Mitchell Dam and Reservoir – Frequently Asked Questions

During the winter and early spring, the reservoir is kept low to permit the North Thames River to freely pass through the structure and ensure ice does not damage the dam or gates.
The reservoir remains at its winter levels through the spring thaw to ensure the dam has the capacity to safely discharge snowmelt and spring rains.
The reservoir is typically filled near the end of April. The timing of this operation is weather-dependent and may vary by a couple weeks.
When the reservoir is at its summer level, rain events that result in high flows in the North Thames River may require municipal staff to operate the dam gates based on protocols established by UTRCA, to permit floodwaters to pass through the structure without causing damage.
In early November, water from the reservoir is released, drawing the reservoir back down to its winter level.

The reservoir follows an annual cycle designed to meet the structure’s key functions without putting the dam at risk.
Drawing the reservoir down allows additional capacity through the dam without requiring gate operations and prevents the dam and gates from being impacted by ice.
Having the gates open allows winter snowmelt and spring rain events, which historically account for the maximum annual flows, to pass safely through the dam.

The spring of 2021 was unusually dry. The Upper Thames River watershed received approximately 61% of the long term normal precipitation for April, and 38% of the long term normal for May. Stream flows across the watershed were also below normal for this time of the year.
Precipitation and stream flow have the biggest impact on when reservoirs are filled. Spring snowmelt and rain events can generate a lot of runoff, as the ground is typically still frozen or saturated and water cannot soak in. While this runoff can be used to quickly fill a small reservoir, if this is done while additional spring runoff events are still a possibility, there is increased risk of flooding and damage to the dam structure, as it may not have the capacity to store and safely discharge the floodwaters.
Structures with automated operations (such as at the RT Orr Dam in Stratford) can be filled somewhat earlier while spring runoff events are still a possibility. The Mitchell Reservoir is typically filled near the end of April, but the unusually dry conditions in the spring of 2021 meant that the reservoir did not fill until the end of June.

We must maintain some flows through the dam to support the aquatic ecosystem downstream. Closing all the gates completely would cut off flow in the North Thames River and threaten aquatic life.
The Mitchell Dam has three large, manual gates (each 7.3 m wide) which are not well suited to controlling low flows to fill the reservoir. To fill the reservoir, the gates are set to allow a minimum amount of flow to pass through and hold back any additional flow created by rain events. While rain can bring the reservoir levels up somewhat, it may not generate the runoff volumes necessary to fill the reservoir completely.
A valve allows finer control over discharge; however, it cannot be used until the reservoir level is high enough to push an adequate amount of water through the valve. Normally, it is not necessary to rely on the valve to provide discharge while filling the reservoir, as late spring events are usually sufficient to fill the reservoir using the gate as described above.

The UTRCA is responsible for the operation and maintenance of the dam.
The UTRCA contracts the Municipality of West Perth to undertake routine operation and maintenance of the dam based on protocols and direction provided by the UTRCA.