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Bass Lake is a small, shallow, moderately developed lake with an area of 0.96 km2 in a watershed area of approximately 6.92 km2.   The length of the lake is approximately 2.7 km while the greatest width is about 600 m. 












The Bass Lake Watershed


Overland flow to the lake is dominated by the area to the south east of the lake.  There are numerous beaver dams in this area.  Some of these dams are more than 2 meters high.  The lake at the head of the watershed is, in fact, an ancient beaver pond!










Left ‘unmanaged’, an extreme rain event could cause a cascading failure of the dams. This has happened in the past, resulting in massive amounts of “beaver” water - i.e. water with high levels of giardia (beaver fever) - flowing into the lake at the south-east end. The flow can and has caused significant damage to Bass Lake Road which is maintained by the cottagers along the road.  It is a duty of the BLA to minimize the risk of both water quality problems related to beavers and road washout.   Refer to this PDF version of a presentation that was put together for a past BLA AGM.  It will be of special interest to any who are unfamiliar with their locations and/or have never hiked back to see the beaver dams in person.


Flow from the lake through the culvert under Hwy 169, is controlled by a natural rock weir close to Lake Joseph. A picture of this rock formation looking from the Stills Bay side to the Bass Lake side is shown following.









During the summer and early fall the water level for Bass Lake is typically below the top of the rock weir (as it is in the picture) resulting in no flow from the lake and the water at the north end of the lake and in the outflow channel being stagnant.  The duration of this stagnant flow period can be controlled or minimized by placing sandbags on the rock weir.  Look closely and you will see some of these.  In the past, Ben Roberts has taken care of this responsibility on behalf of the BLA.  Ben now advises the BLA on this activity. BLA funds are used to support this work. Trying to regulate the water flow from Bass Lake is the responsibility of the association.


Finally, the lake has a maximum depth of approximately 10 m. The bathymetry of the lake in meters of elevation is shown in the following figure.  The dark blue is the deepest part of the lake. 











Bathymetry (m of elevation) of Bass Lake



Bass Lake was formerly classified as moderately sensitive by the District Municipality of Muskoka (DMM). The DMM has a long record of monitoring the water quality of the lake, with their principal interest being phosphorous. Additional monitoring of the lake is with the Muskoka Lakes Association program which started in 2005. The Bass Lake Association is very active in this program; members of the BLA volunteer to take water samples and to coordinate with the lab testing.  We currently have six test sites that are monitored.  The location of these sites is shown in the map that follows.

NOTE: As of July 7th, 2021, Bass Lake has been classified as a 'vulnerable lake'...   Please find more information about that further down this page...












Location of Sample Points

The results for Total Coliform and E.Coli are derived from the samples that our testing team takes and incubates.  

It is important to note that there are a number of factors that impact these results – such as air temperature and the amount of sunlight.  Therefore no definitive conclusions should be attempted.  

Our lake specific data is further analyzed by the Muskoka Lakes Association (MLA) and a full report is made available each spring to participating lake communities.  The E. Coli levels and clarity of the lake are encouraging.  

Total Phosphorus (TP) samples are also collected for analysis.  The MLA report provides a comparison of concentrations in Bass Lake to those of other participating lakes in the Muskoka watershed. The results will be made available to all association members in the annual spring report.

  • Link here to the Spring 2020 MLA Water Quality Report.
  • Link here to the Bass Lake Association 2020 AGM Water Quality Report.


The reality is that this is very likely to be the new normal... with earlier and warmer springs the conditions are established for consequences come the fall each year.

You can always refer to prior Bass Lake Updates for catching up on events, and communications, relating to Blue Green Algae blooms on Bass Lake.

October 2021 Algae Bloom on Bass Lake


And as we know, we experienced a very similar situation again in the fall of 2021.

The story played out almsot exactly as above... the blooming was small, realtively isolated, never approached any levels of concern... and the Public Health notice was lifted in short order, after much hard work documenting conditions, and lobbying, by members of the Bass Lake community.


There are a multitude of factors that can affect the quality of lake water anywhere, and specifically here at Bass Lake.  All of these factors exist within a giant feedback loop… one connected to the next.

In this section we will provide a brief overview of the general and specific factors affecting, and indicators of, the water quality in Bass Lake.  The factors are listed in alphabetical order.

For those who are interested in digging deeper, you are strongly referred to the Muskoka Water Web website… an excellent resource for anything and everything related to water quality in Muskoka.


Bacteria - E-coli

In the post-Walkerton world, E. Coli is closely monitored in drinking water. The Ontario Provincial Standard for E. coli is as follows:

  • Drinking water is 0 E. coli per 100 ml
  • Recreational water (for swimming) is 100 E. coli per 100 ml

Historically the levels of E-coli present in Bass Lake have been acceptable for recreational use. Water from the lake cannot be consumed without treatment because of non-zero E. coli concentrations.  Common cottage treatment methods include filtration to remove sediments and suspended solids and UV light to destroy bacteria.  Periodic testing of treated cottage drinking water is essential and can be done for free by the District of Muskoka in Gravenhurst 


Dissolved Oxygen

Minimum dissolved oxygen levels are necessary for good lake health.  Moving water contains higher levels of dissolved oxygen than stagnant water.  

Dissolved oxygen levels are inversely related to water temperature.  Bass Lake, being a shallow lake which warms up quite a bit in the summer, is therefore susceptible to oxygen-deficient conditions in the mid to late summer period. It is especially important to be mindful of our collective behaviours, as they impact oxygen levels especially during the summer months.

Learn more about Dissolved Oxygen and Water at the USGS Water Science School.


Lake Levels - Water Quantity

The amount of precipitation - and hence the lake level - can impact the water quality.  More water generally means more flow… and more flow is always a good thing as build ups of potential contaminants get washed out with the flow.

This is directly related to the time of year.  Typically for Bass Lake, the early spring (April-May) and late fall (November - ice in) are the two periods of peak flow and higher water.  Lake flow slows during the drier summer months.

To mitigate against dangerously low water levels in the mid-late summer months, water levels are controlled, using sandbags, at the outflow into Lake Joseph.

Refer to this 2003 Report prepared for the AGM - when the Golf Course had a proposal to draw water from Bass Lake for their fairways and greens.  (There was also a proposal for another golf course to the south east of the lake… shown on map on page 3) 

  • If you’ve never seen the natural rock weir that contains our outflow into Lake Joseph, be sure to scan this PDF Report.


pH - Acidity levels

As with most lakes in Muskoka, Bass Lake tends to be slightly acidic (pH of the water is less than 7).  This is a natural condition, given that we live on the Canadian Shield where there are few or no carbonates such as limestone to buffer the acidity. The pH of natural rain is about 5.6!    

We monitor the pH level of Bass Lake as further acidification can cause stress to aquatic plants and animals… which then has a negative impact on water quality.



Phosphorous is a major contributor to the growth of algae; the other important nutrients are nitrogen compounds and potassium.  

In the case of Bass Lake, the greatest contributors of phosphorous load are:  effluent from septic systems, fertilizer run-off and organic matter from trees and other biologic matter.

Our regular testing, at multiple locations around the lake, help keep us on top of any potential issues relating to phosphorous overload.  But all the testing in the world is no substitute for good practice on the part of all cottagers. These practices include maintaining the cottage septic system and not overloading either the septic tank or tile field.  

The US EPA has a great primer page on maintaining a healthy septic system.


Maintaining a healthy septic system has to be our first priority.  

  • Many cleaning products in particular can damage the healthy flora in your septic system. 
  • Make sure that the cleaning products you use explicitly state that they are septic-safe.  
  • The Spruce has an inventory of Best Septic Safe Household Cleaning Products… some of which are available at our local Home Hardware in Mactier.  
  • You can find lots of other information readily available on the web if you do a simple search.  
  • We will be building a locally available resource list in the FAQ section of this website… stay tuned…


Maintaining a healthy shoreline environment - and shoreline buffer zone - on your property is also key.  

  • Leave the natural shoreline as you find it.  
  • Leave the natural vegetation between your cottage and your waterfront as mother nature made it.  The Township of Muskoka Lakes Tree Preservation By-Law 2008-55 was implemented specifically to address the removal of trees in this waterfront zone.  
  • Avoid the use of fertilizers in and around your property.
  • Leave deadfall in the water as it falls (unless, of course, it presents a safety hazard).


Learn more about healthy shorelines here at the Muskoka Water Web.


Shoreline Turbulence

Directly connected to the above discussion, but worth mentioning in its own regard.

Phosphorous hides.......in the sediments of the lake!  Shoreline turbulence will enhance the partitioning of the phosphorous from the sediments into the water column.  For the areas of the lake where there is little flow to transport the water with phosphorous out of the lake, the Cyanobacteria and algae are very happy… and can find conditions suitable for growth.

From an e-mail communication with Judi Brouse (Director of Watershed Programmes for the District of Muskoka and the Muskoka Watershed Council, Retired 2014):

As we discussed on the phone, an emerging issue on many lakes across Ontario is low phosphorus levels as a result of the binding of phosphorus in the soils.  With increased soil acidity due to acid precipitation and reduced precipitation due to climate change, less phosphorus appears to be moving through some watersheds.  It is still too early to really understand the mechanism at play or to determine if this is a long-term or short-term situation.  It is possible that, if climate change models are correct and we eventually see an increase in precipitation, that we might see a flushing of the phosphorus currently being stored in the watershed.  For that reason we continue to strongly recommend that our watershed, especially those areas directly adjacent to lakes, are maintained in as natural a state as possible so that they can cope with these stresses as they we inflict them on the natural system. 


Over the winter, 2021/2022, the Bass Lake Executive was approached by the Muskoka Watershed Council and invited to take part in the Love Your Lake Programme.  




We accepted, with delight, on behalf of the Bass Lake Community.  

Learn more about what this means for Bass Lake here.



Temperature is key, affecting all aspects of lake water health: oxygen solubility; photosynthesis rates; metabolic rates; sensitivity to toxins; life cycle rates; rate of plant growth, and the development of cyanobacteria (blue green algae) blooms.


Turbidity - Water Clarity

Turbidity is the measure of how clear the water is.  Water clarity is generally considered to be a good thing for those who enjoy recreational use of the lake.

Water clarity is also important from a water quality perspective - the clearer the water, the better sunlight is able to penetrate through the water, promoting the beneficial growth of aquatic plants, which in turn oxygenate the lake, and provide fish habitat.  

There is a direct relationship between the clarity of the water and presence of suspended particulates, be they soil particles (from processes like shoreline erosion), matter from decaying leaves or algae.

(Source: US EPA National Aquatic Resource Surveys)


History of Water Testing on Bass Lake

Sampling for water quality dates back at least to the mid- 1970’s. We are working on finding and pulling together historic data.  

For now, we are able to share this snapshot of trends between 1983 and 2006.









October 2019 Algae Bloom on Bass Lake


A complete and technically detailed reporting of the event can be found in the 2020 AGM Water Quality Interim Report.

The following is a summary of events relating to the situation from last October/November.  Fortunately, as you will see from the summary, the occurrence was minor and not long lasting. However, cyanobacteria is something that must be taken seriously and it is important that steps be taken to avoid similar occurrences in the future.  The BLA Executive has already taken a number of steps to address the situation and we will continue to do so.   We have always believed - and emphasised to our members - that good lake stewardship is paramount.


The first sighting of the algae was on October 20, 2019 at the North end of Bass Lake near Hwy 169.   This was reported to the Simcoe Muskoka District Health Unit (SMDHU) shortly after the sighting.  A routine Public Notice with the respect to the occurrence was issued by SMDHU on October 24, 2019. 


Members of the BLA Executive were part of inspections of the lake following the sighting and it was clear that the cyanobacteria were trapped behind a beaver dam and other debris under an old bridge a short distance downstream of the HWY 169 culvert. 

Flow from Bass Lake is through the culvert to Stills Bay.  Because of the beaver dam, the water on both sides of the culvert was very likely stagnant.  The blue green algae bloom referred to in the SMDHU Public Notice was predominantly in the outflow channel. A small amount was also observed on the upstream side of the culvert. 


The initial test results from the Ministry of the Environment, Conservation and Parks (MECP) from samples collected at the North end of Bass Lake on October 21, 2019 were made available to the BLA executive on November 13.   

  • The results for Microcystin LR were 0.1 parts per billion (ppb) the instrument detection limit.  
  • Both MECP and SMDHU consider that concentration of Microcystin LR to be non detect.   
  • The Ontario Drinking Water Standard for Microcystin LR is 1.5 ppb.  Anatoxin-A was also measured and it was also at the instrument detection limit (non detect).   
  • The MECP also took samples from the lake close to the BAS-7 on shore testing site at 1041 Kendon Road because of a very small and very short-lived algae sighting at that location.  
  • This site is tested as part of annual testing because of a drainage run-off from the nearby golf course.   
  • These results were also well below the Ontario Drinking Water Standard.


The beaver dam and debris under the old bridge immediately past the culvert under Hwy 169 were cleared on November 12 and the water flow in the outflow channel was fully restored.   The cyanobacteria were flushed in the usual way through the outflow channel and over the rock weir into Stills Bay. This flow is part of the natural and normal flow from Bass Lake out into Stills Bay.

The MECP conducted further testing on or about November 20, 2019 and provided the results to the SMDHU.  The test results were again at non detect levels.  The Notice from the SMDHU was lifted on November 28, 2019.  


Members of the BLA Executive have contacted a number of government authorities and others, including SMDHU, MEPC, Muskoka Lakes Township (including, Sandy Bos), District of Muskoka, Muskoka Watershed Council and the MLA to seek input and in some cases assistance in ensuring appropriate action is taken going forward.  

We are also keeping you, our membership, informed and have put together a pro-active strategy so that all necessary steps are taken to ensure the continued health of Bass Lake.   


Good lake stewardship is a priority and educating our members and all other property owners and other users of the lake is an important part of this.  We are fortunate to have 2 very active members of the Executive living on the lake full-time, and also to have an expert on environmental matters as a longstanding and regular cottager.   

As a result of the bloom, the MLA initially gave Bass Lake a ‘Red Light’ status in their annual report.  This flags the lake - to the world - as having water quality issues, which can have any number of unintended impacts, including on our ability to draw drinking water from the lake, to use the lake for recreational purposes, and also on property values.


Your BLA Executive believed, and strongly argued - that the red light on the MLA Water Quality Report was not reflective of the current status of Bass Lake and did not take into account the extent or nature of the algae that appeared for perhaps a month at most as a result of lack of water flow past the beaver dam.    

The Bass Lake test results of the samples provided to the MLA 2019 Water Quality Report are consistent with prior years when Bass Lake received a green light.   

Also, the MLA report did not recognise the efforts made, and continuing to be made, to ensure that this situation does not occur again.  

Your BLA Executive was successful in getting the MLA to remove the red flag from the Bass Lake Water Quality report.  We had been hoping for a restoration of our ‘Green Light’… but were satisfied with a ‘Yellow Light’ for this year… with an accompanying explanation.

October 2020 Algae Bloom on Bass Lake


And as we know, we experienced a very similar situation again in the fall of 2020.

The story played out almsot exactly as above... the blooming was small, realtively isolated, never approached any levels of concern... and the Public Health notice was lifted in short order, after much hard work documenting conditions, and lobbying, by members of the Bass Lake community.

  • Additional information about the results of our lobbying will be shared at the BLA AGM on Wednesday July 21st, 2021... and then posted here on the webpage.


The reality is that this is very likely to be the new normal... with earlier and warmer springs the conditions are established for consequences come the fall each year.

  • More information about this 2020 Bloom will be shared at the BLA AGM on Wednesday July 21st, 2021... and additional information will be posted here after that time.


You can always refer to prior Bass Lake Updates for catching up on events, and communications, from the Fall of 2020.

Consequence of Algae Blooms on Bass Lake... Positive steps!!  

Bass Lake declared a vulnerable lake.


District and Area Municipal staff begin implementing the enhanced protection policies as soon as a lake is determined to be vulnerable.  

This is done to be proactive and to apply immediate additional protection to the lake. 

  • In the case of Bass Lake, once the blue-green algae bloom was confirmed by the Province/Health Unit, any development applications received after that date would be and are being requested to meet the enhanced protection policies.  
  • Bass Lake will be formally added to the Muskoka Official Plan as a vulnerable lake on July 7, 2021 provided no appeals are received.  


The enhanced protection policies would apply to development applications such as when a person may wish to divide their property into two parcels.  

Although there are some nuisances on how and when they would apply on a site by site basis, the enhanced protection policies include the following:

  • Increased setbacks from the shoreline for buildings and septic systems
  • A site specific investigation to determine the best location for a new or upgraded septic system, which considers soil conditions
  • The use of septic system technology with an enhanced ability to retain or abate phosphorus (the nutrient most likely to contribute to an algae bloom)
  • Annual monitoring and reporting to the Area... this includes additional benthic moniroting beginning in Summer 2021
  • Municipality for a period of no less than 10 years or until the lake is no longer considered vulnerable to ensure that mitigation measures remain in place


The enhanced policies layer on top of the standard protection policies that apply to all lakes and which address:

  • Appropriate locations of buildings and septic systems, including minimum setbacks from the shoreline
  • Retention or restoration of a shoreline vegetation buffer
  • Maintenance or establishment of native tree cover on a lot
  • Appropriate location and construction materials for roads, driveways and pathways
  • Storm water management and construction mitigation techniques
  • Collection of securities to ensure monitoring and long-term compliance


Should anyone wish to review the policies, the standard protection policies are found on page 25 and the enhanced protection policies begin on page 28 of the Muskoka Official Plan.

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