Wet Weather in Most of the Rideau Watershed but Record Rain and Flooding in North Gower
On January 17, 2017 the Rideau Valley Conservation Authority downgraded the Drought status in the watershed from “Moderate,” which had been carried over from a dry year in 2016, to “Minor.” The “Minor” status was kept in place because of uncertainty about groundwater conditions that couldn’t be checked until spring. The “Minor” status was removed on April 19 after three months of above normal precipitation that went a long way to recharging groundwater aquifers.
Since February, rainfall totals each month have been an average of 1.65 times normal. It is not unusual for there to be above normal amounts of rain a few times each year. What has been experienced in 2017, though, is not rainfall 10 or 20 millimetres (mm) above normal for a few months but well above normal for six consecutive months such that close to a normal year of rain has fallen in half the year at each of the climate stations in and around the Rideau watershed.
An early thaw in March looked like it could be the spring freshet and a minor one at that. However, rain over the first week of April of 70 to 80 mm and warm enough temperatures to melt the remaining snow brought on the spring freshet peak for most of the Rideau system reaching a 1:4 year frequency flow of 456 cubic metres per second (cms) as measured at the Rideau River monitoring station in Ottawa.
Basically, a tumultuous spring followed that didn’t quit. Cooler than normal temperatures persisted along with frequent rainfall. Rain over two weeks in early May caused second, slightly lower, peaks on the Rideau and Jock Rivers and Kemptville Creek. The Tay River reached its highest in 2017 on May 7 as the rain, combined with flood waters from Bobs and Christie Lakes, passed through the system.
On average about 40 percent of days in a year have some rain recorded. In 2017 since April 1, the average is about 55 percent with the Ottawa airport having rain on 62 percent of days. As a result water levels throughout the watershed have continued to be above normal.
The rain that fell in early May over the Rideau watershed was in the order of 135 mm recorded over 10 days. On July 24, rain started to fall in the early morning. By the time the rain stopped on the evening of the 24th, 79 mm had been recorded at the Ottawa Airport, 64 mm at Franktown near the headwaters of Stevens Creek, 71 mm at Kemptville. At Smiths Falls, 100 mm was recorded. These rains caused flows to increase throughout the Rideau watershed.
Of greater significance were the rains that fell in the North Gower area recorded by local gauge readers and City of Ottawa staff — 135 and 127 mm to the west, 120 on the north side, 108 to the south of the village and 131 to the east, all on the 24th. Of the greatest impact, after a thunderstorm cell passed through around 7:00 p.m., was the 190 total mm recorded for the day near the village centre.
The combination of soils that were already saturated from previous rains and had no capacity to absorb the rain and the higher ground around North Gower caused rapid runoff that would have begun shortly after the rain began in the early morning of the 24th. As the rain continued through the day, flows reached flood stage and over topped the banks. The heavy rain in the village core toward the end of the storm combined with the flow passing through to cause the flooding that exceeded the 100-year flood level by as much as 62 centimetres (cm). The rainfall also exceeded the 100-year precipitation amount (123 mm).
The erratic nature of the system that brought the heavy rains to eastern Ontario on July 24 made it difficult for forecasters to predict where the rain would fall. It appeared through the preceding weekend that southwestern Ontario was threatened with a significant storm. Forecasts for eastern Ontario indicated only showers and the possibility of thunderstorms. However, it came clear later on Sunday that the system track would be through eastern Ontario and at 3:30 a.m. the Meteorological Service of Canada (MSC) issued a Severe Rainfall Warning for eastern Ontario as the rain was starting. The Warning stated that 30 to 40 mm could be expected with up to 100 mm in thunderstorms. The Stevens Creek watershed got 35 percent more than that and North Gower almost twice that causing unprecedented flooding of homes and businesses and agricultural fields in the area.
