Rouge River State of the Watershed Report

Surface Water Quantity

Goal: Surface waters of a quantity, volume and naturally variable rate of flow to:

$ protect aquatic and terrestrial life and ecological functions; $ protect human life and property from risks due to flooding; $ contribute to the protection of Lake Ontario as a domestic drinking water source; $ support sustainable agricultural, industrial, and commercial water supply needs; $ support swimming, fishing and the opportunity to safely consume fish; and $ contribute to the removal of Toronto from the Great Lakes list of Areas of Concern. Surface Water Quantity

Key Findings:

The Main Rouge subwatershed has been subject to significant urbanization with an approximate total impervious cover of 18% as of 2002. Several studies suggest that the maximum impervious cover that a watershed can withstand before experiencing severe hydrologic changes and consequent geomorphic and ecological impacts is approximately 10%.

There has been significantly less urbanization in the Little Rouge subwatershed and impervious surfaces make up only 2% of the subwatershed area. As a result, hydrologic impacts and related effects are much less severe than on the Main Rouge River.

Average annual flows in the Main Rouge River show a long-term increasing trend of over 1.3% per year in the past 40 years. This rate of increase is significantly greater than that on the Little Rouge River or nearby rural watersheds and is indicative of the effect of urbanization on the hydrologic cycle.

The Rouge River has become flashy and now generates high flows in response to rainfall events that caused almost no response in the river prior to widespread development. In the same time period, the response of the Little Rouge River has remained essentially unchanged.

The extra stormwater flow generated in the now developed Rouge River watershed has resulted in an approximate 225% increase in summer flow volumes. The seasonal distribution of flow is beginning to resemble that of highly urbanized watersheds such as Highland Creek. Seasonal patterns in the Little Rouge River have been virtually unaffected due to a lack of development.

The shallow Oak Ridges Moraine aquifer contributes between 40-80% of total baseflow in individual tributaries of the Rouge. This is especially noticeable in the northwest portion of the upper Rouge, Bruce Creek and upper Little Rouge.

Significant baseflow losses were observed in the Little Rouge south of Elgin Mills Road, likely due to unreported water uses.

There are one hundred and sixteen water takers in the watershed, of which 34% use surface water sources. The majority of permitted water uses are for golf course irrigation, aesthetics, and industry. Golf courses represent the largest user by volume.

Flood vulnerability in the Town of Markham is greatest in March and April due to spring melt or late fall related to heavy rains from tropical storms.

Summary of Current CondCondiiiitionstions Ratings:

ObjectivesObjectives:::: OverOverallall Rating

Protect and restore the natural variability of annual and seasonal stream flow. Fair

Maintain and restore natural levels of baseflow. Good

Eliminate or minimize risks to human life and property due to flooding and erosion. Good

TABLE OF CCONTENTSONTENTS

5.0 SURFACE WATER QUANTITY ...... 5-1 5.1 Introduction...... 5-1 5.2 Understanding Surface Water Quantity in the Rouge Watershed...... 5-2 5.3 Measuring Surface Water Quantity ...... 5-4 5.4 Existing Conditions...... 5-7 5.4.1 Surface Flow Regime...... 5-7 5.4.2 Baseflow...... 5-15 5.5 Objectives for Surface Water Quantity ...... 5-30 5.6 Summary and Management Considerations ...... 5-30 5.7 References ...... 5-32

LIST OF FIGURES

Figure 5-1: Active Stream Gauging Locations ...... 5-5 Figure 5-2: Streamflow Data from 1962 to Present for Rouge River, Little Rouge River and Highland Creek Gauges...... 5-8 Figure 5-3: Seasonal Flow Patterns: Rouge and Little Rouge Rivers and Highland Creek.... 5-10 Figure 5-4: Summer Flow Volume Trends, Rouge and Little Rouge Rivers and Highland Creek ...... 5-11 Figure 5-5: Area-Weighted Rainfall Event Response: Rouge and Little Rouge Rivers Compared with Highland Creek ...... 5-12 Figure 5-6: Area-weighted daily flows for summer 1965 and summer 1997...... 5-14 Figure 5-7: Location of Baseflow Sampling ...... 5-16 Figure 5-8: Summer Mean Monthly Baseflow from Hydrograph Separation...... 5-17 Figure 5-9: Baseflow Normalized to Stream Length...... 5-19 Figure 5-10: Flood Vulnerable Sites and Special Policy Areas ...... 5-27

LIST OF TABLES

Table 5-1: Known Water Abstractions by Sector-Rouge River, 2006...... 5-21 Table 5-2: Total Water Abstractions by Subwatersheds - Rouge River, 2006 ...... 5-23 Table 5-3: Number and Flood Frequency of Watershed Flood Vulnerable Areas and Roads5-26

Unique Rouge River Watershed Feature

Urban development has increased the volume of stormwater runoff and flow in the Rouge River by about fifty percent and has created a flashy urban hydrologic regime. The Little Rouge River has avoided these effects due to a lack of development in its subwatershed.

CHAPTER SURFACE WATER QUANTITY 555

5.05.05.0 SURFACE WATER QUANTITY

5.15.15.1 Introduction

The accounting of the total quantity of water and its distribution within a watershed is known as the water budget. The input to this budget is the total amount of precipitation occurring in the watershed and the outputs are the proportions of precipitation that return to the atmosphere through both evaporation and transpiration, enter the groundwater system through infiltration, and run overland to rivers and streams. Combined, these input and output components and their movement through the atmosphere and over and through the ground are known as the hydrologic cycle.

Surface water quantity deals with the components of water within the hydrologic cycle that move over land or within surface systems of lakes, streams, and wetlands. Surface flow includes normal low flow in rivers and streams which is comprised of groundwater discharge, overland flow from rain and snow melt and precipitation that falls directly into lakes, rivers and streams and wetlands .

The physical properties of a watershed, such as drainage area, slope, geology and land use affect the distribution of the water budget and the processes that function within a watershed’s hydrologic cycle. This chapter will explain how rural and urban land uses in the Rouge River watershed have altered the hydrologic cycle, including changes in surface flow volumes, annual flow patterns and the risk of flooding. Water uses are described and represented in terms of their significance to low stream flow. The understanding of the processes controlling surface water flow are important to understanding the aquatic systems that have evolved into the existing natural flow patterns. 5-1

5.25.25.2 Understanding Surface Water Quantity in the Rouge WatershedWatershed

Within the Watershed Ecosystem

Climate and surficial geology are key factors in determining the quantity of surface flow within a watercourse, primarily through their effect on precipitation, evaporation and infiltration. Although surface flow varies throughout the year, there is a general tendency in the Rouge River watershed for the highest annual peak flows to occur in the late winter or spring due to snowmelt or combined rain/snowmelt events and for the lowest flows in the summer when precipitation is at a minimum. Flooding is a natural and common occurrence in most watersheds. The Rouge River watershed is typical of temperate watersheds with low to moderate slopes in that most areas have small stream channels whose banks are overtopped on average every one to two years, with flows spilling out onto relatively wide floodplain areas.

A significant percentage of the precipitation within undeveloped watersheds in Southern Ontario and areas of similar climate and geology is cycled back into the atmosphere through evapotranspiration or infiltrates into the soil, leaving a relatively small surface runoff component. In undeveloped conditions, direct surface runoff comprises a relatively small component of the total flow in surface watercourses, elevating stream flows only during larger rainfall or snowmelt events. The majority of total annual flow occurs as baseflow between such events, which is comprised principally of groundwater discharge, with additional contributions in many cases from surface runoff detained and released slowly from wetlands. Even within individual watersheds, the natural proportion of baseflow to total flow in surface watercourses may vary significantly based on geology, topography and vegetation. As a result, baseflows in some watercourses are much more impacted than others by sustained drought or flow withdrawals for human use.

Native aquatic and riparian species have evolved to take advantage of natural patterns of surface water flow, adapting to historic variations in baseflow and surface runoff. Natural watershed conditions act to buffer the effects of natural extremes in climate such as storms and droughts and moderate their effects on the surface flow regime. For example, wetlands within the watershed help to stabilize water levels, retaining surface runoff when it is abundant and releasing it during periods of lower flows. Wetlands therefore help to reduce the frequency of threatening high flow events and moderate the severity of drought conditions. Extreme variations in surface water flow in undeveloped watersheds occur less frequently and are less severe than those in developed landscapes.

Influences of the Rural and Urban Landscape

Consistent with the historical settlement patterns of southern Ontario, land use changes were initially made in the Rouge River watershed to allow for agricultural activities. Flow stabilizing features, such as forested lands, were cleared, wetlands were drained and small streams channelized with riparian zones removed or altered. These activities altered the hydrologic cycle by reducing interception, evapotranspiration and infiltration, which in turn generated increased surface runoff and more frequent high flows in rivers and streams. While these rural land use changes resulted in changes and increases in stream flow, in general the seasonal flow variations that characterize watershed flow regime were maintained. 5-2

Urban land form changes have imposed the most significant stress on the hydrologic system of the Rouge River watershed. Urbanization creates a dramatically increased proportion of impervious surfaces, such as paved roads, driveways, parking lots and roof tops, which eliminate vegetation and eliminate contact between precipitation and the soil. As a result, evapotranspiration and infiltration are significantly reduced and the balance of precipitation is converted into much greater runoff volumes (Shuster et al ., 2005). As a result of these urban land use changes, runoff response from rainfall or snow melt events is much more rapid than in rural watercourses. Urban watercourses generally receive a greater total volume of flow in a shorter time frame, thereby resulting in much higher peak flows. The reduced infiltration resulting from impervious surfaces can also lower local groundwater levels, which can in turn reduce groundwater discharges to baseflow. In some cases, this can cause permanently flowing watercourses to become intermittent, particularly in headwater areas. The increased frequency and quantity of direct surface runoff can also accelerate natural stream channel erosion rates well beyond natural levels, degrading water quality and aquatic habitat (refer to Section 7: Fluvial Geomorphology for additional discussion of these effects).

Seasonal variations in stream flow are also dramatically affected by urban land use changes. Alterations in the pattern of snowpack accumulation in urban areas as compared to rural ones results in modifications to winter and spring snowmelt flows. The reduction in evapotranspiration and infiltration caused by impervious surfaces is most significant in summer, causing significantly increased flows in response to rainfall events and reduced baseflows

Historically, the impact of increased runoff from impervious surfaces in urban areas was exacerbated by drainage engineering practices focused on conveying the increased volumes of surface runoff as quickly as possible off the land through storm sewers and often channelized watercourses. Combined with removal of natural flow attenuating wetlands and encroachment by development on floodplains, this tended to result in a greater hazard from flooding in urban areas. In recognition of this negative outcome, beginning in the early 1970s, the Ontario provincial government developed its floodplain planning policies aimed at minimizing the risk to life and property damage due to flooding. Land use planning tools were used to limit new development in delineated floodplains. Stormwater management policies were introduced in the 1980s to mitigate the impacts of increased runoff on peak flow rates (MTRCA, 1980) through the prescribed use of stormwater detention facilities (ponds). These facilities detain the increased runoff from urban areas and release it more slowly so that peak flows in downstream watercourses are not increased. More recent stormwater management policies ((MTRCA, 1990; OMOE (OMOEE), 1991, 1994 and 2003) have evolved to address control of erosion caused by more frequent high flows, requiring that detention facilities provide extended detention of frequent flows to release them at rates that are thought to be less erosive. However, there is evidence that this approach has not been sufficient to mitigate erosion impacts, as discussed in Section 7.0. The extended detention component of modern stormwater management facilities, which also serves a water quality improvement function, provides short-term enhancement of baseflows after rainfall events as the runoff is released slowly into receiving watercourses. However, this water of poorer quality than groundwater discharge and is not as persistent, and therefore it may not adequately mitigate the loss of groundwater infiltration and baseflow caused by urban impervious surfaces.

5-3

5.35.35.3 MMMeasuringMeasuring Surface Water Quantity

Measurement of flow volume within the river or stream is accomplished by establishing a relationship, known as a rating curve, between the river level (stage) and the corresponding rate of flow at specific locations within the watershed. Stream gauges are then installed at these locations to allow for the measurement of river level and flow volume at any given time. At gauge sites where continuous flow measurements are made, various statistics including average, seasonal, and annual flows can be computed to provide insight into short-term and long-term trends that characterize the flow regime of the watercourse in question. In addition, gauge data can be used to calibrate hydrologic computer models, which can then be used to predict the hydrologic response of the entire watershed area for various types of precipitation and flow events. In the Toronto and Region Conservation Area (TRCA) jurisdiction and in the Rouge River watershed, these modeled flows are used to develop the floodplain mapping used in flood risk management.

Baseflows can be estimated from measured flow gauge data to obtain information regarding interactions between the surface and groundwater systems. However, because gauges are relatively expensive and therefore not widely dispersed throughout watersheds, field measurements timed specifically to eliminate effects from runoff are required to fully understand the distribution of baseflows within the watershed. Numerous, well-distributed baseflow measurements allow a better understanding of the groundwater surface water interaction and identification of natural or man made impacts to these flows. Surface water quantity measurements, particularly those related to baseflow, must also consider human water use, as it can have a significant impact on local stream reaches as well as the overall water budget.

Within the Rouge River watershed, flow is currently measured continuously at seven permanent stream gauges (Figure 5-1) three of which are part of the Federal/Provincial flow network and operated and maintained by the Water Survey of Canada (WSC), an agency of Environment Canada. Data collection at these three gauges, located on the Rouge River near 14 th Avenue, the Little Rouge River near Elgin Mills Road, and the Little Rouge River at Reesor Road began in 1961, 2002, and 1964, respectively. An additional four continuous stream gauges are owned and operated by the Town of Richmond Hill and are located along the Beaver Creek and Upper Rouge tributaries. There are also a number of active temporary gauges operated by local municipalities and a private consultant for specific studies; these do not collect data with the same accuracy or rigor as the WSC gauges nor are they expected to be in place on a long- term basis. The TRCA’s Regional Watershed Monitoring Network (RWMN) has identified a need for more gauging within the Rouge River watershed with a further five stream gauges recommended, as further discussed in the Rouge River Watershed Plan.

Baseflow studies in the Rouge River watershed were first carried out by the Geologic Survey of Canada (GSC), a division of Natural Resources Canada, during the summers of 1996 and 1997 at a total of 126 individual locations (Hinton, 1996-97). TRCA continued sampling the Rouge River watershed at these 126 GSC sites and a further 55 sites throughout the late spring and summer of 2002 (TRCA, 2003). The additional sites were selected from topographic maps, with the final sampling location determined in the field. Baseflows were generally lower in 2002, but there were insufficient data to decide whether the differences were significant and 5-4 suggested a trend or whether they were simply the result of climatic variations, upstream withdrawals, or other factors.

Figure 555-5---1111:::: Active SSStreamStream GaugGauginging Locations

5-5

TRCA applied WSC flow measurement standards and the GSC sampling protocol to ensure accurate sampling occurred during baseflow conditions. Given the geology and climate in the TRCA jurisdiction, a 72 hour period was established as the minimum time to wait following any precipitation event before measuring, to ensure all surface runoff had cleared the system and the measured flow only reflected baseflow. Continued measurements at selected sites at or near specific subwatershed outlets have also been undertaken annually, on a monthly basis through the summer and are planned to be continued indefinitely.

In the late 1990s the Regional Municipality of York began the construction of an expanded sanitary sewer network to accommodate a rapidly increasing population. One of the construction preparation techniques involved the dewatering of the upper portion of the Thorncliffe Aquifer with concomitant discharge of the groundwater into the Robinson Creek tributary system of the Rouge River. This discharge has impacted upon total flow measurements from 2002 and 2003 in several locations within the Rouge River watershed. Dewatering and discharge associated with construction of the portion of sanitary sewer line along 16th Avenue in Markham resulted in a substantial increase in surface water flows within the Robinson Creek system tributary where the operation discharged the groundwater and within the main Rouge River downstream of its confluence. In addition to the measured increase in flows within the Robinson Creek, the lowering of a regional aquifer and the surface water table also impacted baseflows in streams over a wide area. The effect of the dewatering activities is being monitored and it is expected that recovery of groundwater levels and baseflows may take several years.

TRCA has been using hydrologic models on the Rouge River watershed since 1979 (McLaren, 1979) for floodplain management. These models are calibrated using measured flow data and then used to predict the flows from 2-100 year return period storms as well as the Regional Hurricane Hazel storm. These flows are used to predict the depth and extent of flooding using hydraulic models, the results of which are translated to floodline mapping. As part of the development application review process, the TRCA ensures that new developments are located outside of the flood risk area and that existing condition peak flows are maintained through the implementation of stormwater management practices. Regular updates, incorporating field measurements of stream flow and technological advancements, help to keep the models current (Marshall, Macklin, Monaghan Ltd., 1988).

In 2001, the TRCA retained a consultant to update the existing Rouge River watershed hydrology model that reflects new computer modelling software, new flow data, updated land use conditions and integration of existing and approved stormwater management facilities (Marshall, Macklin, Monaghan, 2002). Flows from the updated Rouge River watershed Hydrology model were used to update the hydraulic model in order to generate a new set of digital floodplain maps (Clarifica, 2006). These maps provide an update to the Rouge River watershed map produced in 1959 and updated in 1979, and have been updated and extended to reflect changes in technologies, watershed land use and to utilize the results of the new hydrologic and hydraulic models. An updated database of flood vulnerable sites in the Rouge River watershed is also being developed to utilize these new hydraulic and mapping products.

5-6

5.45.45.4 Existing Conditions

The Rouge River watershed drains approximately 336 km 2 and is comprised of two main branches, the Main Rouge River and Little Rouge River which drain areas of 222 km 2 and 114 km 2, respectively. Stream gradients range from close to 2.5% in the headwaters to 0.25% near the outlet at Lake Ontario. The Main Rouge River receives flow from six additional major tributaries, which have their headwaters in the South Slope of the Oak Ridges Moraine. These tributaries join the main branch of the Rouge River watershed as it flows eastwards, nearly parallel to Highway 7. This west to east drainage pattern is relatively unique, compared to watercourses in the remainder of the Rouge River watershed and in surrounding watersheds, and is likely due to of the presence of an erodible deposit of silts, sands, and gravels through which the river flows bordered by an upland of highly resistant Newmarket Till to the south.

The majority of the development in the watershed is concentrated in its geographic centre, within the Town of Markham, with additional development in the northwest, in the Town of Richmond Hill, the south in the City of Toronto, and the northeast in the Town of Whitchurch- Stouffville. Although much of the development was constructed without the benefit of stormwater management, nearly 200 stormwater management facilities (ponds) have been constructed or are proposed in the watershed. These facilities provide various degrees of mitigation the impacts of urban runoff on the surface flow regime and water quality depending on the time at which they were constructed, and have a significant influence watershed hydrology.

5.4.1 Surface Flow Regime

Flows on both the main Rouge River and the Little Rouge River have been measured for over 50 years, which provides the opportunity to assess the effects of land use changes over that period on the flow regime of both watercourses. Daily flow data (values reported as average flows in m 3/s for each day of the year) is available for the gauge on the main Rouge River near 14 th Avenue and the Little Rouge River gauge at Reesor Road from 1962 and 1964, respectively, to the present. Figure 5-2 shows average annual flows measured at the Rouge River and Little Rouge River gauges, as well as at the gauge on Highland Creek located to the southwest of the Rouge River watershed. Data from Highland Creek is used for comparison throughout the following section, as the Highland Creek watershed is almost completely urbanized with few mitigative stormwater management facilities and therefore represents a ‘worst-case’ outcome of urban impacts to the surface water flow regime.

5-7

Figure 555-5---2222:: Streamflow Data from 1962 to Present for Rouge RiveRiver,r, Little Rouge River and Highland Creek Gauges

Average Annual Discharge Water Survey of Canada Station 02HC022 Increase over period of record: 55% Rouge River near 14th Ave. p = 0.003 (statistically significant) 3.00

2.50

2.00

1.50

1.00 Discharge(m3/s)

0.50

0.00 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Average Annual Discharge Water Survey of Canada Station 02HC028 Increase over period of record: 21% Little Rouge River near Reesor Rd. p = 0.140 (not statistically significant) 3.00

2.50

2.00

1.50

1.00 Discharge(m3/s)

0.50

0.00 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

Average Annual Discharge Water Survey of Canada Station 02HC013 Increase over period of record: 148% Highland Creek near West Hill p = 2.3 x 10 -7 (statistically significant) 3.00

2.50

2.00

1.50

1.00 Discharge(m3/s)

0.50

0.00 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

5-8

Analysis indicates that there has been a statistically significant increase in flows in the Rouge River since 1962, according to the Kendall rank correlation method (Helsel and Hirsch, 2002), with a probability of 0.003 or 0.3% that the increase observed is the result of random chance. The typical maximum p-value attributed to statistical significance is 0.05 or 5%. In a linear sense, average flow in the Rouge River has increased by 55% over the period of record, or approximately 1.3% per year. Flow in the Little Rouge River has not shown a statistically significant increase; although a linear average suggests an increase of 21% over the period of record, the probability that this trend is due to random variability is 14%, well in excess of the significance criterion. The presence of a much larger and statistically significant increase in flows in the Rouge River is an indication of the effects of urbanization in this part of the watershed on the hydrologic water budget. The impervious surfaces created as a result of development in Richmond Hill and Markham over the past few decades have reduced infiltration and evapotranspiration, causing more of the precipitation that falls on the landscape to runoff and enter streams. In contrast, the absence of a significant increase in flow in the Little Rouge River reflects the relative absence of development in this subwatershed over the past 50 years and therefore little disturbance to the water budget. Analysis of flow data from other undeveloped watersheds in the TRCA jurisdiction shows a similar lack of trend.

The flow data from the Rouge and Little Rouge Rivers can be contrasted to the data from Highland Creek; the Highland Creek watershed is similar in size to the Little Rouge River subwatershed, but approximately 80% of the total area is currently taken up by urban development (Aquafor Beech Ltd., 2003). The Highland Creek watershed is generally regarded as one of the most altered and degraded in Canada, and is often cited as a case example of the hydrologic, ecological and geomorphic impacts that result from watershed urbanization. Rapid development throughout the watershed over the past 50 years has resulted in a clear and statistically significant trend of increasing average annual flows, with a linearly averaged increase of 148% over the period of record, in addition to any increases that would have resulted from the development that had already occurred in the watershed prior to the installation of the gauge. This trend is similar to that currently being experienced on the Rouge River watershed, although the rate of increase is greater due to the rapid pace of urban growth in the Highland Creek watershed. However, future increases on Highland Creek watershed will be limited as the watershed is almost fully developed, while increases on the Rouge River watershed can be expected to continue unless mitigated, given that substantial future development is proposed in the north and west portions of the watershed.

Urbanization in the main Rouge River portion of the watershed has also had a significant effect on seasonal flow patterns. Figure 5-3 compares the total seasonal flow volumes for the Rouge River, Little Rouge River, and Highland Creek for two ten year periods, one beginning in 1964 and the other in 1987. From the period 1964 to 1973, the majority of flows in all three watercourses occurred in winter (January to March) and spring (March to June), primarily during the freshet caused by late winter and early spring snowmelt that typically occurs in March and April. However, in the more recent period, summer flows make up a much larger proportion of total annual flows in the Rouge River and Highland Creek, while the seasonal distribution in the Little Rouge River remained essentially unchanged. The increased summer flows on the Rouge River and Highland Creek are a reflection of much greater runoff generation in urban areas during summer rainfall events, as the impervious surfaces in urban areas eliminate the vegetation and surface soils that in undeveloped conditions are at their most 5-9 effective during the summer in utilizing water for infiltration and evapotranspiration. This phenomenon is well documented in urban areas; for example, Hollis (1975) noted that even low levels of impervious cover between 5% and 10% can cause stream flows in response to moderate rainfall events to increase by an order of magnitude. In contrast, flows in winter and early spring periods are much less affected by urbanization as infiltration and evapotranspiration are minimal due to frozen or saturated ground and dormant or absent vegetation.

Figure 555-5---3333:::: Seasonal Flow Patterns: Rouge and Little Rouge RiRiversvers and Highland Creek

Percentage of Total Annual Flows By Season

50%

40%

30% Winter Spring 20% Summer Fall 10%

0% Rouge L. Rouge Highland Rouge L. Rouge Highland 1964-1973 1987-1991,1994-1998

Percent of Total Annual Flows in Winter/Spring vs. Summer/Fall

80% 70% 70% 71% 70% 60% 62% 64% 50% 54% 40% 46% Winter+Spring 30% 38% 36% Summer+Fall 30% 29% 30% 20% 10% 0% Rouge L. Rouge Highland Rouge L. Rouge Highland 1964-1973 1987-1991,1994-1998

Figure 5-4 illustrates the upward trend in summer flow volumes on both the Rouge River and Highland Creek in contrast to the relatively unchanged summer flows on the Little Rouge River. Recorded flow data for the Rouge River and Highland Creek showed statistically significant increases in flow, with linear average increases of 351% and 221%, respectively, over the period of record. The average increase in Little Rouge River was 33% over the period of record, but this trend is not statistically significant and therefore it can be assumed that there has been essentially no change in summer flows. The data indicate that, on the basis of seasonal flows, urban development has caused an increase in summer flows and an alteration in seasonal flow volume patterns on the Main Rouge River at a scale similar to that experienced by Highland Creek, while the absence of development in the Little Rouge River watershed has maintained a generally constant flow regime over a period of forty years. 5-10

Figure 555-5---4444:: Summer Flow Volume Trends, Rouge and Little RougeRouge RRiversivers and Highland Creek

Average Summer Discharge Rouge River, Little Rouge River, Highland Creek

5.00 Rouge River

4.00 Little Rouge River Highland Creek Increase: 351% p = 2.4 x 10 -7 (sig) 3.00 Increase: 221% p = 0.001(sig.) Increase: 33% 2.00 p = 0.9 (not sig.) Discharge(m3/s)

1.00

0.00 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002

In addition to changes in total annual and seasonal flow volumes, urbanization in the Main Rouge River portion of the watershed has also altered the timing and magnitude of flows in the Rouge River and its tributaries in response to individual rainfall events. Figure 5-5 shows the typical flow response of the Rouge River and Little Rouge River at the gauge locations, as well as flows on Highland Creek for comparison. The rainfall event that generated these flows, which have been normalized to the catchment area for each of the respective gauges, was a storm that delivered approximately 33 mm of rain to the Rouge River watershed and surrounding area over the evening of May 10 and the early morning of May 11, 1998. Flows on the Little Rouge River rose very little and descended slowly, reflecting the slower time of travel of runoff to receiving watercourses over the rural land surface as well as the extended release of infiltrated water to streams after runoff processes end. The area-weighted peak flow was about 2.5 m 3/s per 100 km 2. In contrast, flows in Highland Creek rose very quickly to an area- weighted peak flow over 15 times greater than that on the Little Rouge River and then fell rapidly, reflecting the large amount of uncontrolled runoff being generated on the paved surfaces of the watershed, the rapid rate of delivery of that runoff to watercourses via storm sewers, and the near absence of infiltration. The response of the main Rouge River falls somewhere between the Little Rouge River and Highland Creek; the rate of rise is more rapid than on the Little Rouge and the area-weighted peak flow is over 6 times greater, but the rate of rise is slower and the tail of the hydrograph is much more drawn out than on Highland Creek. The response of the Rouge River is the result of a number of influences; the relatively rapid rate of rise and high peak flow relative to the Little Rouge River reflects the substantial urbanization that had occurred by 1997, while the rural areas in the headwaters and the overall size of the watershed have a moderating effect and result in lower overall peak flow and slower response relative to Highland Creek. The effects of stormwater management on newer development are also reflected as runoff is detained and released slowly, resulting in an extended period of elevated flows lasting for several days after a storm.

The storm described in Figure 5-5 is typical of the type of rainfall event that causes the most hydrologic change in an urbanizing watershed. Hydrologic response to moderate-sized storms such as this one are most affected by urbanization, particularly during the growing season 5-11 because in undeveloped conditions the majority of the rainfall from these storms infiltrates, evaporates, or is transpired by vegetation, and therefore volume of runoff generated from urban areas can be up to an order of magnitude greater than rural ones (Schueler, 1995). During larger storms, the proportion of runoff from rural catchments is much larger as infiltration and evapotranspiration mechanisms are eventually overcome by the volume of precipitation and therefore the relative impact of urbanization is reduced, although still significant. Similarly, as noted above, storms or snowmelt events that occur while the ground is frozen and/or while vegetation is dormant are not as significantly affected by urban development. Therefore, in seasonal climates such as Southern Ontario, flows during the summer (as well as late spring early fall) are the most dramatically increased by urbanization, as illustrated in Figure 5-3 and Figure 5-4 as a result of changes in hydrologic responses to these types of typical moderate rainfall events.

Figure 555-5---5555:: AreaArea----WeightedWeighted Rainfall Event Response: RouRougege and Little Rouge Rivers Compared with Highland Creek

Area-Weighed Flow - May 10-11 1998 Event, Rouge River, Little Rouge River, Highland Creek 50 Rouge Little Rouge Highland 40 ) 2 30 /s per 100per /s km 3 20 Discharge(m

10

0 9-May-98 11-May-98 13-May-98 15-May-98 17-May-98

In addition to the impacts of human activities, watershed hydrologic response is also affected by catchment shape and size, vegetation, soils, geology, and slope. While it is possible that some of the differences in the responses of the Rouge River, Little Rouge River and Highland Creek watersheds are the result of variability in these characteristics, historic information suggests that urban development is the dominant control on flow volume and peak flow responses to rainfall events. Figure 5-6 compares area-weighted summer daily flow data from for all three watersheds for 1965 and 1997, two years with roughly equivalent summer rainfalls. For comparison, the approximate daily rainfall associated with various storm events is displayed. In 1965,,,, area-weighted flows at the Main Rouge River gauge, with a relatively 5-12 undeveloped upstream catchment, are typically lower after rainfall events than in the Little Rouge and in many cases the peak flows occur later than those in the Little Rouge River, contrary to the current response shown in Figure 5-5. In many cases, neither watercourse shows any significant response after rainfall events of 15 mm or less. In contrast, the Highland Creek watershed had already experienced significant development in 1965 and already exhibited a much more dramatic flow response than the Rouge or Little Rouge Rivers. By 1997, a major portion of the Main Rouge River catchment had been developed, resulting in area-weighted flows of the Rouge River that were significantly greater than in the Little Rouge River and much larger in proportion to the magnitude of individual rainfall events than in1965. The Main Rouge River also exhibited appreciable flow responses to small events of 15 mm by 1997. In Highland Creek, continued urban development resulted in even more dramatic hydrologic responses. The changes in response of the Main Rouge River and Highland Creek to these smaller events and the absence of change on the Little Rouge River can be confirmed statistically through assessment of flow-frequency characteristics. For example, there have been statistically significant increases in the 10 th percentile daily flow, or the flow that is exceeded 10% of the time (36 days of every year) on the Main Rouge River and Highland Creek, while there has been no statistically significant trend on the Little Rouge River. Assuming a linear trend, the 10 th percentile flow on the Main Rouge River has increased by over 50%, from 2.8 m 3/s to 3.9 m 3/s over the period of record. In comparison, the increase on the more highly urbanized Highland Creek has been greater than 200%, from 1.1 m 3s/ to 3.6 m 3/s.

It is difficult to assess the historic impacts of urbanization on individual event response on the scale shown in Figure 5-5 because hourly data is required, while the historic flow data is generally only available in daily format. However, from the information presented in Figure 5-6 it is reasonable to conclude that the pre-development hydrologic response of the Main Rouge River was similar to the historic and current response of the Little Rouge River. Urbanization has almost certainly decreased the response time of the Rouge River and it is obvious from the available information that rainfall events now generate much more runoff and watercourse flow in the Main Rouge River, in a pattern similar to highly urbanized watersheds such as Highland Creek. The absence of such an increase in the Little Rouge River further confirms that the change on the Main Rouge River is the result of urban development impacts.

Currently, new developments in the Rouge River incorporate stormwater management practices that are designed to mitigate the impacts of development to the surface water flow regime and to water quality. Of the approximately one third of the Rouge River watershed land area that is currently urbanized, about 22% was constructed prior to the advent of stormwater management controls and 18% was constructed with older end-of-pipe stormwater management facilities (ponds) that only provided a flood control function. The remaining 60% consists of newer development constructed since the early 1990’s, incorporating stormwater management end-of-pipe facilities for flood, water quality and some degree of erosion control. The performance of these various types of stormwater management facilities in mitigating impacts to flooding are not ell understood and are difficult to establish as flood events are infrequent and unique. There is also evidence that more modern facilities that address erosion control through extended detention may not be sufficient to maintain important aspects of the natural flow regime and to mitigate the impacts of development on erosion. Detailed discussion of this issue is also provided in the Fluvial Geomorphology state of the watershed characterization in Section 7.0. 5-13

Figure 555-5---6666:: AreaArea----weightedweighted daily flows for summer 1965 and summer 19971997

Area Weighted Daily Flow - Summer 1965 8.00 Rouge

) Little Rouge 2

6.00 Highland /sper100km 3 50 mm 4.00 35 mm 35 mm 25 mm 20 mm 2.00 50 mm 15 mm Area-Weighted Flow (m Flow Area-Weighted

0.00 1-Jun 1-Jul 31-Jul 30-Aug 29-Sep 29-Oct

Area Weighted Daily Flow - Summer 1997 8.00 Rouge 15 mm Little Rouge ) 2 Highland 6.00 /sper 100km

3 30 mm 4.00 15 mm 25 mm 30mm 15mm 15 mm

15 mm 2.00 Area-Weighted Flow (m Flow Area-Weighted

0.00 1-Jun 1-Jul 31-Jul 30-Aug 29-Sep 29-Oct

Based on the magnitude of impact to surface flows already observed under existing conditions and the potential for additional impacts as a result of ongoing development, an objective of “to protect and restore natural variability of annual and seasonal stream flow” was adopted for the Rouge River Watershed Plan.

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This is described below, with the applicable indicators, measures, targets and a rating for existing conditions in the watershed.

Objective: PProtectrotect and restore the natural variabilityvariability of annuaannuall and seasonal Overall Rating stream flow. Fair

Indicator Measure Target

Streamflow timing and Average annual and seasonal flow volumes Maintain existing annual magnitude (m 3/yr) and seasonal flow volumes* * per long-term Water Survey of Canada gauge measurements and additional gauges recommended for installation.

The rating for this objective reflects the existing condition of the watershed where the historical pattern of conversion of rainfall to runoff has been altered significantly in developed areas and the surface flow regime increasingly resembles that of a degraded urban system. However, compared to other watersheds within the TRCA jurisdiction, the Rouge River watershed is ranked fair as substantial portions of the watershed remain undeveloped and some subwatersheds, in particular the Little Rouge River, continue to exhibit a more natural response. As such, there is an important opportunity to arrest further degradation by minimizing the impact of future development and in some cases, to restore a more natural surface flow regime in developed areas or areas downstream of significant development through retroactive application of better water management practices.

5.4.2 Baseflow

Summer baseflow data measured at 126 locations; 92 sites in 1996 and 67 of the sites resampled in 1997, with an additional 34 sites (Hinton, 1996/97). The majority of these sites and an additional 55 sites were sampled in late spring and summer of 2002 (TRCA, 2003). These data provide the basis for an assessment of baseflow conditions throughout the watershed. Figure 5-7 illustrates the locations of the 181 baseflow sampling sites. The findings contribute to an understanding of the influence of surficial geology on the ground and surface water interactions in the watershed and the impacts of water withdrawals.

Several factors influenced analysis of the baseflow data. The summer sampling period necessary to define baseflows coincided with the period when many water users are known to be active in removing surface flows from this system for local activities such as irrigation and livestock watering. The active de-watering at 16th Avenue to facilitate the construction of a major trunk sanitary sewer was also impacting baseflow levels in the surrounding tributaries during the 2002 survey. The size of the Rouge River watershed relative to the staff resources meant that data were collected over a period from May through August, such that seasonal baseflow differences needed to be considered within the data set. These challenges in data interpretation were addressed to the extent possible in the analysis.

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Figure 555-5---7777:: Location of Baseflow Sampling

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Baseflow fluctuates highly throughout the year, with spring and fall baseflows found to be two to three times higher than those during the midsummer months (Figure 5-8). These seasonal fluctuations are a natural function of a watercourse which was confirmed through an analysis of daily base flow data at stream gauges located on the Main Rouge River at 14th Avenue and the Little Rouge River at 10th Line.

The identification of groundwater recharge and discharge zones along the watercourse is important when considering the potential local or regional effects of land use changes and water use. Analysis of the relative gains and losses in baseflow from one sampling station to the next gives an indication of the stream reaches where ground-surface water interactions are more pronounced. Significant increases in baseflow were observed along the lower areas of the South Slope, coincident with the outcrop of the shallow Oak Ridges Moraine (ORM) aquifer. This upper aquifer was found to contribute between 40 to 80% of total baseflow in individual tributaries of the Rouge River. Groundwater inputs were especially noticeable in the Upper Main Rouge, Little Rouge River and Bruce Creek.

Figure 555-5---8888:: Summer Mean Monthly Baseflow from Hydrograph SepSeparationaration

Summer Mean Monthly Baseflow 1961 - 2004 Little Rouge @ Locust Hill (02HC028)

0.35

0.30.30.3

0.25 0.20.20.2

0.15

Discharge(m3/s) 0.10.10.1 0.05

000 May June July August September

Summer Mean Monthly Baseflow 1963 - 2004 Rouge River @ Markham (02HC022)

0.80.80.8 0.70.70.7 0.60.60.6

0.50.50.5

0.40.40.4

0.30.30.3

Discharge (m3/s) Discharge 0.20.20.2

0.10.10.1 000 May June July August September

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Figure 5-9 shows the change in baseflow volume normalized to stream length for each stream reach. This analysis assumes that any changes to baseflow volume occur in a linear fashion, equally distributed along the stream length. The linear distribution assumption enables quick identification of potential recharge/discharge zones which can be investigated further. The major geologic influences of both the ORM and the Iroquois Shoreline are well defined in this figure.... Baseflow derived from the ORM aquifer are shown as the blue reaches in the northwest area of the upper Rouge River, Main Bruce Creek and in the upper Little Rouge Creek. These inputs range from 5-20 l/sec/km. The red reaches at the southern portion of the watershed display the downward movement, or loss, of surface water into the porous Iroquois Shoreline. These geologic influences are echoed in other TRCA watersheds that have similar geological characteristics (TRCA, 2003). The results also validate groundwater discharge locations and volumes estimated in the groundwater modelling study, described in Chapter 4. Most differences between the observed and modelled conditions can be explained by known water withdrawals or interferences in the natural system.

The central portion of the watershed has several stream segments with no data displayed, because of high levels of uncertainty in the data. Uncertainties in the results for these streams were due to seasonal discrepancies in the data set or effects of the de-watering activities associated with the 16th Avenue trunk sewer construction. Therefore their observations were not representative of natural conditions in the Rouge River watershed and thus were omitted from this figure. Further baseflow sampling will be required to characterize normal conditions in these areas.

Significant baseflow losses were observed in one section of the Little Rouge River south of Elgin Mills Rd. These losses are not directly due to the 16th Avenue de-watering activities or seasonal influences. This significant reduction in baseflow was measured in this reach in both 2002 and 1997. An initial review of the Ontario Ministry of Environment (MOE), Permit-To-Take- Water (PTTW) database indicated that there are 15 water use permits in the upper west portion of the Little Rouge River watershed, but unfortunately only a few of these water users were accounted for in TRCA’s 2003 field water use assessment study (TRCA, 2005). Further water use surveys and groundwater monitoring are underway to ascertain the causes of these losses.

A seasonal fluctuation study has been initiated by the TRCA where seasonal baseflow is being extracted from hydrographs of all long-term stream gauges within the Rouge River watershed. The goal of this study will be to determine the distribution of mean monthly baseflows within the watershed to assist in reviewing impacts of water takings and other watershed management scenarios. To supplement the mean monthly gauge data, a number of indicator sites were selected for baseflow sampling to monitor monthly fluctuations in our watersheds. These sites were chosen as being representative for the watersheds/sub-watersheds in which they are located based upon collected baseflow data. Target volumes and variations of baseflow will be established using historic and current data collected at long-term stream gauge and indicator site locations.

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Figure 555-5---9999:: Baseflow Normalized to Stream Length

5-19

An objective, “to maintain and restore natural levels of baseflow”, was adopted to address life cycle needs of aquatic communities and ensure a sustainable water supply. This is provided below along with the indicators, measures, targets and a rating for existing conditions in the watershed.

Objective: Maintain and restore natural levels of baseflow Overall Rating

Good

Indicator Measure Target

Baseflow Seasonal and annual average Maintain or enhance baseline seasonal baseflow discharge at indicator and annual baseflows*. sites (m 3/sec) * Per TRCA and GSC baseflow measurements (TRCA, 2003; Hinton, 1996/97)

An overall rating of “Good” for surface water use reflects the general level of impact of water users to the aquatic and terrestrial ecosystems reliant on the Rouge River watershed. Preservation of the existing baseflow volumes is the primary target for management of water users. A threshold value of 60% of mean summer baseflow is currently used to define minimum environmental flow to maintain existing aquatic health. Conservation Ontario and the MOE are conducting studies in an attempt to develop an “environmental flow” threshold level for surface water.

Surface Water Use Assessment

A complete analysis of the surface water flows and baseflows and the development of a water budget cannot fully take place without a sound knowledge of current surface (and ground) water uses. To locate and identify the various regulated and non-regulated water users within the Rouge River watershed, data were collected from three sources. The OMOE PTTW Program regulates all water use of amounts in excess of 50,000 litres/day for purposes other than livestock watering, fire control and domestic use (OMOE, 2004). To locate and verify both regulated and non-regulated uses under the PTTW Program, the TRCA initiated a water use assessment survey within the Rouge River watershed, and utilized a previous study completed by Golder Associates and Marshall Macklin Monaghan Ltd. for the Regional Municipality of York, (2003). Through the Water Use Assessment study, the TRCA located and identified 72 individual water users. Domestic, self supply water use was not within the scope of this assessment however, data were collected regarding municipal use. The amalgamated data from both studies were used to develop a PTTW and water use database for the Rouge River watershed (TRCA, 2005). This new database indicates there are 116 individual water takers within the Rouge River watershed () with almost an even split between surface water and groundwater users.

5-20 Table 555-5---1111:: Known Water Abstractions by SectorSector----RougeRouge River, 2006

Total Annual Withdrawals by Source (m3 / year) Percent by Purpose Number of Users Percent of Users Surface Ground Both Unknown Total Sector Aesthetics 27 23% 188,534 41,896 - 52,370 282,800 7.2% Aggregate Washing 2 2% - 461,192 - - 461,192 11.8% Aquaculture 2 2% - 1,612,059 - - 1,612,059 41.1% Field and Pasture Crops 4 3% 26,525 - 8,176 26,492 61,193 1.6% Flood Control 2 2% 63,072 - - - 63,072 1.6% Golf Course Irrigation 21 18% 435,451 457,724 208,111 - 1,101,286 28.1% Heat Pumps 1 1% - - - - - 0.0% Livestock Watering 18 16% 569 8,595 74,491 1,139 84,794 2.2% Market Gardens / Flowers 2 2% 300 - 1,526 - 1,826 0.0% Nursery 15 13% 13,191 90,530 85,152 7,791 196,663 5.0% Other - Agricultural 8 7% 19,222 6,336 8 5,724 31,290 0.8% Other - Commercial 1 1% - - - 19,080 0.5% Other - Industrial 1 1% - 2,650 - - 2,650 0.1% Other - Miscellaneous 4 3% 178 322 - - 501 0.0% Sod Farm 1 1% - - - 2,059 2,059 0.1% Unknown 7 6% - - - 530 530 0.0% 116 Total 116 766,122 2,681,305 2,681,305 377,464 377,464 96,104 96,104 3,920,995 3,920,995 Self Supply Domestic 10,066 642,953 642,953 not included (Potable) Municipal (Potable) 6 2,034,145 2,034,145 not included Source: TRCA, 2005; Golder Associates and Marshall, Macklin, Monaghan Ltd., 2003; Beatty and Associates, 2003; Gartner Lee Ltd., 2004.

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General statistics were calculated to describe distributions, sources, and volumes of water users within the Rouge River watershed. Of the 116 users, 34% rely solely on a surface water source. Groundwater takings comprise 37% and those taking both surface and groundwater represent 12%; the remaining 16% could not be confirmed, and their use is currently unknown. With more than one third of the total water users withdrawing from surface water sources, there is a potential for stress to aquatic habitat and communities. This is demonstrated in the Little Rouge River where the usage of a single surface water user, when withdrawing water, was found to be more than 40% of the available baseflow in the creek.

A total of 57 individual water users were identified who either do not fall under PTTW regulations, or who have not obtained a PTTW permit. Although these unregulated users comprise nearly one half of the number of water users in the Rouge River watershed, they account for approximately 11 % of the total volume of water withdrawals cumulatively. Of these 57 non-regulated users, only one was identified to be withdrawing for purposes / amounts requiring a MOE PTTW permit, without holding a valid permit. An analysis of information completed for the current study showed a considerably higher number of un- permitted withdrawals, however recent changes to the PTTW regulations (O. Reg. 387/04), and related public outreach has brought many previously un-permitted takers into compliance.

When considering water use by sector, it was found that the majority of permitted water uses are for aesthetics, golf course irrigation, and livestock watering. Aquaculture operations were found to be removing the largest quantities of water among all permitted users; this was followed by golf course irrigation and aggregate washing respectively. This breakdown by sector is an important tool when assessing overall water use within a watershed. Not only does it show where increased conservation practices would provide the largest benefit, but the consumptive factor of these uses can vary significantly. The “consumption factor” refers to that part of water withdrawn that evaporates, transpires, or is otherwise incorporated into products that do not return to the streams or groundwater source immediately after use. Irrigation is a highly consumptive use of water, where the loss rates can range from 70% - 90% of withdrawn amounts; in contrast, industrial use consumption is on average 10% of withdrawn amounts (Conservation Ontario, 2003; Golder Associates and Marshall, Macklin, Monaghan Ltd., 2003).

Water use budgets were created based upon the amalgamated data and TRCA baseflow measurements (Table 5-2). These budgets provide a general overview of the Rouge River watershed, and indicate subwatersheds that are under some level of stress due to water use. Overall water use on a watershed scale represents approximately 3.2% of the available baseflow. As seen in Table 5-2, the Bruce Creek has the highest potential for stress at 9.2 %. However, these water budgets reflect an overall rate of withdrawal for the entire watershed and subwatershed. When considering smaller tributary watercourses, the percentage of baseflow removed may reflect a significantly higher amount of the baseflow at the actual withdrawal location. For example, in the Little Rouge River the overall subwatershed level of withdrawal represents only 6.4 % of available baseflow; however, when focusing in on the upper reach of the River north of Major Mackenzie Drive a major local withdrawal of 64% of baseflow was measured in both 2002 and 1997 (GSC, 1996/97) in this location. There are known to be several active and expired PTTWs in these headwaters which cumulatively are the likely cause of this loss. Through TRCA Field Surveys, and the amalgamation of similar datasets, 47 individual water users were identified in the headwaters of the Little Rouge River. While only 13 of these users rely on surface water sources, their cumulative withdrawals amount to approximately 118 l/s; almost 75 % of this measured withdrawal.

5-22 Table 555-5---2222:: Total Water Abstractions by Subwatersheds --- Rouge River, 2006

Total Annual Withdrawals by Source (m3 / year) % of Measured Measured Number of Percent of ToToTotalTo tal Annual Discharge Subwatershed Surface Ground Both Unknown Baseflow Users Users Withdrawals (m3) (surface Discharge (m3/y) takings) Berczy Creek 9 8% 47,596 30,000 27,532 - 105,129 804,168 5.9% Bruce Creek 20 17% 139,389 269,108 132,217 5,724 546,554 1,523,189 9.2% Little Rouge 61 53% 235,369 2,126,213 176,907 84,351 2,622,840 3,689,712 6.4% Lower Rouge 8 7% 252,237 4,168 - 2,059 258,464 20,372,256 1.2% Middle/Beaver 8 7% 40,253 162,787 - 76 203,115 15,673,392 0.3% Upper Rouge 10 9% 51,277 89,029 40,808 3,895 185,009 3,778,013 1.4% 116Total 116 766,122 766,122 2,681,305 2,681,305 377,464 377,464 96,104 96,104 3,920,995 3,920,995 24,061,968 24,061,968 3.2% 3.2%

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Conservation practices should be an increased priority for high consumptive users, to keep the necessary water volumes in the local system be it surface or groundwater. As well as protecting and maintaining baseflow volumes, a secondary target for surface water use is the removal of all online surface water takings from the Rouge River. Large volume online water withdrawals facilitated by dams and weirs across watercourses are a major contributor to baseflow losses. There are currently 44 surface water users in the Rouge River system, 23 of which are not using online structures to facilitate their withdrawals; or that use permanent online structures that exist for flood protection purposes. Three users are currently operating with fixed intake systems, which protect instream flows. Of the remaining online users, many larger operations are working with the TRCA and the MOE to remove their online structures, as protection of minimum instream flows is now a requirement of the PTTW Regulations.

Objective: Maintain and restore natural levels of baseflow (co(cont’d)nt’d) Overall Rating

Good

Indicator Measure Target

Surface Water Seasonal baseflow volumes at • Maintain or enhance baseline Withdrawals withdrawal locations (Litres / seasonal and annual baseflows. 1 Second)

Number of online vs. offline • All surface water users offline from surface water withdrawals watercourse.2

1 Per TRCA and GSC baseflow measurements (TRCA, 2003; Hinton, 1996/97) 2 Per TRCA and MOE water use databases (MOE, 2002; TRCA, 2005)

Flooding

Flooding is a natural component of any watershed’s hydrologic response. The Rouge River, like all watercourses, regularly experiences streamflows of sufficient size to overtop its banks. Early accounts of flooding in the Rouge River watershed date back to the early 1870s. The most serious flooding in the recorded history of the Rouge River watershed occurred in October of 1954 as a result of Hurricane Hazel which brought approximately 210 millimeters of rain to the Toronto region in less than 12 hours. Historic flow records indicate that most flooding in the main Rouge and Little Rouge Rivers has occurred either in the late winter or early spring, as a result of snowmelt or rain-on-snow events and in some cases is exacerbated by ice jams, as described in additional detail below. However, flooding has also occurred as a result of spring or summer thunderstorms or late fall tropical storm events. The largest flood event recorded since data collection began on the Main Rouge River in 1962 was as a result of a major thunderstorm in May 2000.

During the Hurricane Hazel flood, the Rouge River destroyed or displaced over 125 homes and buildings, destroyed 11 bridges in Markham Township, washed out over three kilometers of roads and flooded both the east and west approaches to the village of Markham, leaving the 5-24 village marooned. As a result of the destruction caused by Hurricane Hazel, the 1959 Plan for Flood Control and Water Conservation (MTRCA, 1959) identified the need for 15 large control dams as well as four major flood control channels within the TRCA. Among the dams identified in the plan was the Milne Dam which was constructed on the Rouge River in Markham in 1973. More importantly the Plan identified the acquisition of lands in the lower Rouge Valley as one of the most effective means of reducing flood risks. This recommendation led to the assembly of a significant area of lands in public ownership, which laid the foundation for the establishment of Rouge Park.

In Ontario, policy established by the Ontario Ministry of Natural Resources (OMNR) requires that a regulatory flood event be used to define the floodplain area that is potentially subject to flooding and in which development should be prohibited or strictly managed. In south central Ontario, the regulatory flood is the greater of the flows expected to result as the outcome of either the 100-year design rainfall event or from Hurricane Hazel in reflection of the extensive damage caused by Hazel and the potential for such an event to be repeated (OMNR, 2001). In accordance with this policy, the TRCA has established a flood management program which has the objective of minimizing or preventing the risks associated with flooding through the protection of valley and stream corridor systems, the restriction of any development activities in floodplain areas necessary to convey flood flows and the operation of a Flood Forecasting and Warning program. As a result of this program, the Authority has been largely successful in the past fifty years in ensuring that new development does not occur within regulatory floodplains, and has also acquired many flood prone properties and constructed flood remedial works. As a result, the risk to life and property from flooding in the Rouge River watershed has significantly decreased since the time of Hurricane Hazel despite the extensive development that has occurred within the Towns of Richmond Hill, Markham, Whitchurch-Stouffville and the City of Toronto.

Despite major strides in the reduction of flood risk over the past half-century, a significant number of flood-vulnerable roads and flood vulnerable areas (properties and buildings) and roads remain in the Rouge River watershed, as shown in Table 5-3 and Figure 5-10. The majority of these are located within the Town of Markham, along the Main Rouge River, Bruce Creek, and Fonthill tributary on lands that have been designated Special Policy Areas (SPA) by the Government of Ontario. The SPA designation recognizes the social and economic significance of historic urban areas that were constructed in flood-prone locations prior to the development of flood hazard policy. As such, a certain amount of new development or redevelopment is permitted in these areas, subject to stringent conditions. The relatively flat topography and the lack of valley definition of the Rouge River and its tributaries as the river flows east-west through the Town of Markham make this area particularly vulnerable to flooding. Over 500 flood vulnerable areas within Markham would be expected to be inundated during the Regional Storm (Hurricane Hazel) and a number of roads and homes are predicted to flood in flows as low as the 2-year flood event. Table 5-3 indicates the number of flood vulnerable areas and roads in each municipality within the Rouge River watershed that are prone to flooding in various return period flood events.

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Table 555-5---3333:: Number and Flood Frequency of Watershed Flood VuVulnerablelnerable Areas and Roads Flood Frequency Flood Vulnerable Areas 2-year 5-year 10-year 25-year 50-year 100-year Regional Regional Markham 7 32 19 23 28 49 533 Toronto 1 1 1 1 1 1 6 Richmond Hill 1 1 1 1 1 2 13 Whitchurch-Stouffville 11 13 14 14 15 15 21 Total 202020 474747 353535 393939 454545 676767 573

Flood Vulnerable Roads Markham 4 11 18 27 30 38 129 Toronto 0 0 0 0 0 0 6 Richmond Hill 4 5 6 7 7 7 10 Whitchurch-Stouffville 2 3 3 4 4 4 6 Total 444 111111 181818 272727 303030 383838 129

To avoid increasing the total flood risk, new development or redevelopment within the Town of Markham SPAs is subject to minimum floodproofing standards and the prediction and management of flooding in this area is a significant focus of the TRCA flood warning program. Flood vulnerable areas exist in other areas of the watershed, although these have not been designated as SPAs and are strictly regulated with no new development permitted within the regulatory floodplain. One area of particular significance is the Glen Rouge campground, which is located just below the confluence of the Rouge River and Little Rouge River at Highway 2 immediately adjacent to the river and is estimated to be susceptible to flooding as soon as the river overtops. No history of problems exists as flooding in this area is generally the result of spring runoff and ice jams when the campground is not open. However, major rainfall events in summer or fall could present a significant hazard and this area is closely monitored.

As discussed previously, urban development and the associated creation of impervious surfaces results in an increased volume of runoff and streamflow as a result of rainfall events. This effect can increase flood flows downstream of developed areas and significantly increase flood risk. In recognition of this issue, in the late 1970’s and early 1980’s the TRCA and its constituent municipalities began to require that development within the Rouge River watershed incorporate end-of-pipe stormwater reservoirs to detain excess runoff so that it would not increase peak flood flows downstream. This approach continues to be used for mitigating the impacts of new development on flood risk, although the original end-of-pipe reservoirs have evolved into the currently employed wet ponds and wetlands that provide water quality and erosion benefits in addition to flood control.

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Figure 555-5---11110000:: Flood Vulnerable Sites and Special Policy Areas

5-27

It is difficult to determine, based on empirical data, whether the frequency or magnitude of flooding has changed over time as a result of either changes in climate or urban development. As flood events occur relatively infrequently, (on average about every 1-2 years) and major floods even less often, statistical trends cannot be established unless there are dramatic changes or very long periods of record. As a result, for the relatively short 40 years period that flows have been consistently measured on the Main Rouge River and Little Rouge Rivers no, statistical upward or downward trend is present in the annual maximum instantaneous peak flood flow. However, there are statistically significant increases on the Main Rouge River of high flows that would not cause flooding but that would be very near to spilling over the banks. These occur more frequently and therefore it is easier to establish statistical trends. Similar but more dramatic trends in these types of flows have been observed on Highland Creek, which has also experienced large and statistically significant increases in flood flows. It can therefore be inferred that flood magnitude and frequency may be increasing the Rouge River, but that changes have not occurred quickly enough for them to be observed in the current historical record. The Little Rouge River and other nearby rural watersheds have not experienced statistically significant increases in these near bank full flows, suggesting that the increases observed on the Main Rouge River and Highland Creek are the result of urbanization and not of climatic variability. Indirectly, the above evidence suggests that flooding has likely increased in magnitude and frequency on the Main Rouge River but that the effects cannot yet be observed with the readily available data. However, the extent to which this has occurred and the efficacy of stormwater management in mitigating the impact to flood risk of more recent development would require much more extensive analysis of archived historic data, which may be difficult to obtain or may not exist.

A key tool in managing flood risks is the TRCA Rouge River hydrology model (Marshall, Macklin, Monaghan, 2002), which is used to estimate peak flows in the Rouge River and its tributaries that would be caused by the Regional Event (Hurricane Hazel) and return period storms (2, 5, 10, 25, 50 and 100 year return periods). The resulting flows are used in hydraulic modelling to generate the regulatory floodline maps for the watershed (Clarifica, 2006) and to allow identification of the location and vulnerability of flood sites within the watershed. To generate a conservative regulatory floodline, the mapping is based on a scenario in the hydrology model that reflects a developed condition for the Rouge River watershed which would result in slightly higher Regional Event flows. The Rouge River watershed hydrology model is periodically updated to improve calibration as additional data becomes available and to reflect changes in land use. It is also used in the development review process to simulate the potential effects of new development and to design stormwater management facilities to prevent any increase in peak flows. The importance of flood control for new development was illustrated in the Rouge River hydrology model study, in which a scenario of complete watershed development was modelled without flood flow controls, resulting in significant increases in return-period peak flows that in some locations exceeded 100% (see ). Other scenarios confirmed that the ongoing use of stormwater management end-of-pipe facilities for peak flow control continues to be an effective means of preventing increases to peak return- period flows downstream.

As noted above, ice jams can occasionally cause or exacerbate flooding during winter and early spring melt periods in the Rouge River watershed. Ice jams most commonly occur on the Little Rouge River at the 9 th Line Crossing and on the main Rouge River near the Glen Rouge 5-28

Campground. However, ice jams at these locations rarely result in threats to inhabited areas as the river valleys are deep and well defined. Historically, ice jams have rarely occurred in inhabited flood-vulnerable areas such as the Town of Markham SPA’s.

Based on existing conditions, an objective to “Eliminate or minimize risks to human life and property due to flooding and erosion” was adopted to encourage initiatives for ongoing reduction of flood risk in the Rouge River watershed as well as the continuation of existing flood management policies and programs. Erosion related risks are identified here, in consideration of shared management approaches; however erosion is also discussed further in Chapter 7 (Fluvial Geomorphology). This objective is indicated below, along with indicators, measures, targets and a rating for existing conditions in the watershed.

Objective: Eliminate or minimize risks to human life and propepropertyrty due to Overall Rating flooding and erosionerosion....

Good

Indicator Measure Target

Peak flow Peak flow rate Maintain or reduce existing peak flows (2-100 year events)1

Water level Water level (flood lines) Maintain or reduce baseline flood levels 2

Flood vulnerable Number of flood vulnerable Maintain or reduce existing flood vulnerable areas and roads areas and roads areas and roads 3

Ice jams Number of ice jams Monitor the formation of ice jams 4 1 Per Rouge River Watershed Hydrology Model Update (Marshall, Macklin, Monaghan, 2002) 2 Per Rouge River Watershed Floodplain Mapping Updates (Clarifica, 2006; Burnside, 2007) 3 Per TRCA Flood Vulnerable Area / Flood Vulnerable Roads Database (TRCA, 2007) 4 Per qualitative observations of ice jams by TRCA and others

An overall rating of ‘Good’ for flooding reflects that the watershed has met some but not all of the targets outlined above and that there is room for improvement. Since the advent of floodplain management policy, practices have been successfully implemented to reduce flood risk in the Rouge River watershed to acquire flood vulnerable properties, restrict development within the regulatory floodplain and to mitigate the effect of new development on peak flood flows and flood levels. However, the watershed continues to contain a number of flood vulnerable areas and roads that require careful management and ongoing development has the potential to significantly increase flood flows and levels if it is not strictly controlled. Further opportunities to reduce flood risk through acquisition of land and remedial works may also still exist and should be identified. While ice jams are not currently a significant contributor to flood risk in the Rouge River watershed and its tributaries, their formation should continue to be monitored in the event that effects of climate variability result in new ice jam locations or increased frequency at existing locations.

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5.55.55.5 Objectives for Surface Water Quantity

Four objectives have been adapted for surface water quantity in the Rouge River watershed:

1. Protect and restore the natural variability of annual and seasonal stream flow. 2. Maintain and restore natural levels of baseflow. 3. Sustainable rates of surface water use. 4. Eliminate or minimize risks to human life and property due to flooding.

For the purposes of this chapter, these objectives and ratings of current conditions are presented in Section 5.4.

5.65.65.6 Summary and Management Considerations

Urban development over the past fifty years in the Rouge River watershed has caused significant changes to the hydrologic water budget, resulting in impacts to the surface flow patterns of rivers and streams. In areas downstream of development, the surface flow regime increasingly resembles those of highly degraded urban watersheds. The historic data suggests that stormwater management practices have not fully mitigated these impacts, although further analysis is required to determine the effectiveness of modern stormwater management measures. Additional management measures beyond those currently employed will likely be required to prevent future development from exacerbating existing impacts to surface hydrology. To increase the level of understanding of the surface water system and our abilities to develop appropriate management solutions, the existing stream flow monitoring network needs to be expanded to provide a sufficient base for monitoring and to ensure targets are met.

Baseflow in the Rouge River watershed is fundamentally linked to the pattern of groundwater recharge and discharge, with the majority of baseflow originating in a few primary areas. It is important to maintain these patterns and protect key recharge and discharge areas to maintain an adequate baseflow regime. On a watershed basis, surface water takings represent a relatively minor impact to baseflows, but in some localized areas permitting takings comprise a major portion of the available baseflow, resulting in major impacts in summer months. The creation by TRCA of a detailed, all encompassing water use database is only a first step. It will be important to keep data management as a high priority, and maintain it by updating permit records, and permit enforcement. This too appears to be the goal of the Ministry of Environment with the onset of new PTTW regulations, studies, and guidelines (O. Reg. 387/04). These new guidelines aim to protect the ecological needs of watercourses, ultimately bringing surface water users offline from the watercourse during low flow periods. It is important to note that encouraging water users to move to groundwater sources is not a solution to the protection of surface water streams and rivers, but merely passes potential problems from one hydrologic system to another, potentially masking the impacts of water use on the overall resources. It is in the best interest of all involved to work with water users to determine the optimum local solution, based on watershed information.

When assessing new PTTW applications, studies have proven that both individual and cumulative impacts need to be addressed. A cumulative impact assessment should be 5-30 undertaken to model worst case scenarios (where all users are withdrawing at the same time) and utilize stream flow information not just from gauging locations, but spot measurements at withdrawal locations, and watershed wide. Cumulative assessments, however, need to be based on realistic withdrawal rate and volumes, as well as permitted maximums. This would provide for both a conservative estimate of remaining allocation, as well as an actual allocation estimate. Through the TRCA Water Use Surveys, it was found that a considerable discrepancy exists between permitted maximum withdrawals and actual typical usage. In general, actual usage on average comprised 30% - 40% of the permitted volumes.

Significant reductions in flood risk have been achieved in the Rouge River watershed since Hurricane Hazel struck in 1954, primarily through the acquisition of property in river valleys and the prevention of new development within the regulatory floodplain. However, a significant number of flood-prone properties and buildings remain, particularly along the Main Rouge River through the Town of Markham and some areas will be impacted by flooding as low as or below the 5-year event. Older urban development upstream of these areas did not incorporate stormwater controls and likely increased peak flood flows and flood risk downstream. Since the late 1970’s and early 1980’s stormwater detention facilities have been incorporated into new development in the Rouge River watershed to prevent increases in peak flows and flood risk. Recent modelling has reconfirmed that such facilities are effective in preventing increases in peak-return period flows in downstream areas. However, the effect of development beyond currently approved Official Plan boundaries has not yet been assessed, and has the potential to dramatically increase flood flows if it is not properly managed. A detailed analysis of potential flood impacts and required mitigative measures for future development and urban expansion should be undertaken. Ice jamming, while not a significant cause of flooding in the Rouge River watershed should continue to be monitored and documented.

Ratings of existing conditions in the Rouge River watershed stand at “Fair” for surface flow regime and “Good” for baseflow maintenance and flood protection. The “Fair” rating for surface flow regime reflects both the negative impacts of development and the consequent shift towards a degraded condition within and downstream of urban areas, as well as the positive condition of the Little Rouge River and other streams within rural or protected areas north of development centers. The “Good” baseflow rating reflects the generally minor impact of water withdrawals on the watershed although there are some significant impacts to headwater areas. The “Good” rating for flood protection reflects the significant reductions to flood risk that have been achieved since Hurricane Hazel but recognizes the remaining flood vulnerable properties and buildings.

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5.75.75.7 RRReferencesReferences

Aquafor Beech Ltd. 2003. Toronto Wet Weather Flow Management Master Plan . Submitted to the City of Toronto. Project 63497.

Beatty and Associates. 2003. Water Use Assessment Study - Regional Municipality of Peel. Prepared for Regional Municipality of Peel.

Burnside and Associates, 2007. Updated Flood Plain Mapping, Selected Areas, Rouge River Watershed. Prepared for TRCA

Clarifica. 2006. TRCA Updated Flood Plan Mapping, Rouge River Watershed. Prepared for TRCA.

Conservation Ontario. 2003. A Framework for Local Water-Use Decision-Making on a Watershed Basis . Government of Ontario. 88 pp.

Environment Canada. 2001. HYDAT CD_ROM. Version 2.01.

Golder Associates Ltd. and Marshall Macklin Monaghan Ltd., 2003. Regional Municipality of York - Water Use Assessment (DRAFT) (incl. Database). Produced for Regional Municipality of York. 29 pp.

Government of Ontario. 2002. Provincial Policy Statements (Section 3.1 Natural Hazards Policy) under the Planning Act.

Helsel and Hirsch. 2002. "Chapter A3: Statistical Methods in Water Resources ." In "Book 4, Hydrologic Analysis and Interpretation." United States Geological Survey.

Hinton M. 1996/97. Baseflow measurement database . Geological Survey of Canada.

Hollis, G. 1975. The effect of urbanization on floods of different recurrence intervals . Water Resources Research 11(3): 431–435.

Marshall, Macklin, Monaghan Ltd. 1988. The Rouge River Urban Drainage Plan Study Phase 2. Prepared for the Metropolitan Toronto and Region Conservation Authority.

Marshall, Macklin, Monaghan Ltd. 2002. The Rouge River Hydrology Update Study . Prepared for the Toronto and Region Conservation Authority.

McLaren, T. F. 1979. Hydrologic Model Study Humber, Don and Rouge rivers, Highland, Duffins, Petticoat and Carruthers creeks . Prepared for the Metropolitan Toronto and Region Conservation Authority.

MTRCA. 1959. Plan for Flood Control and Water Conservation .

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MTRCA. 1980. Watershed Plan - Flood Control Program.

MTRCA. 1990. A Comprehensive Basin Management Strategy for the Rouge River Watershed.

Ontario Ministry of the Environment. 2002. Permit to Take Water Database (2002).

Ontario Ministry of the Environment. 1991. Interim Stormwater Quality Control Guidelines .

Ontario Ministry of the Environment. 2003. Stormwater Management Planning and Design Manual.

Ontario Ministry of Environment. 2004. Water Taking and Transfer Regulation - Ontario Regulation 387/04

Ontario Ministry of the Environment and Energy. 1994. Stormwater Management Practices Planning and Design Manual.

Ontario Ministry of Natural Resources, 2001. Natural Hazards Technical Guides: River and Stream Systems Hazard Limit Technical Guide, in Adaptive Management of Stream Corridors in Ontario, Watershed Science Center, Trent University, Peterborough.

Schueler, T. R. 1995. Site Planning for Urban Stream Protection, Center for Watershed Protection , Ellicott City, MD, http://www.cwp.org/SPSP/TOC.htm.

Shuster, W. D., J. Bonta, H. Thurston, E. Warnemuende, and D. R. Smith. 2005. Impacts of impervious surface on watershed hydrology: A review. Urban Water Journal, Vol. 2 (4), 263–275. 2005.

TRCA. 2003. Low Flow Measurements in the Rouge River Watershed, database.

TRCA. 2003. A Watershed Plan for Duffins Creek and Carruthers Creek.

TRCA. 2005. Water Use Assessment in the Rouge River Watershed, database.

TRCA, 2007. Flood Vulnerable Areas/Flood Vulnerable Roads Database, database.

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