Good morning, sir: Our colleagues at CCIW’s Boundary Waters Issues Unit relayed your message onto me for a response. I am the Canadian Regulation Representative of the International Lake Ontario-St. Lawrence River Board.

Attached, please find a series of plots showing the precipitation anomalies to date this year, by month, including July’s to date. It is very clear to see that precipitation has been very high this year, particularly for Lake Erie and especially Lake Ontario. The very good news is that, to date, July has been quite dry. Let’s all hope that trend continues.

That said, precipitation is but a single piece of a complex jigsaw puzzle that helps to explain why Lake Ontario levels are, say, high. Let’s take a quick look at some of the main pieces to this whole picture below, comparing values from early on in 2000 to those so far in 2017…

LAKE ONTARIO MONTHLY MEAN WATER LEVELS (m IGLD 1985)
***** 2000 2017
JAN 74.50 74.62
FEB 74.45 74.82
MAR 74.58 75.00
APR 74.82 75.35
MAY 75.10 75.80
JUN 75.26 75.81
AVG 75.78 75.23

LAKE ONTARIO MONTHLY MEAN OVER-BASIN PRECIPITATION (mm)
*** 2000 2017
JAN 64 73
FEB 62 61
MAR 52 74
APR 109 115
MAY 132 152
JUN 142 109
AVG 94 97

LAKE ONTARIO MONTHLY MEAN OUTFLOWS (m3/s)
***** 2000 2017
JAN 5940 6440
FEB 5900 6890
MAR 5820 7390
APR 5940 7610
MAY 6770 8560
JUN 7520 10310
AVG 6320 7870

LAKE ONTARIO MONTHLY MEAN TOTAL INFLOWS [NET TOTAL SUPPLIES] (m3/s)
**** 2000 2017
JAN 5940 8360
FEB 6040 8150
MAR 6930 8580
APR 8240 11020
MAY 8540 11030
JUN 8420 9640
AVG 7350 9460

At first glance, yes, the average precipitation values from 2000 compare relatively well to those from 2017. It has only been slightly “wetter” over the Lake Ontario basin for the first six months this year. So why has the lake been an average of 55 cm higher and why is it currently 44 cm higher than at this time in 2000? Let’s break down the much-higher inflows above and have a closer look. Total inflows are comprised of the outlows from the upper Great Lakes (i.e., through Lake Erie, down the Niagara River and Welland Canal into Lake Ontario) plus what is known as the net basin supplies (the algebraic sum of precipitation over the lake surface, plus runoff from streams tributary to Lake Ontario, minus evaporation directly off the lake surface)).

LAKE ERIE MONTHLY MEAN WATER LEVELS (m IGLD 1985)
***** 2000 2017
JAN 173.82 174.29
FEB 173.75 174.40
MAR 173.83 174.47
APR 173.95 174.64
MAY 174.07 174.82
JUN 174.18 174.83
AVG 173.93 174.58

LAKE ERIE MONTHLY MEAN OUTFLOWS (m3/s)
***** 2000 2017
JAN 5390 6470
FEB 5190 6690
MAR 5240 6650
APR 5540 7040
MAY 5800 7440
JUN 5960 7350
AVG 5520 6940

LAKE ONTARIO MONTHLY MEAN NET BASIN SUPPLIES (m3/s)
***** 2000 2017
JAN 550 1900
FEB 860 1460
MAR 1690 1930
APR 2700 3890
MAY 2740 3570
JUN 2440 2290
AVG 1830 2510

You can clearly see that the outflows from Lake Erie have been much higher this year than in 2000, owing to the fact that Lake Erie has been an average of 65 cm higher and is currently also 65 cm higher than at this time in 2000. On average, outflows from the upper Great Lakes have contributed an additional 19 cm or so a month to Lake Ontario’s level this year compared to in 2000. Also, net basin supply values this year have mainly been higher. This is mainly due to higher runoff in all likelihood (as evaporation tends not to differ much year-to-year during these particular months, and, as noted above, the overall precipitation values are about the same). This difference also constitutes an additional average of 9 cm or so a month to Lake Ontario’s level since the beginning of the year.

It’s worth noting, too, that Lake Ontario outflows this year are much higher than those in 2000, and have recently been eclipsing them by about 2800 m3/s. This factor alone constitutes an additional 38 cm of water recently removed from Lake Ontario within a month’s time. So, in short, one can expect a more rapid decline in water levels this year.

Hopefully this short comparison of conditions now to those of 2000 sheds some light on a large part of the reason why water levels this year have been so much higher than then.