Chapter 7 – Streamflow

Release Date: February 2013

“Accurate information on the condition and trends of a country’s water resource–surface and groundwater; quantity and quality–is required as a basis for economic and social development, and for maintenance of environmental quality through a proper perception of the physical processes controlling the hydrological cycle in time and space…. almost every sector of a nation’s economy has some requirement for water information, for planning, development, or operational purposes.” (WMO/UNESCO 1991)

Streamflow is a central component of the IWMP at CVC because of strong interactions with the physical, chemical, and biological structure and function of the Credit River ecosystem. Extreme streamflow values, either through flooding or drought, can also greatly impact residents and infrastructure in the watershed. The amount of water flowing in a stream is influenced by two primary sources: water running off the land (i.e. rainfall or snowmelt) and groundwater discharge (Figure 1). The streamflow of the Credit River and its tributaries varies considerably from season to season and year to year; consequently this influences other components monitored through the Integrated Watershed Monitoring Program (IWMP). In the Credit River streamflow peaks in March and April with the spring snowmelt then declines to lower levels between July and September with warmer and drier conditions. Streamflow can potentially alter water and sediment quality of the watercourses, affect wetlands adjacent to rivers and impact fish habitat. Streamflow monitoring provides critical information regarding potential flooding hazards and low water warnings for communities in the Credit River Watershed.

 Figure 1. High streamflow in the Credit River at Erindale.

Figure 1. High streamflow in the Credit River at Erindale.

Streamflow Monitoring

CVC has partnered with Environment Canada and its Water Survey of Canada (WSC) to monitor streamflow in the Credit River Watershed. Although the responsibility for streamflow data collection lies with WSC, CVC provides local assistance to WSC in maintaining streamflow gauges and associated equipment. CVC also communicates flood and low water status to municipalities within the Credit River Watershed to warn of potentially hazardous situations based on extreme streamflow values. For example, CVC uses real time streamflow, water level and precipitation information to anticipate flooding and warns municipalities of potentially hazardous situations.

WSC currently operates 10 streamflow gauges in the Credit River Watershed (Figure 2). Five of these streamflow gauges are along the main branch of the Credit River, four stations are located along tributaries that flow into the Credit River, and one station is located along Cooksville Creek, a neighbouring stream east of the Credit River that drains directly into Lake Ontario. Most of these streamflow monitoring stations have operated for many years providing a valuable long-term record of streamflow in the Credit River Watershed. In fact, one streamflow station near Cataract on the Credit River provides nearly 100 years (1916 to present) of historical streamflow information. Data on instantaneous flow and daily maximum, minimum and average flow rates are recorded at the streamflow stations. From these data, average monthly, seasonal and annual streamflows are calculated for each monitoring station for their respective period of record. Three of the 10 WSC streamflow gauges (two in the Lower Watershed and one in the Upper Watershed) have operated for less than 10 years. These shorter records have been omitted from trend analysis.

Figure 2. Water Survey of Canada streamflow monitoring locations in the Credit River Watershed.

 Figure 2. Water Survey of Canada streamflow monitoring locations in the Credit River Watershed.

Streamflow Status (2011)

Streamflow status in 2011 for all WSC stations was within the range of streamflow values recorded from 1999 to 2010 (see Figure 3 for an example from the Cataract streamflow gauge). In addition, streamflow at the Cataract gauge exceeded the minimum environmental flow requirements in 2011 as calculated using the Tessmann method (Tessmann 1980), suggesting sufficient streamflow at this station to support ecological functioning throughout 2011. The Tessmann method estimates minimum environmental streamflow requirements and is intended as a first order approximation of ecological flow requirements (Conservation Ontario 2005). CVC will be working on refining these estimates of minimum environmental flow requirements by incorporating greater detail on streamflow needs of aquatic organisms in the Credit River. It is also important to note that these results are for the Credit River whereas smaller streams in the watershed may experience greater variation in streamflow.

Figure 3. Monthly streamflow for 2011 from the Cataract Water Survey of Canada station compared to the average and range of streamflows during the IWMP (1999-2011), average annual streamflow before the Island Lake Dam, and environmental flow requirements calculated by the Tessmann method.

Figure 3. Monthly streamflow for 2011 from the Cataract Water Survey of Canada station compared to the average and range of streamflows during the IWMP (1999-2011), average annual streamflow before the Island Lake Dam, and environmental flow requirements calculated by the Tessmann method.

Streamflow Trends

Variability in annual streamflow in the Credit River Watershed is pronounced over the monitoring period. For example, for the Cataract streamflow gauge (which has the longest continuous record of streamflow) average annual streamflow ranged between a maximum of 2.7 m3/sec in 1918 to a low of 0.9 m3/sec in 1958. Although streamflow has been variable at all streamflow gauges over the monitoring record, time series analysis indicates there are few statistically significant trends in streamflow through time with some notable exceptions (See Table A1-A3 at the end of this chapter for results of statistical trend analysis for the Upper, Middle and Lower Watershed respectively).

The most striking trends in streamflow are evident in the Upper Watershed. The Cataract streamflow gauge records statistically significant trends of decreasing daily maximum streamflow (i.e. the greatest daily average streamflow of the year) and increasing daily minimum streamflow (i.e. the lowest daily average streamflow of the year) between 1916 and 2010 (Figure 4). These trends are largely the result of lower spring flows and greater summer and fall flows (Table A1). In part, the change in streamflow evident in the Cataract record appears to be driven by hydrological control at the Island Lake Dam. This dam was constructed in 1967 to ensure adequate water supply to the Orangeville Water Pollution Control Plant by retaining water in Island Lake during high flow periods (e.g. spring) and releasing water during lower flow periods (e.g. summer and fall). Resulting changes to streamflow from this dam and discharge from the Orangeville Water Pollution Control Plant appear evident in the streamflow record from the Cataract WSC (Figure 3 and 4). Streamflow control may provide an overall benefit by reducing flood damage and providing sufficient water for sewage treatment during low flow periods. Regulating natural flow regimes may also result in reduced sediment flushing from the river and less flooding of adjacent wetland communities, possibly influencing the ecological structure and function of the Credit River and associated wetland communities.

 

Figure 4. Daily maximum (i.e. the greatest daily average streamflow of the year) and minimum (i.e. the lowest daily average streamflow of the year) flows from 1916 to 2011 for the Credit River near Cataract WSC station located in the Upper Watershed.

In the Middle Watershed the only statistically significant trend in streamflow through time is a modest increase in winter and summer streamflow recorded at the Silver Creek WSC station (Table A2).

The lone station in the Lower Watershed of sufficient length for trend analysis (Norval) showed no statistically significant trends in streamflow through time (Table A3).

The number of flood related incidents, although variable, showed no statistically significant trend between 1957 and 2008 (CVC 2012; Figure 5). Although there was no significant trend in the number of flooding incidents, the timing of the maximum daily water level has become increasingly variable (Figure 6), possibly as a result of climate change. In the early to mid-1900s maximum daily water levels most often occurred in spring. Increasingly from the mid-1900’s forward maximum daily water levels are occurring during winter and fall. This shift in timing of maximum water levels may result in increased risk of winter or fall flooding. This trend will continue to be closely monitored by CVC.

Figure 5. Number of flood related incidents in the Credit River Watershed (1957 and 2008).

Figure 5. Number of flood related incidents in the Credit River Watershed (1957 and 2008).

Figure 6. Month of maximum daily streamflow from 1916 to 2011 for the Credit River near Cataract WSC station located in the Upper Watershed.

Although flooding incidents can be a concern for municipalities and residents, low water conditions are also problematic for aquatic plants and animals and may limit recreational opportunities such as fishing or canoeing. In an effort to better communicate low water conditions, the Ontario Ministry of Natural Resources has developed an Ontario Low Water Response Plan based on three response levels. In general, low water conditions have been rare in the Credit River. Climate change may, however, increase the frequency and severity of low water conditions in the Credit River. In 2012, the Credit River Watershed was raised to a level 1 low water condition where residents were asked to voluntarily reduce water use by 10 percent. As part of the Ontario Low Water Response Plan, CVC will continue to monitor streamflow to inform municipalities of potential low water conditions.

Conclusions

Streamflow variability on timescales from days to decades and across the Credit River Watershed is a natural occurrence and current streamflow measurements are in line with historical trends. The most striking trend through time has been reduced daily maximum and spring flows and increased daily minimum and summer and fall flows in the Upper Watershed. This trend is likely driven, in part, by streamflow management through flow control at the Island Lake Dam and discharge from the Orangeville Water Pollution Control Plant. Although the number of flooding incidents in the Credit River Watershed does not appear to be significantly increasing, the timing of daily maximum streamflow has become more variable, possibly in response to climate change. It remains important to continue to monitor streamflow so that CVC can plan for and communicate potentially hazardous situations to municipalities. Given the steady increase in water takings from the Credit River Watershed future analyses will also include examining cumulative effects of streamflow change in relation to other IWMP components. Streamflow monitoring is especially pertinent to help municipalities predict and mitigate the impact of climate change on water quantity in the Credit River.

Chapter 8 examines Fluvial Geomorphology of the Credit River. What is fluvial geomorphology and why is it important? How is the shape of the Credit River and its tributaries changing and what impact, if any, are human activities having on its shape?

Did you know?

Streams in the Credit River Watershed are getting wider. Why? Read on in Chapter 8.

References

Conservation Ontario. 2005. Establishing Environmental Flow Requirements Synthesis Report.

Credit Valley Conservation (CVC). 2012. Flood Related Incidents 1957 to 2008. Draft Report.

WMO/UNESCO. 1991. Report on Water Resources Assessment.

Table A1. Results from linear regression analysis of select streamflow variables versus monitoring year for the Upper Watershed. Statistically significant results are shown in bold.

Streamflow station

Sample size (n)

Coefficient of determination (r2)

Slope

p-value

Orangeville (1967-2010)

Daily Maximum

42

0.00

0.002

0.92

Daily Minimum

41

0.38

0.004

<0.001

Average Annual

42

0.00

0.000

0.95

Average Winter

42

0.02

0.002

0.41

Average Spring

42

0.02

0.003

0.37

Average Summer

42

0.03

0.001

0.30

Average Fall

42

0.006

0.001

0.63

Cataract (1915-2010)

Daily Maximum

94

0.25

0.200

<0.001

Daily Minimum

92

0.57

0.006

<0.001

Average Annual

94

0.01

0.002

0.31

Average Winter

96

0.03

0.003

0.07

Average Spring

95

0.07

0.009

0.007

Average Summer

96

0.24

0.006

<0.001

Average Fall

96

0.25

0.007

<0.001

Erin (1983-2010)

Daily Maximum

25

0.06

0.036

0.24

Daily Minimum

25

0.04

0.001

0.33

Average Annual

26

0.002

0.001

0.80

Average Winter

26

0.05

0.003

0.28

Average Spring

26

0.01

0.002

0.60

Average Summer

26

0.01

0.001

0.55

Average Fall

26

0.05

0.003

0.25

Table A2. Results from linear regression analysis of select streamflow variables versus monitoring year for the Middle Watershed. Statistically significant results are shown in bold.

Streamflow station

Sample size (n)

Coefficient of determination (r2)

Slope

p-value

Boston Mills (1982-2010)

Daily Maximum

26

0.01

0.130

0.62

Daily Minimum

25

0.07

0.010

0.18

Average Annual

27

0.002

0.005

0.81

Average Winter

27

0.001

0.004

0.89

Average Spring

27

0.006

0.016

0.70

Average Summer

27

0.044

0.030

0.29

Average Fall

27

0.04

0.001

0.63

Black Creek (1987-2010)

Daily Maximum

22

0.001

0.002

0.91

Daily Minimum

23

0.02

0.000

0.53

Average Annual

22

0.004

0.001

0.78

Average Winter

22

0.01

0.001

0.67

Average Spring

22

0.001

0.001

0.67

Average Summer

22

0.03

0.001

0.41

Average Fall

22

0.005

0.001

0.75

Silver Creek (1960-2010)

Daily Maximum

46

0.03

0.053

0.25

Daily Minimum

46

0.07

0.001

0.08

Average Annual

48

0.02

0.003

0.40

Average Winter

49

0.08

0.025

0.049

Average Spring

50

0.001

0.81

0.81

Average Summer

50

0.093

0.005

0.03

Average Fall

49

0.011

0.003

0.47

Table A3. Results from linear regression analysis of select streamflow variables versus monitoring year for the Lower Watershed. Statistically significant results are shown in bold.

Streamflow station

Sample size (n)

Coefficient of determination (r2)

Slope

p-value

Norval (1988-2010)

Daily Maximum

20

0.02

0.41

0.56

Daily Minimum

19

0.01

0.01

0.64

Average Annual

21

0.07

0.07

0.25

Average Winter

21

0.05

0.09

0.32

Average Spring

21

0.09

0.13

0.18

Average Summer

21

0.04

0.04

0.38

Average Fall

21

0.01

0.03

0.68

Please note 3 streamflow monitoring records were fairly short, for example only 5 years of data, and were excluded from time series analysis.

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