
Lake Simcoe Science
Stream Flow: A Tale of Two Rivers
From Rain Fall to River Flow
Water moves in natural cycles. It falls from the sky, it
filters through the ground, it travels across the land
through rivers to larger bodies of water, it evaporates,
and then the process starts all over again.
As part of the natural water cycle, rivers play an
important role in the health and function of our
watershed. When it rains, water moves across the
landscape and into the river. Along the way, some of
this water is slowed and consumed by vegetation and a
portion soaks into the ground to become groundwater.
The amount that flows directly into the river is known as
event flow.
When urbanization occurs and hardened surfaces
such as roads and roof tops replace vegetation, more
water flows directly into the river during a rainstorm
and makes it there faster. What we see are water flow
patterns that change because of urbanization.
Hardened surfaces also prevent water
from soaking into the ground to
replenish groundwater. Groundwater
contributes to the flow in rivers
through seeps and upwellings in the
river bed. This source of river flow is
called baseflow and is what sustains
flow in the river between storms
and during dry periods. A reduction
in groundwater due to hardened
surfaces can also result in a reduction
in the baseflow of a river.
River Flow into Lake Simcoe
The Lake Simcoe watershed is made up of 35 river and creek systems
that annually contribute 10% of the total Lake volume. The map above
displays the percent of total river flow each sub-watershed contributes to
the lake annually.
Did You
Know...
Annual
evaporation
from Lake
Simcoe totals
535 billion
litres of water,
approximately
equal to
the total
daily flow of
Niagara Falls!
Comparing Streams in
Different Landscapes
The Lake Simcoe Region
Conservation Authority
(LSRCA) measures the change
in stream flow resulting from
urbanization by comparing
how streams respond
in different landscapes.
Monitoring a stream’s
response to rainfall in an
urban setting and comparing
it to a nearby stream in a rural
setting helps us understand
the effects of urbanization.
An excellent example of this
can be seen by comparing
the Tannery Creek with its
neighbour, the East Holland
River at Vandorf Sideroad.
Tannery Creek runs through
the most densely populated
areas of Aurora and has a
slightly smaller watershed.
The East Holland River at
Vandorf runs through a landscape that is primarily
rural. The two areas are located close to each other
and typically receive the same amount of rainfall and
weather conditions.
Although these two river systems should behave much
the same way, this is not the case, primarily because of
the increase in hardened surfaces associated with urban
areas. Hardened surfaces include things such as roofs,
roads and parking lots. These surfaces do not allow
water to soak into the ground and instead cause larger
volumes of rainwater to move to the river much more
quickly.

Comparison of the land use for two reaches of the East Holland River. Tannery Creek is a more urban
catchment located to the west, while the East Holland River at Vandorf drains a much more rural
catchment to the east
This change is clear when comparing our two study
river systems. During and shortly after a rainstorm,
the Tannery Creek’s water levels and volume increase
dramatically, while the water levels in the East Holland
River at Vandorf experience a much smaller response.
The larger and higher flow in Tannery Creek impacts
the stream banks which are more eroded, have poorer
vegetation and murkier water.
In contrast, the banks of the East Holland River at
Vandorf are much more stable, have better vegetation,
and the water is much clearer.
Event Flow
Stream flow for Tannery Creek
and East Holland River at Vandorf
showing the different way these
two streams respond to similar
storms. The urban river (Tannery)
has large quick peaks in water flow
due to the runoff from hardened
surfaces. In contrast the rural river
(East Holland at Vandorf) only
shows slightly elevated flows due to
the greater amount of interception
by vegetation and infiltration to
groundwater.

Comparing 40 Years of Baseflow
in Two Rivers
Baseflow Index is a method of comparing the proportion,
or percent of baseflow to total stream flow in a stream
over a year. Over the long term, changes in the Baseflow
Index show how the stream responds to changes such
as climate or urbanization.
A comparison of the historical baseflow indices for the
Beaver River and East Holland River sub-watersheds
illustrates how an increasing urban area can impact the
volume of baseflow in a stream.
The East Holland River drains two large urban centres,
Newmarket and Aurora. The Beaver River is a primarily
rural / agricultural sub-watershed with large wetland
areas in the eastern portion of the Lake Simcoe
watershed.
In the late 1960s through mid 1970s both subwatersheds had similar baseflow contributions with 5
year average baseflow indices of approximately 75%.
By the late 1980s the baseflow indices for the East
Holland River dropped to approximately 70%. This
decrease coincided with urban growth in the Town of
Aurora that occurred during this same period.
Again in the early 1990s and most notably by 1995,
the baseflow indices for the East Holland River dropped
again to approximately 65%, here coinciding with
increased growth in Newmarket that occurred through
the 1990s. In contrast, the Beaver River saw very
little urban growth during this period. As a result, the
baseflow index remained relatively stable with natural
climatic variability having the greatest impact on
baseflow.
Baseflow Index of the
East Holland River and
Beaver River for a
40-year period
The East Holland shows a
declining Baseflow Index
associated with growth in the
urban areas of Aurora and
Newmarket. The Beaver River
has seen very little increase in
urban area and as a result, the
Baseflow Index has remained
relatively stable.
Why Baseflow is Important
We can see, storm by storm, that an increase in hardened surfaces affects the amount of stormwater running into
the river.
By looking at monitoring data over a long period, we can also see how it affects the volume of baseflow in a
river.
Baseflow is very important for a stream because:
- it helps regulate water temperature which is crucial for many plants and animals,
- it is typically clean and helps to dilute contaminates from runoff,
- it improves water quality and,
- it provides a stable source of water to help augment flow during dryer periods which are
typical during late summer or more severely during drought.
A river with good stable baseflow is more resilient to drought and to the anticipated effects of
climate change. Unfortunately the increased runoff volume generated by hardened surfaces
in urban areas also serves to reduce the amount of water that is able to soak into the ground,
replenishing groundwater. As this groundwater is the primary source of baseflow, urbanization
can have the added impact of reducing baseflow volumes in streams.
Event Flow = river flow
generated by
rainfall runoff
Baseflow
= river flow
generated by
groundwater
upwellings
Low Impact Development
Low Impact Development (LID) is an approach to
managing stormwater that mimics a site's natural
drainage pathways as the land is developed. LID
encourages water to seep into the ground to help
manage the groundwater and stream levels.
LID principles include preserving natural landscape
features, minimizing hard surfaces, and creating
drainage systems that treat
stormwater as a resource rather
than a waste product. In essence,
Low Impact Development is about
designing spaces that let water
travel through its natural cycle
as close to the way it did before
development took place.
Low Impact Development is not
a new concept, but it has not yet
been widely adopted in Ontario. At
LSRCA, we hope to change this and
are working with the development
community and government
partners to implement LID design
into new developments. As well,
LSRCA’s “Landowner Environmental
Assistance Program” (LEAP) works
with municipalities and residents
to incorporate LID features into
existing properties and businesses.
Low Impact
Development
involves
designing
spaces that let
water travel
through its
natural cycle
as close to
the way it did
before development took
place.
Reduce Your Water Footprint and Help
Your Local Rivers
The old saying that even one person can make a
difference still holds true. If you think you are too small
to make a difference, try sleeping with a mosquito in the
room.
- Anytime you use water outside, try to divert the
runoff onto grass or soil. For example, if you wash
your car at home, wash it on the grass and not on
the driveway. Divert downspouts off driveways and
onto your lawn.
- Build a rain garden on your property. Rain gardens
help encourage water to collect and slowly seep
back into the soil. It means less water in the
storm sewers.
- Sweep your driveway to clean it rather than hosing
it down.
- Consider alternatives to asphalt for your driveway,
such as permeable pavement.
How LSRCA is working to improve the
quality of Lake Simcoe
The watershed is a complex and dynamic system, that
changes over time in response to both human activities
and natural events.
LSRCA works with its many partners, including our
communities and municipal, provincial, and federal
governments, to protect and restore the environmental
health and quality of Lake Simcoe and its watershed. We
do this by leading and supporting programs and projects
in science and research, protection and restoration, and
education and engagement.