Hydrology of the Don (GWC 2)

Water Levels in the Don

Given the cold weather, I did not venture out to assess the water levels in the field. However, I did take a look at the hydrometric data provided by Environment Canada’s gauges. Three stations exist along the Don River, namely one along Don’s East Branch, York Mills, and Todmorden (Figure 1).  To see if the water levels have changed at any of these stations, I used paired t-tests to compare recent water level means (2010 – 2015) versus the past decade (2005 – 2015).

Figure 1. Water gauges monitored by Environment Canada in the Don River

East Branch


The t-test did not show a statistically significant difference in water level between the means of the past decade (2.0738 metres) compared to the past 5 years (2.07365 metres) at the east branch of the Don River.

Group   2005-2015   2010 – 2015
Mean 2.07380 2.07365
SD 0.06890 0.05292
SEM 0.00154 0.00119
N 2000 1983



The t-test did not show a statistically significant difference (p = 0.56) in water level between the means of the past decade (12.22 metres) compared to the past 5 years (12.23 metres) at the Todmorden Station in the Don River.

  Group   2005 – 2015   2010-2015
Mean 12.22334 12.22589
SD 0.14227 0.13708
SEM 0.00318 0.00307
N 2000 2000

York Mills


The t-test showed a statistically significant difference (p <0.01) in water level between the means of the past decade (0.68 metres) compared to the past 5 years (0.58 metres) at the York Mills Station in the Don River. Note that 2010 data was not available so the second group (2010 – 2015) did not include data from that year which could have had an influence on the results. The remarks on this missing data are that it is “currently undergoing a data review” (GOC, 2017).

Group   2005-2015   2011*-2015
Mean 0.67632 0.57998
SD 0.14887 0.19917
SEM 0.00333 0.00477
N 2000 1745

To perhaps understand why there was such a large drop in water levels, I looked at satellite images on Google Earth Pro but could not distinguish by the naked eye any substantial changes in land cover immediately upstream.Note that in the water level graph there is a marked decrease in levels from 2013-2014 so there may be a change in the instrumentation that effected the measures.

I looked into TRCA’s “Baseflow and Water Use Assessment” Report from 2009 to try and get an idea of what is going on. In the Lower West Don River where York Mills is situated, half of the flow comes from runoff meaning the urban area has a large influence on the river. Also, the TRCA found a decreasing trend of 1.6% in summer median baseflow discharge from the Lower West Don between 1995 and 2005 (though they note that this range is within normal range of variance). This may imply that the water level has been decreasing over a longer period of time given the TRCA’s findings from 1995-2005 and mine from 2005-2015. I believe what could be causing the lower water levels at the York Mills gauge would have to do with hydrological changes in the Upper West Don River. This could be contributed to urbanization in the Upper West Don River since there would be more water users over the years. The TRCA also offers other ideas which can decrease water levels, including groundwater being circumvented into newly built sewer infrastructure away from the river and short term decreases in precipitation.

Aquatic invasive species in the Don

In my last post for the Great Waters Challenge I spoke about the fish species in the Don River. Just to recap:

– before the 20th century there used to be larger fish like salmon and lake trout

-now there only exist fish that can tolerate an urban environment (smaller species)

-the river is stocked with salmon for sport fishing

Invasive species are a whole other issue regarding the quality of the river as a whole. They come and disturb an ecosystem by outcompeting other native species. Those that have been found in the Don include carp (common carp, goldfish, grass carp, koi), sea lamprey, round goby, alewife, and rusty crayfish (TRCA, 2009b). Alewife can be called a “naturalized” species because it was introduced in the 1800’s but I find that placing this label on the species takes away from people’s understanding of the damage/changes it may have incurred on the environment when it was introduced. It probably moved into Lake Ontario and eventually into the Don River when Toronto had established itself as a port and people decided to either stock connected lakes/rivers with alewife or they accidentally brought it in their ship’s ballast waters.


GOC. 2017. Historical Hydrometric Data Remarks for DON RIVER AT YORK MILLS. Retrieved from https://wateroffice.ec.gc.ca/report/remarks_e.html?type=h2oArc&stn=02HC005&mode=Graph&reportType=Daily

TRCA. 2009a. Don River Watershed Plan: Baseflow and Water Use Assessment – Report on Current Conditions. Retrieved from http://trca.on.ca/dotAsset/55387.pdf

TRCA. 2009b.Don River Watershed Plan: Aquatic System – Report on Current Conditions. Retrieved from http://trca.on.ca/dotAsset/55393.pdf


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