The Water Management Program
The spring freshet is the most challenging time for the water management team. Throughout the spring freshet, the TSW has two difficult and sometimes competing objectives – to store as much water as possible for use in maintaining navigation along the canal in the summer while reducing or eliminating flooding. Care is also taken to maintain appropriate water levels for walleye spawning.
Flooding is a natural event. In an average year, flooding is not much of an issue. Spring conditions occasionally result in flooding in the “flood-prone” areas along the system. Each of these areas is affected by one or both of the following conditions: uncontrollable peak flows or levels, and natural restrictions downstream that impede high flows. Specific flood-prone locations include:
- Above and below Lock 7 (Glen Ross)
- Percy Reach and Meyers Island
- Upstream from Healey Falls around the Trent River bridge
- Below Lock 18 (Hastings)
- Below Lock 19 (Peterborough)
- Above Cameron Lake on the lower Burnt River
- Shadow Lake north of Coboconk
- Below the Talbot Dam on the Talbot River in the town of Gamebridge
- The Holland River and Marsh area at the south end of Lake Simcoe
- Below dams C, D, E in Washago
- Lake St. John
- The Black River above and below the Highway 169 bridges
- Sparrow Lake, between Hydro Glen and Wasdell Falls
- Below the Big Chute Generating Station (upstream of Little Chute, especially in the Six Mile Channel below the Six Mile Dam)
- Six Mile Lake
Each of the watersheds has its own management parameters, based on its unique characteristics. These parameters are outlined in the following paragraphs.
The Reservoir Lakes
Snow surveys are carried out during the winter to better anticipate spring runoff. As spring arrives on the reservoir lakes and they are rising with the runoff, stoplogs are placed in the dams. Typically there is more inflow than is required to fill the lakes, and some surplus is allowed to run off. If a small freshet is predicted, water may need to be caught earlier to fill the lakes. Water management in the reservoir lakes is summarized in Appendix 1
As the lakes are nearing their full levels, snow survey data and all available sources of information are checked in order to anticipate whether or not a larger volume of water is still coming. If only a little water is expected, then the lakes are topped up for the summer. If a lot of water is expected, then the lakes are allowed to discharge more freely. Heavy inflows can easily result in pulling stoplogs out again at dams on these lakes to expel surplus water.
Kennisis Lake Dam
If two logs (60 cm) are pulled at the Kennisis Lake dam with the lake full and no inflow or other outflow, it takes 14 days for the lake to drop 30 cm. If there is an additional 5 cm rainfall (representing 82 hectare-metres of extra water), it takes 35 days for the lake to drop to the level of the stoplogs.
It is often suggested that removing an additional stoplog would alleviate spring flooding on some reservoir lakes because it would immediately lower the water level by the depth of the stoplog. However, when a 30 cm deep timber stoplog is removed from a dam, the lake level may still rise or take several days to drop, depending upon inflow volumes and the length of time the dam is opened. When the lake’s volume is increased by rainfall, the increased outflow is relatively small. Rainfall is thereby automatically caught without immediate stoplog re-insertion. Observations and adjustments are made on a weekly basis so any gains from rainfall are taken into account.
The Kawartha Lakes
When considering the number of lakes and structures that must be managed on a watershed such as the Trent River, the challenge is understandably complex. Much of the time it is only necessary to deal with a few individual flow changes to rectify a situation where one lake is too high, is rising too fast, or is dropping too low. Occasionally, however, especially in spring, many sites need attention at the same time. For example, if Balsam Lake is rising too high due to rainfall, the obvious response is to increase the flow from Rosedale at its outlet. But this flow goes to Cameron Lake, which is much smaller in area. If no further adjustment is made, Cameron Lake would flood. Therefore the flows must be passed on downstream at Fenelon Falls. A cascading effect usually occurs all the way to Trenton in sudden melt or heavy rain situations.
In spring conditions, water flow increases accumulate as they move downstream. At some point, the increases exceed the ability of water control crews to complete the changes in one day and still be able to manage the discrepancies that occur. Therefore, a maximum daily change is worked out that may not fix every lake in one day and may require a second, third, or fourth day. The larger lakes have to rise to absorb water while flow increases are worked through the system. This is a key reason for lowering water levels before the freshet begins. The inflow to the Kawarthas, for example, can exceed 600 cubic metres per second (m3
/s), while outflows are still rising from 250 to 350 m3
/s. The challenge is compounded by the fact that some Kawartha Lakes are considerably smaller in both area and volume than others and thus may rise very rapidly with inflow surges. Water management in the Kawartha Lakes is contained in Appendix 2
Once the freshet starts, some reservoir lakes and many Kawartha lakes fill or overfill even with all the stoplogs out of their dams. Downstream conditions are also critical considerations. During extreme flood conditions (flows of 400 – 450 m3
/s at Peterborough), it may be necessary to flood the Kawartha Lakes above normal in order to prevent much more serious flooding downstream of Peterborough. After the flow peak has passed, logs are placed back in the Kawartha dams and gates are lowered as the lakes decline, until they are slightly under filled. Adjustments are then made to bring the lakes to their optimal level for water management.
Some years produce mild weather where the snow begins to melt and fill the lakes before the heavy winter ice cover has had sufficient time to soften around docks and boathouses. The rising lake level can shift some structures not designed for these conditions. At the same time, further removal of stoplogs from control dams may be insufficient to slow the rise in the lake level or may worsen conditions downstream. As with many water management situations, actions must be decided day-by-day as the conditions develop, and it takes time for lake levels to react to these actions. Often ice damage is unavoidable.
During flood events, at times dams are not fully open (all the stoplogs removed or gates raised wide open above the water), even though the lakes they hold back are still rising and flooding. This can occur for one or more of the following reasons:
- The dam has more capacity to pass water than dams upstream or downstream that already have all logs out. This is the case with the dam at Buckhorn.
- The lake downstream is already experiencing high water, so releasing more water will only worsen conditions (for example, Pigeon-Buckhorn Lake below Sturgeon Lake).
- A natural obstruction such as a rock ridge in the lake or river bed upstream of the dam is controlling the flow to such an extent that releasing more water only lowers the water immediately above the dam with little or no effect on the amount of flow coming over the rock ridge. This is the case with the Hastings dam at the end of Rice Lake and Lock 7 at Glen Ross.
Areas of flood hazard in the Trent Basin generally result from restrictions and shallow gradients. The Trent River is not typically prone to rapid changes in flow because of the natural storage in the lakes, the soil infiltration in the Kawartha Basin, and lower average snowfall. The peak flow is generally attained slowly and lingers considerably during its decline. Tributary streams, such as the Burnt River, Eels Creek, and Jack Creek, are more volatile and sometimes cause flooding in low-lying areas. The system is drawn down in the fall and winter, and efforts are made during the spring to mitigate flooding, although it cannot be prevented altogether.
The Severn River
In the Severn River basin, the two main spring objectives are to fill Lakes Simcoe and Couchiching without overfilling, and to minimize the effect of the peak flow from the uncontrolled Black River that joins the Severn River below the outlets of Lakes Simcoe and Couchiching at Washago. A fact sheet on the Severn River basin is contained in Appendix 3
A rule curve indicates the most desirable water level in a specified water body for each day of the year.
Lakes Simcoe and Couchiching are very large bodies of water, presenting a different management challenge. They are managed using a rule curve in effect since 1918, which serves as a target or guide for water levels throughout the year. Because of the size of the lakes and the limited inflow and outflow relative to the effects of evaporation, it is very difficult to reverse the trend if water levels depart from the rule curve. For example, a decline in lake levels on Simcoe-Couchiching cannot be stopped by shutting off all the dams in Washago, nor can a rise be halted by opening the dams in Washago for maximum outflow.
The rule curve for Lake Simcoe has been studied extensively over the years and no better method has ever been developed. Below is a graph showing water level elevations for the rule curve, as well as average water levels over a 90-year period.
Figure 11. Lake Simcoe Rule Curve and 90 year average levels© Parks Canada
Most of the flooding on the Black and Severn Rivers is due to the unregulated nature of the Black River. It lies in the Georgian Bay snow belt and drains a large rocky basin area that absorbs very little melt water. This causes rapid and high runoff of spring flows from an area with very little controllable storage. The sudden release of water is often great enough to overtop the Lake St. John Dam, aggravating local conditions and flooding residents along the Black River down to Washago. During peak flows from the Black, even if flows from Lakes Simcoe - Couchiching are completely cut off, the levels can rise to flooding stage in the Village of Washago as well as on Sparrow Lake. Sparrow Lake flooding is mainly caused by natural constriction in the Severn River at Sparrow Lake Chute, McDonald’s Rapids, and Hydro Glen. The cost of removing these constrictions is prohibitive and would not wholly eliminate the flooding.
In spring, Lakes Simcoe-Couchiching often begin to rise, but are not full when the Black River peaks. To reduce the flooding effect on Sparrow Lake and on the Severn River, the peak freshet of the Black River is monitored and flows from Lakes Simcoe-Couchiching are reduced as much as possible until the peak passes. Simultaneous peaks on Lakes Simcoe-Couchiching and the Black River can result in flooding on Lakes Simcoe-Couchiching, Sparrow Lake, and Six Mile Lake downstream of Lake Simcoe.