The overall flood risk in the Grand River watershed this spring is moderate, with the highest potential for flooding from heavy rain on saturated or frozen ground and along the Lake Erie shoreline.
Stephanie Shifflett, Water Resources Engineer, Grand River Conservation Authority (GRCA), presented the forecast at the GRCA’s annual meeting of municipal flood co-ordinators on February 22. The meeting is one of a series held each year, as the GRCA works closely with municipal officials to operate, test and improve the flood warning system. The GRCA has the responsibility of monitoring weather and river conditions and issuing warnings to municipal flood co-ordinators. When a flood message is issued, municipalities implement their local flood response program by warning residents, closing roads, managing evacuations or taking other actions.
Weather Outlook and Flood Prognosis
This past fall was very dry, but higher than normal precipitation returned to the watershed in December and has continued over the past three months. Along with higher precipitation there have been fluctuating temperatures, resulting in cycles of melt and freeze accompanied by cycles of rain and snow.
“Wet conditions and warm temperatures have led to saturated ground conditions in many parts of the watershed”, explained Shifflett. “With warm temperatures over the past weekend, the southern and central parts of the watershed are virtually snow free at this time, while the northern watershed is still fairly covered with snow. Conditions are changing rapidly and are being reassessed daily.”
Warm temperatures are predicted to stay for the remainder of the week followed by a brief return to seasonal temperatures at the end of the weekend before warm temperatures return next week. Periodic precipitation is forecast with a larger system at the end of the week. This system is not anticipated to result in flooding, but will be watched closely and reassessed as needed.
Environment Canada is predicting warmer than normal temperatures for the watershed for the next three months and above normal precipitation. With warm temperatures there is little chance of rebuilding a stable snowpack in southern parts of the watershed, although the northern parts of the watershed may keep their snow for longer. Fluctuating temperatures and high precipitation can increase the chance for large runoff from heavy precipitation on wet or frozen ground. There is also a risk that heavy rain may fall before the remaining snowpack melts, which could also produce large runoff events and flooding.
River, Reservoir and Lake Erie Conditions
Watercourses are mostly ice free, with the exception of some short reaches in the northern part of the watershed. With warm temperatures forecast to continue for the next few weeks, it is anticipated that the remaining ice will melt gradually, reducing the risk of ice jams.
The major reservoirs are at their normal levels for this time of year. The reservoirs were low late in the fall as a result of heavy augmentation during the dry summer and fall. Runoff from early winter melts in December and January helped to return the reservoirs to their normal levels early in the year. The normal filling cycle of the reservoirs will start in the next few weeks. During the filling cycle, reservoirs are operated to capture water and reduce downstream flooding throughout the spring. The water in the reservoirs is then released gradually during the summer and fall, augmenting the flow in the river to ensure there is enough water to meet the needs of downstream municipalities.
Lake Erie levels are well above the long-term average for this time of year, and are slightly above the level they were at when compared to this time last year. Lake Erie is virtually free of ice, including the mouth of the Grand River. There is a moderate risk for flooding along the Lake Erie shoreline due to the high lake level and open water conditions.