What’s Happening Now ?
Your lake association is resuming its weekly advice to Members on projected lake levels for this spring. This is the first issue of 2020 and applies to the week of March 26th to April 2nd.
The Ministry of Natural Resources and Forestry, who are responsible for the management of water levels on the lakes guided by the Muskoka River Water Management Plan, have been drawing the lake levels down to levels in the lower part of the operating range.
About the 10th of March there was some 20mm of rain. This rain curbed what had been a steady lake drawdown since mid-January and commenced what appears to be an early gradual freshet. Ten days later there was 22mm more rain which continued the rise in lake levels. There is much less snow than last year. MNRF advises that the current average snow water equivalent [SWE] for March 23rd is 132 mm or slightly above normal for this time of year. With this level of snow no triggers in the Muskoka River Water Management Plan [MRWMP] have been activated.
Future Weather
For the coming week, daytime thawing and accumulated rain in the 10 to 20mm range is forecast for Muskoka. Daytime highs are forecast to range between 4oC and 9oC and lows are forecast below 0oC. Neither the temperatures nor the rain activate Muskoka River Watershed Management Plan triggers.
Flooding Forecast
With gradual melting and moderate rain in the coming week, plus slightly above normal snow on the ground, there is little risk of flooding over the next week.
| Data for March 23rd |
Lake Muskoka |
Lakes Rosseau & Joseph |
| Gauge [m] |
9.00 |
8.58 |
| Elevation [m] |
224.95 |
225.70 |
| Target Level [m] |
224.95 |
225.7 |
| Normal Zone Range [m] |
224.65 to 225.2 |
225.45 to 225.9 |
| Summer Zone Range [m] |
225.28 to 225.65 |
226.0 to 226.65 |
|
|
LAKE MUSKOKA STATUS:
In Figure 1 [below] lake levels are shown for the current year - January 1 through March 23th. The right side of the graph shows that drawdown reached a low of 8.85 m before rainfall caused lake levels to rise in March 10th. This minimum lake level was 5 cm [about 2”] lower than at that date last year. Figure 2 [below] shows lake levels versus those required by the MRWMP. Note that, until the recent rain events, levels had been below the “Target Line”. The Rule Curve allows lake levels to rise a further 0.75 m [30”] above current levels and still remain in the “Normal Operating Zone” [NOZ] during Spring Freshet. Spring levels can be some 0.3m [12”] above summer level without causing flooding .
Figure 1: Lake Muskoka – 2019 Water Levels [meters above gauge 02EB018 Beaumaris]
Beaumaris Gauge [m] Jan 1 - Mar 23
Figure 2: Lake Muskoka Actual Levels vs MRWMP Rule Curve
Actual vs MRWMP – Elevation [m]
|
|
LAKES ROSSEAU & JOSEPH STATUS:
Figure 3 [below] shows lake levels for the current year – January 1 through March 23rd for the Upper Lakes. Here drawdown had been below last year’s minimum level before the 20 mm and 22 mm rain events caused levels to rise. Figure 4 shows levels are currently at MRWMP Target level.
There is currently a 0.75m water elevation difference at Port Carling allowing some 35 cms flow through the dam at the small lock channel.
Figure 3: Lakes Rosseau & Joseph – 2019 Water Levels [meters above gauge 02EB020 Port Carling]
Port Carling Gauge [m]: Jan 1 – Mar 23
Figure 4: Lake Rosseau Actual Levels vs MRWMP Rule Curve
Actual vs MRWMP – Elevation [m]
|
|
CAUSES OF FLOODING:
Following last year’s record setting flood there have been several studies, including:
• a report by the Province’s Special Flood Advisor;
• a Flood Plain Mapping study by Hatch for the District Municipality of Muskoka;
• a white paper on Integrated Watershed Planning by Muskoka Watershed Council (MWC)
Yet to come is the $ 5 Million Muskoka Watershed Initiative that the Muskoka Watershed Advisory Group (MWAG) is advising the province on.
Floods happen whenever the volume of water entering the watershed exceeds its physical ability to transport the water volume downstream. Or, more simply, when inflow greatly exceeds outflow for several days. During those days the water level of affected lakes rises dramatically. In most cases the water level in these lakes does not recede until after inflow has peaked and diminished to the point where outflow exceeds inflow.
Inflow comes from snow melt and from rain. Of particular concern are combination events where temperatures rise above 10oC, snow melts rapidly and heavy rain falls at the same time. Frozen ground contributes by preventing water from soaking into the soil. A review of past flood events [Table 1] back to 1970 shows very heavy rainfall [greater than 2”] combined with heavy rainfall [greater than 1”] within a 6 day period was associated with every flood event.
Table 1:
Heavy snowpack [– snow water equivalent (SWE) in the 190 mm range] appears to be a secondary factor. Snowpack is the only factor that can be measured ahead of time. The rest is weather which, at best, has a short lead time of 2 to three days. The current MRWMP contains triggers to activate additional drawdown when snowpack is 50% above normal and extraordinary drawdown when snowpack is 100% above normal. While triggers are in place for rapid snow melt and heavy rain, the inability to accurately forecast weather prevents these triggers from being effective.
In addition, very heavy precipitation storms were rare during the weather period [1970 to 2000] used to formulate the MRWMP. Table 2 shows three 2” rain storms in that 30 year timeframe, one of which produced the 1985 flood. Since 2000 there have been six such storms in a twenty year period, three of which have combined with other factors to produce floods [2013, 2016, 2019]. We now know when such storms are on their way. These cross continental storms are usually called “Colorado Lows”, though last year a “Texas Low” was added to this category. Climate change forecasts show that extreme storms will become more frequent [see Figure 5].
Table 2:
Figure 5:
More Frequent Severe Storms
Returning to the inflow/outflow problem, last year inflow to Lake Muskoka exceeded 500 cubic meters per second. Outflow downstream of Bala did not reach 400 cubic meters per second until 6 days later [Figure 6]. During this time the water level in Bala Bay was 0.4m to 0.5m lower than in Lake Muskoka. This was due to flow restrictions in the three narrow channels entering Bala Bay [Wallis Cut, Coulter Narrows & Jannock Narrows]. The North and South Bala control dams can and did pass water more quickly than the three narrow channels. Downstream of the Bala Reach another flow restriction – Moon Chutes – limits water flows.
Figure 6:
Moon River at Hwy 400
Figure 7 shows how the water level in Bala Reach increases as flow through the Bala Control dams increases. At the time of writing total flow through Bala was 167 cubic meters per second [cms]. The graph translates this into a water level of 220.0 m which is 0.75m below flood stage.
Figure 7:
Since last year the Bala North Generating Station has gone into service. This station increases the ability of Bala dams to pass water by up to 96 cms. This will neither increase the ability to pass flow into Bala Bay at the three narrows nor increase flows past the Moon Chutes.
There has been discussion about the ability of upstream lakes to store additional water to lessen the impact on Lake Muskoka. Table 3 shows that all major lakes in the watershed were above flood levels during the three recent floods. No major lake was spared. For 2019 the MWC paper calculated the flood inflow to be 1.0 cubic kilometer and maximum combined lake storage to be 0.6 cubic kilometer, leaving 0.4 cubic kilometer of water [equivalent to 160,000 Olympic swimming pools] to be routed through the watershed. This leaves non-conventional storage, such as upstream wetland areas, or infrastructure improvements to choke points as options for further evaluation.
Table 3:
Watershed Lake Flooding 2003-2019
Meantime, 2020 appears headed for a “manageable” inflow year. Stay posted for weekly updates. Stay well in these troubled times.
|
|
|
|
|
