record-highs of the mid-1980s. 

Lake Erie exceeded historical records starting in early-May 2019 and these record-highs continued through summer (Figure 5).  This resulted in above-record inflows from Lake Erie into Lake Ontario.  In terms of magnitude, inflows from Lake Erie in May 2019 alone added the equivalent of 113 cm (44 inches) of water to Lake Ontario. This is 24 cm (9.4 inches) more than the average May input, and compared to May 2017, Lake Erie added an extra 2 cm (0.8 inches) per week of water to Lake Ontario in May 2019.

Question 4. How much precipitation was received around Lake Ontario and the St. Lawrence River prior to spring 2019, and how does this compare to 2017?

Answer:

While total precipitation across the Lake Ontario – St. Lawrence River basin was less than during the record-breaking January – May period of 2017, precipitation was again well-above normal and for a prolonged period, starting around late-fall of 2018 and continuing through spring 2019. 

Explanation: 

The most significant precipitation totals were observed along the northern and eastern shores of Lake Ontario, and downstream along the St. Lawrence and Ottawa Rivers. 

For example, from November through May (Table 1), 555.6 mm (21.9 inches) of precipitation was recorded at Toronto, ON, the 4^th highest total recorded at this location over this seven-month period since 1938, while in Watertown, NY, a total of 823.7 mm (32.4 inches) was recorded during this same seven-month period, the 2^nd highest total at this location since 1898.  Downstream, the 717.8 mm (28.3 inches) recorded at Montreal, QC, was the 5^th highest November to May total since 1942, while the 613 mm (24.1 inches) recorded in Ottawa, ON was the 7^th highest on record since 1890. 

Other areas around the basin, particularly those to the south of Lake Ontario in the state of New York, did not see as much precipitation through the fall, winter and early-spring.  For example, Rochester, NY, saw 499.6 mm (19.67 inches), which is above-average, but only the 29^th highest November to May total on record since 1926.  This differs from 2017, where the entire Lake Ontario, Ottawa and St. Lawrence River basin saw very high precipitation amounts over a shorter period, with many locations setting records for the period of January through May 2017. 

Nonetheless, while conditions across Lake Ontario and the St. Lawrence River basins were generally wetter overall in 2017 than in 2019, inflows from Lake Erie and the Ottawa River were substantially greater in 2019.

Question 5.    Did Lake Ontario total inflows set records in 2019 and how do they compare to 2017?

Answer: 

Yes, new record monthly inflows were set in February and May 2019, and the combination of generally wet weather and high inflows from Lake Erie caused Lake Ontario’s net total supply (total inflows) to be near or above historical records for several months prior to the record-high water levels being reached in spring.

Explanation:

Water levels on Lake Ontario began rising in November 2018 in response to wet weather and rising Lake Erie levels.  Net total supplies (Figure 6) to Lake Ontario in November and December 2018 were the 4^th and 5^th highest for those months on record, respectively.

January and February saw high inflows continue.  Record-high net total supplies were recorded for the month of February 2019, following just one year after the previous February record set in 2018. 

The spring started somewhat drier in March and early-April, but inflows from Lake Erie remained high and ensured net total supplies remained well above-average. 

The remainder of spring was exceptionally wet.  Total inflows to Lake Ontario during April 2019 were very high, the 8^th highest April on record and the 13^th highest net total supply recorded in any month of the year.  Inflows to Lake Ontario are typically at their annual peak in the month of April, but instead of decreasing in May, net total supplies increased.  According to provisional data, May 2019 exceeded the previous record set for the month of May in 2017.  May 2019 recorded the second highest total inflow for any month of the year dating back to 1900, and April-May 2019 combined are the 2^nd highest two-month total ever recorded, behind only April-May 2017.

Question 6. Why did Lake Ontario start rising in November 2018?

Answer: 

Water levels of Lake Ontario began rising in November 2018 in response to wet weather and rising Lake Erie levels, which caused increasing inflows to Lake Ontario through the Niagara River. 

Explanation:

Net total supplies to Lake Ontario in November and December 2018 were the 4^th and 5^th highest on record for those months, respectively.  

Lake Ontario outflows, which were already well above average, also increased at the same time that inflows began to rise (Figure 7).  The high outflows resulted in continued low levels on Lake St. Lawrence during this period, and yet despite the increase, outflows could not fully offset the rising inflows at that time.