A Wet Spring Floods into the Upper Thames River Watershed
This spring, the Upper Thames River watershed experienced a drawn-out 10-year flood. A spring flood with this total runoff volume occurs approximately once every 10 years, or has about a 10% chance of occurring in any given year. 听
This year, the spring freshet (i.e., flooding due to snowmelt and rain) spanned three weeks, with the UTRCA issuing eight Flood Bulletins for five separate events between March and April. The first bulletin was issued on March 16th as a strong low-pressure system entered the province, bringing warm air and rain.听听
At that time, the snowpack across the watershed contained the equivalent to about 75 mm of water. A typical spring snowpack in the Upper Thames River watershed contains 100 mm of water equivalent (based on records dating back to 1957).
Frozen and saturated ground meant the runoff would cause water levels in streams and rivers to rise with some low-lying flood plain areas being inundated. With the rain and above freezing temperatures, water levels remained high for several days.
On Friday, March 31st, a low-pressure system moved into the province, bringing 30-55 mm of rain accumulation with more to come over the weekend. By April 3rd, up to 70 mm of rain had fallen with an additional 30-55 mm still expected.
During this time, water levels rose again, causing minor flooding in some areas. The UTRCA operated the flood control reservoirs at Fanshawe, Wildwood, and Pittock Conservation Areas to reduce downstream flooding. Fanshawe Reservoir reduced flows in the North Thames River by 55%, as 450 m鲁/sec were flowing into the reservoir, and 250 m鲁/sec were being discharged from the dam. Similarly, Wildwood reduced flows on Trout Creek from 35 m鲁/sec to 15 m鲁/sec, and Pittock reduced flows on the South Thames from 60 m鲁/sec to 30 m鲁/sec.
By the end of this 21-day event, 115 mm of rain had fallen, and 75 mm of snow water equivalent had melted, making this spring particularly wet. This combined 190 mm of rain and snowmelt resulted in 140 mm of runoff.听 Typically, we expect spring runoff of 75 鈥� 100 mm.
In February 2018, a flood with similar runoff volumes occurred over a period of days rather than weeks, resulting in higher peak flows and much more damage to property. For example, peak discharge leaving the Upper Thames River watershed, as measured at the hydrometric gauge in Byron, was about 600 m鲁/sec in听 2023, in contrast to 1,020 m鲁/sec in February 2018.
The recent rain and snowmelt also increase groundwater recharge, defined as water that percolates from the surface of the ground downward into aquifers. Groundwater recharge mainly occurs during spring months. Due to warmer temperatures, snowmelt, and the high volume of rain recently experienced, the soil is mostly saturated. As a result, more water travels across the surface of the soil and into waterbodies, instead of seeping into the soil.
