Rouge River State of the Watershed Report
Fluvial Geomorphology
Goal: Natural, stable stream channels and corridors that allow for natural stream flow patterns, support diverse aquatic habitat, limit sediment loadings and protect human life, property and infrastructure from risks due to erosion and slope instability.
Fluvial Geomorphology
Key Findings:
Within and downstream of developed areas, the Main Rouge River and its tributaries are experiencing unnatural rates of erosion and channel change from the hydrologic changes associated with the conversion of rural to urban land uses.
The process of adjustment and instability on the Main Rouge River and tributaries in response to urbanization is only partially complete. On tributaries where no further upstream development will take place, instability may persist for decades. On the Main Rouge River and other tributaries where more development is proposed, existing instability will be exacerbated and will persist longer.
The subwatershed of Little Rouge River has experienced little urban development and therefore the physical stability of watercourses is generally better than in subwatersheds contributing to the Main Rouge River. However, more information than is currently available is required to assess the physical stability and geomorphic condition of main and tributary watercourses throughout the watershed.
Compared to other developed watersheds, watercourses in the Rouge River watershed have experienced relatively little direct physical alteration such as channelization, and the corridors adjacent to watercourses have been protected from development in many areas. As such, there is a greater potential to rehabilitate the physical condition and geomorphic processes of watercourse channels if a more natural hydrologic regime can be restored.
Erosion impacts to infrastructure and private property have been identified as a major concern in Markham and although they have not been investigated in detail by other municipalities it can be expected that problems are occurring in most other parts of the watershed where development has occurred adjacent to or within valleys and streams. These problems exist largely because infrastructure site selection and limits delineated for development did not consider the natural migration of the watercourse and the creation of appropriate buffers, and have been exacerbated by the increasing instability of watercourse channels as described above.
The TRCA Regional Watershed Monitoring Network (RWMN) includes 37 fluvial geomorphology monitoring sites within the Rouge River watershed. These sites provide a characterization of the physical form of watercourses in various parts of the watershed. In time as more monitoring data is acquired these stations will also provide an understanding of the rates of geomorphic change that these watercourses experience. However, more monitoring sites would be required if initiatives are undertaken to track the impacts of current and future development on watercourse stability and geomorphic condition
The RWMN sites provide localized measurements for a specific purpose and do not allow characterization of the condition of watercourses over their entire length. While some studies have been undertaken to characterize watercourses in more detail in some areas, there are many parts of the watershed where the condition and operative processes of watercourses are unknown.
Summary of Current Conditions RatingsRatings::::
Objective:Objective ::: Overall Rating
Protect and restore natural channel morphology and stability. Fair
TABLE OF CONTENTS
7.0 FLUVIAL GEOMORPHOLOGY...... 7-2 7.1 Introduction...... 7-2 7.2 Understanding Geomorphic Processes ...... 7-3 7.3 Measuring Fluvial Geomorphology ...... 7-5 7.4 Existing Conditions in the Rouge River Watershed...... 7-8 7.5 Objectives for Fluvial Geomorphology ...... 7-19 7.6 Summary and Management Considerations...... 7-20 7.7 References ...... 7-23
LIST OF FIGURES
Figure 7-1: Rouge River Watershed – Stream Order and Geomorphic Monitoring Stations .... 7-7 Figure 7-2: Rouge River Watershed – Topography...... 7-10 Figure 7-3: Bruce Creek at Stouffville Road (RMN Site GR-14) ...... 7-11 Figure 7-4: Upper Rouge River Near Highway 404 (RMN Site GR-7)...... 7-11 Figure 7-5: Bruce Creek at Major Mackenzie Drive (RMN Site GR-17)...... 7-12 Figure 7-6: Rouge River West of McCowan Road (RMN Site GR-21)...... 7-12 Figure 7-7: Little Rouge River at Major Mackenzie Drive (RMN Site GR-38) ...... 7-13 Figure 7-8: Rouge River Upstream of Sheppard Avenue (RMN Site GR-27)...... 7-13 Figure 7-9: Little Rouge River Upstream of Sheppard Avenue (RMN Site GR-52) ...... 7-14 Figure 7-10: Rouge Marsh ...... 7-14
LIST OF TABLES
Table 7-1: Rouge River Watershed Stream Length by Order ...... 7-8 Table 7-2: Morphologic Characteristics of the Rouge River and Tributaries* ...... 7-15
Unique Rouge River Watershed Feature ::: The Rouge River system contains some of the most pristinepristine but also some of the most degraded rivers and streams in the Toronto area
CHAPTER FLUVIAL 777 GEOMORPHOLOGY
7.07.07.0 FLUVIAL GEOMORPHOLOGGEOMORPHOLOGYYYY
7.17.17.1 Introduction
Fluvial geomorphology is a study of the processes responsible for the shape and form, or morphology , of watercourses. In simple terms, fluvial geomorphology describes the processes whereby sediment (e.g. silt, sand, gravel) and water are transported from the headwaters of a watershed to its mouth. These processes determine and constantly change the form of river and stream channels and determine the stability of the channel morphology. Fluvial geomorphology provides a means of identifying and studying these processes, which are dependent on climate, land use, topography, geology, vegetation and other natural and anthropogenic influences.
Protecting and restoring the morphology of watercourses requires a thorough understanding of fluvial geomorphology and the effects of urbanization on geomorphic processes. Urbanization has a dramatic impact on the movement of water and sediment in a watershed and therefore on geomorphic processes. Aquatic species which have adapted to the natural shape and form of watercourses are threatened as channels destabilize. Human lives, property and infrastructure are also put at risk as a result of increased and accelerated erosion. To avoid these results, the impacts of urbanization must be addressed prior to development, as experience has shown that it is extremely difficult, if not impossible, to repair urban watercourses after the damage has occurred.
The Rouge River watershed is rapidly urbanizing and the river and stream network is reacting to this change. It is estimated that the channel of the Main Rouge River downstream of Markham and Richmond Hill has been destabilized and that the channel is in a process of enlargement and adjustment as a result of hydrologic changes caused by upstream urbanization. To date, the channel cross-section has enlarged 25% or more as a result of these changes, and even if development was to end immediately, the channel would continue to enlarge at least as much again due to the time needed for its total response to the impact of urbanization. Other watercourses within the watershed have been much more significantly impacted by urbanization,
7-2 and could be expected to enlarge by up to 500% based on research in other areas. The process of enlargement and adjustment to urban impacts can take centuries during which there is constant channel instability and alteration of aquatic habitat, which ends with a channel form that is no longer suitable for a diverse aquatic ecosystem. These impacts on the Rouge River watershed are largely the result of past and current stormwater management and erosion management practices that are not effective in protecting the channel morphology of the rivers and streams from the impacts of urban development on the surface flow regime.
7.27.27.2 Understanding Geomorphic Processes
A watercourse, by its very nature, is a dynamic system responding to a constant change in flow and sediment supply. The amount of flow in a natural watercourse is determined primarily by climate and geology. Climate controls the amount of water delivered to the surface of the watercourse and how and when it arrives. Geology exerts a fundamental control on what happens to the water once it arrives at the ground surface. Through its effect on infiltration, geology establishes the volume and proportion of groundwater and surface water available to flow through a drainage basin. Geology also determines the volume and properties of sediment supplied to the channel, and the strength and erodibility of the surficial material through which the watercourse flows. A complex underlying geology and topography can result in considerable variation in channel character, as well as sensitivity to potential impacts, within the same drainage system.
Natural rivers and streams respond to constant changes in flow and sediment supply with frequent adjustments in channel position and shape accomplished through erosion and deposition. This self-regulating ability is an inherent characteristic of natural watercourses that allows channel morphology to remain relatively constant. The state in which flow and sediment supply are balanced to achieve this stable channel form is referred to as Adynamic equilibrium @. In a condition of dynamic equilibrium, channel morphology is stable but not static, since it changes gradually as sediment is deposited and re-mobilized throughout the watercourse. For example, in many natural watercourses the outsides of channel bends tend to erode, but there is corresponding deposition of material on the insides of bends. This gives the channel the appearance of slowly >migrating = across the floodplain or in a downstream direction over time. This kind of controlled erosion and deposition is natural and is essential to maintaining the balance between flow and sediment supply in the system. Dynamic equilibrium is also critical for riparian and aquatic biota, which are adapted to the habitats provided by this constantly evolving but stable condition.
Geomorphic Processes and the Human Landscape
Over periods of centuries or even decades, human activities can affect geomorphic processes on a scale that far exceeds natural fluctuations over the same time frame with an effect likened to a major global climate change (Knighton, 1998). Deforestation for agricultural purposes reduces evapotranspiration and infiltration and increases runoff and sediment supply to watercourses. Urban development results in extensive compression and paving of land surfaces, which significantly reduces infiltration and dramatically increases total and peak runoff to watercourses. The changes in flow regime and sediment supply from land clearing and urbanization typically exceed the thresholds for self-regulation in affected watercourses, upsetting the dynamic
7-3 equilibrium and causing the channel to become unstable. In such circumstances, the watercourse responds with large physical changes that occur much more rapidly than the controlled adjustments of the natural dynamic equilibrium. These changes are rapid, extensive and often catastrophic, resulting in destruction of aquatic and riparian habitat, damage to infrastructure and property, and risks to public safety.
Research into the effects of urbanization on watercourses has indicated that the critical threshold at which channel destabilization begins typically corresponds to a total drainage basin imperviousness of three to five percent (Hammer, 1972; Booth, 1990). Significant enlargement of the channel cross-section begins once the drainage basin reaches five to ten percent imperviousness. It is generally estimated that the channel will continue to enlarge, in response to urbanization, for a period of 35 to 65 years after the end of development in the watershed. Once adjustment of the channel to urbanization is complete, the cross-sectional area may be up to 6 times greater than that of the channel prior to disturbance (e.g. Hammer, 1972). This enlargement can occur by both erosion of the channel banks and incision of the channel bed; the degree of each being determined by the relative resistance of their material to erosion. In addition to cross-section enlargement, urban watercourses also experience adjustment of their plan form as the channel attempts to evolve a new meander pattern that is compatible with the new hydrologic and sediment regime. This adjustment process is thought to take an order of magnitude longer than cross-section change, resulting in a total period of instability as a result of urbanization that may be measured in centuries. It is theorized that urban watercourses will eventually achieve a new form of dynamic equilibrium through these adjustments, but even if this should occur, experience suggests that the ultimate form of an urban watercourse will bear little resemblance to a natural river or stream and will not possess the stability or structure required to support diverse aquatic ecosystems (Booth and Jackson, 1997; Fuerstenberg, 1997).
In addition to the effects of land use change, human induced change can also include activities that result in the directly modification to watercourse channels themselves. Agricultural practices can sometimes result in the realignment and channelization of watercourses resulting in loss of natural channel forms and habitats, while tillage immediately adjacent to watercourses causes channel instability as bank vegetation that would normally control erosion rates is lost. In urban areas, channels have commonly been realigned and straightened to facilitate development, eliminating aquatic habitat and intensifying channel instability as the resulting artificial channel form lacks natural adjustment mechanisms. Further, historic approaches to flood control have emphasized the rapid removal of water from the landscape, generally via the realignment, enlargement, and hardening of river and stream networks. The resultant increase in flow velocities and reduction in flow attenuation from the disconnection of channelized watercourses from their floodplain has amplified the increase in flows caused by urban land uses and exacerbated the resultant erosion.
Historically, the management of channel instability and increased erosion in impacted urban watercourses has been addressed using engineered erosion protection. This has involved a variety of modifications to river and stream channels including hardening of bed and/or banks with concrete, riprap, gabion baskets or armour stone as well as the installation of weirs and other grade control measures. However, in many cases such works have failed because they are undermined or circumvented by the watercourse channel as it adjusts either to continue its natural evolutionary path or to respond to urbanization. Such works generally also eliminate
7-4 aquatic and riparian habitat within and adjacent to the watercourse. Further, the hardening of the watercourse channel increases velocities and decreases the natural routing attenuation of flows, exaggerating the urban land use impacts on physical channel form that are described above. As a result, these conventional engineering approaches have typically resulted in a cycle of failure of the installed protection and ongoing channel degradation, leading to the constant repair of existing works and the need for construction of new protection works elsewhere.
In recognition of the negative outcomes of traditional erosion management, there is an increasing consideration of geomorphic processes, ecological considerations, and potential impacts on upstream and downstream areas when designing and constructing erosion protection works. In some cases, large sections of watercourse are reconstructed in an attempt to restore equilibrium conditions through a practice referred to as “natural channel design”. However, the complexity of geomorphic processes in urban systems and the constraints created by infrastructure and private property in valley systems make it difficult to truly recreate natural channels, and the performance of such projects in restoring natural physical and ecological function of watercourses is still unknown. Further, conventional erosion protection works continue to be constructed for areas immediately at risk where there is insufficient time, space or funding to examine more comprehensive solutions.
Over the past two decades, development has increasingly incorporated stormwater management measures to attempt to mitigate the unbalance between the urban hydrologic regime and the natural channel form. By far the most popular and widely-used approach, including in Southern Ontario and the Greater Toronto Area, is the design of end-of-pipe stormwater ponds or wetlands to detain the excess runoff from urban developments and release it slowly at a rate that is considered to be safe to the stability of the receiving watercourse. The design of such facilities is predicated on the assumption that flows in the watercourse below the level required to initiate sediment transport of the median substrate particle size will not result in erosion. Currently, there is increasing evidence that these facilities may not be protecting receiving watercourses downstream of new developments (Booth and Jackson, 1997). It is speculated that this may be due an oversimplification of complex mechanisms of erosion and sediment transport in current design practices in that the release of flows at low rates may not be sufficient to mitigate their impacts (TRCA, 2007). In addition, there is evidence that these facilities may not perform to detain flows in real-world conditions even to the degree for which they are designed (Bengtsson and Westerstrom, 1992). Such results suggest that current stormwater management approaches based on detention may not be sufficient to manage the watercourse impacts from increases in runoff and flow volume in urban areas.
7.37.37.3 Measuring Fluvial Geomorphology
Measurement of fluvial geomorphology involves both examination of channel morphology and investigation of the flow regime and sediment supply that drive geomorphic processes in the watershed. The combined information resulting from these measurements allows the geomorphic condition of a watershed to be determined, and repeated measurements over a period of time allow for the impacts of land use change and urbanization on geomorphic processes to be evaluated.
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Characterization of conditions in a large area with respect to fluvial geomorphology is made difficult by limitations in the ability to collect information. As there are hundreds of kilometers of defined rivers and streams in the Rouge River watershed, it is not practically or economically possible to maintain current data describing the condition of each segment of watercourse within the system. However, the Toronto Region Conservation Authority (TRCA) has initiated long-term geomorphic monitoring as part of its RWMNP, focusing on a limited number of sites that were selected to be representative of the broader range of conditions within each watershed. In the Rouge River watershed 37 geomorphic monitoring stations have been established, as shown in Figure 7-1. Monitoring was initiated on all sites in 2002, using standard fluvial geomorphology documentation and measurement techniques to characterize channel form and dimensions. Reference points were set up to allow TRCA to monitor changes of some parameters over the course of repeated measurements, which are planned on a three-year cycle. In addition to geomorphic measurements, upon initiation of the geomorphic monitoring, all sites were subject to rapid assessments which provide a qualitative measure of watercourse condition, as well as basic historic air photo analysis to provide some measure of the types of change that have occurred over the past several decades.
Measuring channel morphology involves an examination of the complex three-dimensional geometry of a watercourse. Typically, channel morphology is defined in three different planes: plan form (the form of the channel when viewed from above); longitudinal profile (the elevation and gradient of the bed in a lengthwise direction); and cross-section (the size and shape of the channel in cross-profile). Measurements in these planes are taken using topographic survey equipment and then transferred into two-dimensional representations that can be interpreted and compared with subsequent surveys. The rate of erosion and morphological change is also monitored using erosion pins that are driven horizontally into the channel banks. Another fundamental aspect of channel morphology is the composition of the bed material, or substrate, which is an important factor in forming the channel geometry and can provide insight into the watershed sediment supply. Bed material is characterized by sampling the substrate and analyzing the particle size distribution of the sampled material.
Flow regime can be measured directly by gauging flow in a watercourse at discrete locations. However, as the installation and calibration of gauges is time consuming and expensive, the flow regime is often predicted using computer hydrologic models. This method also allows the effects of land use changes on flow regime to be predicted by modifying the input parameters of the model. Empirical gauge data or modelled flow data can be related to geomorphic conditions using various indicators relate the effect, or potential effect, of changes in flow to sediment transport and erosion. One type of indicator used is an erosion index, which is an indicator of the length of time in which flow in the creek exceeds a level at which erosion is assumed to occur, and the magnitude of the flow during that time. In theoretical terms, an erosion index can be used comparatively to examine the change in erosion potential as a result of different flow conditions. However, the results of such analyses must be used with caution as complex erosive processes cannot be described through the designation of a simple threshold, and therefore the amount of erosion or channel instability that will actually occur may not relate directly to the calculated erosion index.
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Figure 777-7---1111:: Rouge River Watershed ––– Stream Order and Geomorphic Monitoring Stations
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The degree of disturbance to river and stream channels and corridors within a watershed can provide a measure of their resilience and ability to self-regulate. Where river and stream channels have been disturbed through artificial confinement, straightening or hardening, they are unable or less able to modify their form in a controlled manner in response to changes in flow regime or sediment supply and as a result often erode catastrophically or transfer erosion problems to downstream areas. Further, where river valleys and stream corridors are constricted by infrastructure or development, the watercourse channel cannot migrate naturally across its floodplain and is typically hardened or altered to prevent this from occurring. While watercourses may still be impacted by flow changes from urbanization, where channels and corridors remain undisturbed the resultant changes are more contained and controlled, and present less of a threat to life, infrastructure and property when they occur.
7.47.47.4 Existing ConditiConditionsons in the Rouge River Watershed
There are approximately 430 kilometers of defined river and stream channel in the Rouge River watershed. Figure 7-1 and Table 7-1 illustrate the channel length associated with each stream order in the Rouge River watershed, and in the Main Rouge and Little Rouge Rivers respectively. Stream order is a measure of the degree of stream branching within a watershed; a first-order stream is an unbranched tributary, a second-order stream is a tributary formed by the connection of two or more first-order streams, a third-order stream is a tributary formed by two or more second-order streams and so on. The principal stream order of a watershed is the largest stream order present within it, and therefore the Rouge River watershed is a sixth order system, which is typical of many watersheds on the north shore of Lake Ontario. Bifurcation ratio is a measure of the degree of branching of tributaries within a watershed and is defined as the average ratio of stream length of a particular order to the length of streams of the next greatest order. The average bifurcation ratio for the Rouge River watershed is 5.9, a value which is very high and would typically indicate the absence of major tributaries and an uneven distribution of different stream orders within a watershed. However, in the case of the Rouge River watershed the average is artificially inflated by the short length of the Main Rouge River between the confluence with the Little Rouge River and the lake which is the only sixth order stream in the watershed. If the sixth order stream length is not considered, the average bifurcation ratio is 1.8, which is more typical of Southern Ontario watersheds.
Table 777-7---1111:: Rouge River Watershed Stream Length by Order Stream Order Total (km) Main Rouge Little Rouge 1 174.6 105.8 68.8 2 90.0 55.9 34.1 3 78.0 55.2 22.8 4 21.3 17.5 3.8 5 61.2 29.1 32.1 6 2.7 2.7 0.0 Total 427.8 266.2 161.6
The drainage density of the Rouge River watershed, which is the ratio of the total length of river and stream channels to the overall watershed area, is 1.3. This is a relatively low value, which typically indicates that a watershed naturally experiences relatively low runoff and greater infiltration of fallen precipitation. However, the current drainage density of the Rouge River may 7-8 be lower than its historical natural condition due to the elimination of many first and second order streams during development of the urban portions of the watershed.
Like most fluvial systems in Southern Ontario, the Rouge River and its tributaries are referred to as ‘semi-alluvial’ in nature (Ashmore and Church, 2001). A purely alluvial stream is one in which the channel and floodplain are created entirely by conveyance of flows through alluvium, the material that has been eroded and transported by the stream itself, In Southern Ontario, streams are relatively young in geologic terms and most were created at or after the end of the Wisconsin glaciation around 10,000 BC. Because of insufficient time and/or erosive capacity, these streams have not manufactured enough alluvium to completely fill the corridors and valleys through which they flow, particularly in the northern half of the watershed. As a result, the morphology of stream channels is at least partially controlled by the properties of the glacial deposits through which they flow, rather than by the characteristics of alluvium transported from upstream as is the case in many non-glaciated and mountainous geographic areas. In the case of the Rouge River watershed, this controlling material includes glacial till, glacial lake and pond deposits and glacial outwash material. Further, the large and well-defined valleys of the Main Rouge and Little Rouge Rivers and some of the major tributaries were likely formed by glacial outwash rather than the current streams that reside in them, and therefore the plan form development of these watercourses is affected as developing meander bends contact the sides of the much larger outwash channel. While the morphology of semi-alluvial streams can superficially appear similar to alluvial streams, they do not develop features such as pools and riffles with the same regularity or predictable form (Foster, 1999). This is important as the majority of research in fluvial geomorphology has been conducted on alluvial streams, and therefore conventional models and theories must be applied with caution to semi-alluvial systems.
In addition to glacial influences, the varied topography (Figure 7-2), geology and land use of the watershed result in a varied geomorphic character of the Rouge River and its tributaries between the headwaters and the lake as well as from the eastern to the western boundaries. In the northwest corner of the watershed, the small headwater tributaries of the upper Rouge River are generally intermittent due to the high infiltration potential of the surficial sands and gravels in this area; watercourses originate as dry valleys through the steep, hummocky terrain. The headwaters of the middle tributaries and the Little Rouge River are more well- defined (Figure 7-3) due to greater runoff from the mixed silt, sand and clay soils of the moraine in this region. Moving south off the moraine on to the relatively flat Peel Plain, watercourses become larger as they collect additional drainage and become slower moving and sinuous in appearance . The major tributaries flow in partially defined valleys of little depth due to the relatively resistant glacial till and relatively low erosive energy of the low slope environment (Figure 7-4 and Figure 7-5). The Main Rouge River is forced to the east near the confluence with Beaver Creek as it encounters the outcropping of resistant glacial deposits that separate the Rouge River and Highland Creek watersheds. Both the Rouge and Little Rouge Rivers maintain this sinuous character while crossing the Peel Plain as they collect flow from the major tributaries and slowly become larger (Figure 7-6 and Figure 7-7). However, as the Rouge and Little Rouge River cross the former Lake Iroquois shoreline the topographic slope increases significantly and the speed of flow increases dramatically. From this area to past their confluence, the valleys of both rivers are deep and well defined, having been cut into less resistant lacustrine sediments and alluvium in the past, a process that is continued by the current fast-flowing rivers (Figure 7-8 and Figure
7-9
7-9). Both rivers continue to meander and migrate laterally through this area, creating dramatic natural erosion scarps where the channels contact the valley walls. Downstream of Highway 401, the gradient of the 6th order Rouge River channel begins to be controlled by backwater from Lake Ontario, slowing flows and creating a depositional environment that sustains the estuary marsh (Figure 7-10). Table 7-2 summarizes basic morphologic characteristics of watercourses throughout the watershed, including cross-section shape, channel slope, and median bed substrate material, as measured at the Regional monitoring stations shown in Figure 7-1.
Figure 777-7---2222:: Rouge River Watershed ––– Topography
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FigurFiguree 777-7---3333:: Bruce Creek at Stouffville Road (RMN Site GRGR----14)14)
Figure 777-7---4444:: Upper Rouge River Near Highway 404 (RMN Site GRGR----7)7)7)7)
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Figure 777-7---5555:: Bruce Creek at Major Mackenzie Drive (RMN Site GGRR-R ---17)17)
Figure 777-7---6666:: Rouge River West of McCowan Road (RMN Site GRGR----21)21)
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Figure 777-7---7777:: Little Rouge River at Major Mackenzie Drive (RMN SiSitete GR-GR ---38)38)
Figure 777-7---8888:: Rouge River Upstream of Sheppard Avenue (RMN SitSitee GRGR----27)27)
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Figure 777-7---9999:: Little Rouge River Upstream of Sheppard Avenue ((RMNRMN Site GR-GR ---52)52)
Figure 777-7---10101010:: Rouge Marsh
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Table 777-7---2222:: Morphologic Characteristics of the Rouge River aandnd TributariesTributaries****
Drainage Average Average Median Average Bankfull ID#ID#ID# Area Bankfull Bankfull Substrate Bank Height Gradient (%) (km(km(km 222))) Width (m) Depth (m) (cm) (m) GR – 2 17.3 3.74 0.69 0.15 0.56 1.2 G4R – 4 6.7 2.78 0.38 0.03 0.59 1.0 GR5 – 5 12.9 3.49 0.48 0.395 0.887 0.9 GR - 7 33.4 6.42 0.62 0.465 1.66 1.1 GR – 10 24.4 5.64 0.51 0.58 1.53 0.9 GR – 11 22.3 4.26 0.48 0.62 2.48 0.9 GR – 12 30.3 5.33 0.71 0.225 0.72 1.1 GR – 13 13.1 2.81 0.28 0.73 0.32 0.6 GR – 14 16.1 6.86 0.42 0.2 0.0064 0.5 GR – 16 24.1 4.21 0.47 0.19 0.23 1.0 GR – 17 31.5 5.66 0.55 0.41 0.586 1.1 GR – 21 147.5 15.65 1.09 0.02 0.61 1.3 GR – 24 11.5 8.07 0.49 0.37 0.57 1.0 GR – 25 186.9 16.69 0.90 0.23 1.86 1.7 GR – 26 192.8 15.54 0.72 0.25 6.19 1.6 GR – 27 215.3 16.43 0.71 0.85 6.94 2.7 GR – 32 4.7 4.61 0.27 0.21 0.0044 3.3 GR – 33 10.3 8.76 0.55 0.45 0.74 1.0 GR – 37 71.3 7.89 0.58 0.15 1.9 1.3 GR – 38 72.3 10.81 0.62 0.05 0.0008 1.6 GR – 39 2.5 3.87 0.24 0.65 0.54 0.5 GR – 45 87.4 13.43 0.60 0.33 7.08 1.0 GR – 47 95.7 10.86 0.59 0.425 8.21 1.2 GR – 51 108.7 13.74 0.61 0.81 0.019 2.3 GR – 52 113.5 18.12 0.55 1.04 6.31 1.7 GR – 53 116.6 16.78 0.70 0.42 5.21 2.3 *from TRCA Regional Watershed Monitoring Network, 2002
As with the majority of watersheds in southern Ontario, the Rouge River watershed has been dramatically altered by human activity and stream geomorphology is experiencing varying degrees of unnatural adjustment as a result. Historic settlement and clearing of the watershed for agriculture, which took place mostly in the 1800’s, increased runoff from precipitation that would previously have been intercepted and evaporated by tree cover, and would have dramatically increased sediment supply to watercourses as soil was laid bare and exposed to erosion. Although there are no reliable historic records of stream condition for the Rouge River watershed for this time period, based on observations in other areas this would have altered the character of the rivers and streams through deposition and accumulation of sediment eroded from the landscape, with consequences to aquatic and riparian habitat. The process of adjustment to these changes would have taken decades before achieving a new stable equilibrium.
Urban development in the Rouge River watershed is now widespread, primarily in the central and northwest areas within the Towns of Markham and Richmond Hill, but also in Toronto 7-15
(Scarborough) to the south and the Town of Whitchurch-Stouffville to the northeast. Dating back to the 1950's, a large portion of the development within the watershed has been constructed without the benefit of modern stormwater management technologies (Chapter 13 provides further description of the extent and timing of historic development within the watershed). As a result, there have been dramatic hydrologic changes to the rivers and streams located within and downstream of these developed areas, as discussed in Chapter 5 (Surface Water Quantity) which are increasingly affecting their morphology and stability as discussed in further detail below. Streams to the north and east of Markham and Richmond Hill are relatively unaffected by urbanization and still possess flow regimes that are more or less natural. As a result, natural physical stability of the watercourse channels in these areas has been largely maintained.
Channel conditions throughout the Rouge River watershed were investigated in 1987 as part of the Rouge River Urban Drainage Study (RRUDS - MTRCA, 1997). The study examined channel enlargement using aerial photographs taken in 1965 and 1987. Increases in channel width were reported for most sites with the largest increases in width observed on tributaries. Robinson Creek showed the greatest degree of enlargement with a doubling in channel width in some areas. Increases in channel width on the Main Rouge River ranged from 1.01 to 1.41 times. Little or no enlargement was observed on the Little Rouge River, which was at the time a primarily rural watershed with very little urban development. The RRUDS concluded that changes to hydrology from urbanization were accelerating erosion in the watershed, suggesting that changes to the flow and sediment regime were beyond the ability of most of the watercourses to self-regulate. It also concluded that traditional stormwater management practices of controlling runoff peaks were largely ineffective in preventing increased erosion from urbanization. The study recommended that current stormwater management policy for erosion control be discontinued, and that detailed erosion control studies be carried out at the Master Drainage Plan level to determine the most effective means to control erosion.
Detailed geomorphic surveys were conducted at all of the RWMN stations in 2002 and repeat measurements of erosion pins, one control cross-section, and bed substrate were conducted in 2005. Preliminary analysis of the 2002-2005 data for evidence of channel change related to urbanization was not conclusive, and there were no trends identified that can relate specific types of channel change with upstream development or other land use impacts. However, as noted previously, the physical configuration of virtually all watercourses, natural or otherwise, is in constant flux and the impacts of urbanization require decades to be manifested. As such, it can be expected that urbanization effects will be reflected in long-term monitoring data, but that it may require two or three more cycles of monitoring to detect. However, the evidence of urban stream instability may be less obvious in watercourses downstream of older development, as many of the more dramatic morphologic changes associated with channel instability would already have taken place. Further, the repeat measurements conducted at the three-year repeat monitoring intervals are limited in scope; more detailed measurements are likely required to capture the true extent of morphologic change.
In general, the corridors of the Rouge River, Little Rouge River and major tributaries have not been subject to direct physical alterations through practices such as straightening or concrete lining, as the negative impacts of these practices were beginning to be understood as development in the watershed took place. As a result, most of these watercourses have
7-16 maintained their natural valley form and some amount of riparian vegetation, which has preserved the natural hydrologic flow attenuation and routing properties of the corridors and reduced the exacerbation of flood impacts that is seen in many other urban watersheds as a result of channel alteration. However, many smaller watercourses in the developed areas of Markham, Richmond Hill and Toronto have been completely removed and replaced with storm sewer networks, which would have eliminated the natural flow attenuation effects of these stream corridors and contributed to increased peak flows and erosion in the larger receiving watercourses. Further, although the physical structure of the main tributary corridors has been maintained in urban areas, in many locations infrastructure and buildings have been constructed within the corridors resulting in damage to public and private property from channel movement. Preliminary GIS analysis by the TRCA indicates that at least 20 percent of stream corridors within the Rouge River watershed are comprised of non-natural or non-open space land uses, where buildings, infrastructure and vegetation removal associated with residential, commercial/industrial, or agricultural land uses may encroach upon the corridor. In many of these areas, local channelization and engineered erosion protection measures have been constructed to protect property and infrastructure from largely natural channel movement, which has impacted physical and ecological processes and in many cases created barriers to fish movement. Such areas have been investigated in further detail by the Town of Markham in a study described below.
Other studies and data sources that describe fluvial geomorphology conditions in the Rouge River watershed or specific areas within the watershed include the following:
City of Toronto Wet Weather Flow Management Master Plan (Aquafor-Beech Ltd., 2003); University of Guelph Master’s Thesis – Change in Channel Morphology Due to Urbanization in Morningside Creek, Ontario (Badelt, 1999) Town of Markham Development Charges Study for Stormwater Management (Aquafor Beech Ltd., 1996, updated in 2004); Town of Markham Erosion Restoration Plan – Environmental Assessment (Aquafor Beech, 2007) Town of Markham Burndenet (Eckardt) Creek Erosion Control Optimization Study (Aquafor Beech Limited, draft report 2005, final report in progress)
In their Wet Weather Flow Management Master Plan, the City of Toronto investigated the geomorphic condition of the portions of the Main Rouge River and Morningside Creek located within the city boundaries. They determined that development impacts had already been manifested in the Rouge River watershed through unnatural enlargement of river and stream channels. At a study site on the main branch of the Rouge River downstream of Meadowvale Road, river channel was 25% larger than its natural pre-development size (Aquafor Beech Limited, 2003). It was also concluded that the river was only partway through its process of enlargement and would continue to enlarge over several decades to approximately 1.5 times its original width in response to the current level of development, which was estimated at 12% impervious cover at the time of study. Were development to continue, the amount and duration of future enlargement would increase. For example, watershed development to 20% impervious cover would result in channel enlargement to about 2.5 times the natural size, and 50% impervious cover would result in a predicted enlargement factor of 8 times. Although not explicitly assessed in the study, many of the tributaries of the Rouge River within the City of
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Toronto, such as Morningside Creek, already have impervious coverage of greater than 20% and as such would be expected to be incurring significantly greater impact than at the study location on the main branch of the Rouge River. The study estimated that the unstable cross- section enlargement process of the main Rouge River would require approximately 45-55 years after the occurrence of land use change, followed by the longer period of instability associated with plan form change. A major recommendation of the study was that innovative stormwater management approaches be promoted in the 905 portion of the watershed north of the City to reduce the impact of development in those areas to the Rouge River, in recognition that the majority of development in the watershed has and will take place outside of Toronto boundaries.
The study of Morningside Creek by Badelt in 1999 determined that the west branch of Morningside Creek was highly unstable due to the effects of extensive upstream development that had incorporated few or no stormwater management measures for erosion control. The research also determined that the north branch of the creek was still relatively stable, due in large part to the retrofit creation of a major on-line stormwater management pond upstream of the study area, which provided substantial attenuation of flows from upstream development. However, detailed modelling determined that further development of the north branch catchment area would exceed the capability of the pond to provide erosion control and that the channel would become unstable. It was recommended that stormwater management measures for the control of runoff volume, such as infiltration source controls, be incorporated into future development in the Morningside Creek subwatershed.
Geomorphic processes and channel condition were assessed for watercourses within the Town of Markham in the determination of development charges for stormwater management and erosion (Aquafor-Beech, 1996). The study determined that under existing (1996) conditions, 30% of the length of watercourse channels within the Town of Markham, the majority of which are located in the Rouge River watershed, were either unstable or only moderately stable in response to impacts from urbanization. Further, it was also predicted that this figure would increase to 90% in response to proposed future development if current stormwater management practices were continued, as watercourse impacts were found to be related to the increase in runoff volume from upstream development which is not addressed by current stormwater management practices. The study concluded that the cost to the Town to restore these unstable watercourses would be approximately $50 million. An update to this study was conducted in 2001 (Aquafor Beech, 2001) to account for new information regarding existing and future development. The update determined that there would be greater degradation to Markham watercourses than originally estimated, resulting in a future cost to the town of approximately $80 million. It is important to note that the costs determined through these studies are based on estimated one-time costs of reconstructing the impacted watercourse channels using ‘natural’ channel design techniques. However, as the practice of ‘natural’ channel design is still in development, it is unclear whether it can be successful in restoring the long-term stability of watercourses in urban settings. As a result, there may be additional costs incurred by the Town in repairing or replacing these works in future.
Subsequent to its development charges studies, the Town of Markham prepared an Erosion Restoration Implementation Plan (Aquafor Beech, 2007) that was intended to identify priority restoration sites in Markham watercourses and to develop a system for prioritizing sites and
7-18 guiding capital expenditures for erosion remediation. Within 85 km of the Rouge River, Little Rouge River and the major tributaries, the study identified 200 locations where erosion was either acutely exacerbated by the hydrologic impacts of development, or where infrastructure or private property were placed without regard to future channel migration and therefore impacted by erosion, and in some cases, both. In addition, 32 locations were identified where the above conditions had created barriers within watercourses to fish movement and migration. Of these sites, 18 were identified as priority projects requiring intervention within a 10-year period to remove unacceptable risks to health and safety, at an estimated cost of $8 million.
The Town of Markham is currently undertaking a study of Eckardt Creek (formely Burndenet Creek) to assess the effects of urbanization and to recommend stormwater management measures to address negative impacts. Although the results have not yet been published, the findings to date show that development in the Eckardt Creek subwatershed has caused the creek to enter the process of enlargement and instability that is typical of urban watercourses. This observation is particularly significant because the development in the subwatershed is relatively recent and incorporates state-of-the-art stormwater management ponds that include detention storage for erosion control. The study has also determined that while modification of the existing ponds may provide some minor improvements to the current erosive condition, the key issue is the increase in runoff volume created by development which ponds cannot address. Detailed hydrologic modelling conducted for the study indicated that source controls providing infiltration of the runoff associated with a 10 mm rainfall event are required to reestablish a hydrologic regime for Eckardt Creek that will restore channel stability.
7.57.57.5 Objectives for Fluvial Geomorphology
In recognition of ongoing and potential future impacts of urbanization on the physical stability and structure of watercourses within the Rouge River watershed, as well as the opportunities to protect watercourses that have not yet been impacted, the following Goal for Fluvial Geomorphology was developed for the watershed plan:
Natural, stable stream channels and corridors that allow for natural stream flow patterns, support diverse aquatic habitat, limit sediment loadings and protect human life, property and infrastructure from risks due to erosion and slope instability.
Based on existing conditions, an objective of Ato protect and restore natural channel structure and stability, and stream corridor integrity @ was adopted to address fluvial geomorphology in the Rouge River watershed. This is provided below, along with indicators, measures, targets and a rating for existing conditions in the watershed.
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Objective: PPProtectProtect and restore natural channel morphology and Overall Rating stabilitystability.... FFFairFairairair Indicator Measure Target Channel morphology Channel geomorphic surveys at Maintain or restore RWMN stations. natural channel structure and rates of morphologic change.1 Hydrologic regime and Erosion indices and flow frequency. Maintain or restore erosion potential. pre-development flow regimes and erosion potential.1 Stream corridor integrity Proportion of protected stream All stream corridors in and continuity. corridors. public ownership or otherwise protected from interference.
Proportion of natural cover in Complete natural stream corridors cover in stream corridors. Risk to public and private Number of locations where Reduce or eliminate property from channel infrastructure, buildings and private buildings, evolution and change property are located in stream infrastructure and corridors at risk from channel private property at risk erosion and evolution (either from channel natural or exacerbated by evolution. urbanization). 1 Target channel structure and rates of change to be defined from baseline Rouge River watershed RWWN data as well as other local and provincial reference data sets
An overall rating of AFair @ for the fluvial geomorphology condition in the Rouge River watershed reflects that many of the watercourses in the watershed remain in a relatively natural, stable condition. However, this is largely due to the limited extent of development to date; the watercourses that are located in or downstream of urbanized areas have been become unstable in response to the hydrologic impacts of land use change and are experiencing negative changes in their morphology. It may be challenging to maintain this “fair” grade, given that the stream form is still adjusting to the current level of urbanization and that major future development is likely. Further, while a relatively high proportion of stream corridors in the watershed are protected from development and have maintained a natural character, there are many locations where incompatible land uses or infrastructure intrude into corridors, resulting in damage to both to property and infrastructure as well as negative impacts to streams.
7.67.67.6 Summary and Management Considerations
Current data and understanding indicate that urbanization is affecting the geomorphic processes and physical stability of the Rouge River and its tributaries. The impacts of existing development have not yet been fully manifested and will be exacerbated by future development if urbanization continues without improvement to stormwater management practices. From a 7-20 positive perspective, much of the Little Rouge River and the headwaters of the Upper Rouge River and Middle Tributaries remain relatively unaffected by urbanization to date and exhibit good physical condition. The preservation of these areas is both an opportunity and a challenge when considering the potential extent of future urban development.
While it may not be possible to reverse the impacts to channel form that have already occurred, the ongoing process of channel degradation could potentially be arrested by implementing measures to reverse the hydrologic impacts of existing development. This effort would involve the retrofit implementation of detention and runoff reduction practices at a large scale. The City of Toronto proposes such retrofit measures in the WWFMMP, for the portion of the Rouge River watershed and other watersheds within its boundaries. The cost of undertaking a watershed- wide program of stormwater retrofits would need to be weighed against the ongoing costs of remedial works to address the impacts of urban channel instability and erosion to public safety, private property, infrastructure, and aquatic ecosystems. To this end, further study of the relative effectiveness of SWM retrofit schemes was a component of the modelling and analysis phase of this watershed study. Regardless, the additional impact of future development can be minimized if measures to prevent changes to the hydrologic regime can be incorporated into development design. This preventative approach is likely to be less expensive and more effective than retrofitting these measures to existing areas or paying for remedial works in perpetuity.
Detailed monitoring of flows and channel form will be required to track the ongoing and future effects of development on the geomorphology of watercourses within the watershed and the success of any mitigative measures that are implemented. The extent of current monitoring activities, including the TRCA RWMN, is not sufficient to capture these effects and more detailed monitoring will be required.
If existing development and watershed management practices continue, enlargement and degradation of watercourse channels in the Rouge River watershed will continue for the foreseeable future. Even if new development is constructed in such a way as to eliminate hydrologic impacts, the effects of existing development will continue to cause increased erosion and channel change for decades, if not longer. As a result, it can be expected that the instance of erosion damage to property and infrastructure will increase as channels continue to enlarge and migrate erratically within their corridors. Experience has shown that local repairs and hardening of watercourses are only temporary solutions that can transfer problems elsewhere, necessitating a perpetual cycle of maintenance of existing works and construction of new ones. In most cases, the removal of infrastructure, buildings, and private property located in stream corridors at risk to natural or unnatural watercourse erosion is a more sustainable long-term solution that eliminates the risk while reducing direct impacts to watercourses from construction of erosion control works. However, the capital costs of removal are often difficult to justify, even when the return on investment from annual maintenance costs savings is high. The cost and effort of managing these existing erosion issues, which are expected become worse in future, underscore the importance of the TRCA role in protecting watercourse corridors in new development areas and regulating the construction of infrastructure within or across corridors.
It has been suggested that damage to watercourses from urbanization and the resultant increase in risk to infrastructure and property could be addressed through the reconstruction of
7-21 affected watercourses using natural channel design principles. For example, the Town of Markham Development Charges Studies described above determined development charges for new development under the premise that the impacts to downstream watercourses from new development would be addressed in this manner. The premise of such an approach would be to construct channels with a form that is stable and/or more compatible with the modified urban flow regime. However, it has not yet been demonstrated that it is possible to construct a channel that reproduces the self-sustaining dynamic stability of natural watercourses in an urban setting. In most cases, attempts at urban watercourse reconstruction have resulted in channels that have a somewhat natural appearance but that require the use of unnatural materials to withstand the erosive forces of the modified hydrologic regime. These channels do not adjust or renew their morphology naturally in response to high flow events and will require maintenance in perpetuity to sustain their constructed form. As such, it should be acknowledged that in most cases urban impacts on receiving watercourses cannot simply be mitigated by reconstructing channels after the damage has already occurred. While in some instances constructed, human-maintained channels that have some aspects of natural form and function may be preferable to a degrading natural watercourse, it must be recognized the only means of preserving a naturally functioning watercourse with its attendant ecosystem and self- maintaining properties is to maintain or restore the natural hydrologic and sediment transport regime.
Although there is an appreciable quantity of information to describe the existing condition of the Rouge River watershed with respect to fluvial geomorphology, as described above, the overall understanding of the physical processes and impacts that affect individual subwatersheds and watercourses is not highly developed. Such information is required to establish the sensitivity of watercourses in various areas of the watershed to change in the hydrologic and sediment regime, in order to prioritize efforts with respect to the mitigation of impacts from both ongoing and existing development. It is recommended that more detailed subwatershed or watercourse-based studies be conducted for areas that are or will be affected by urbanization prior to the preparation of future development plans or the implementation of mitigative strategies for existing development.
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7.77.77.7 References
Ashmore P. and M. Church. 2001. The Impact of Climate Change on Rivers and River Processes in Canada, Geological Survey of Canada Bulletin 555 , Ottawa.
Aquafor Beech Ltd. 1996. Development Charges Study for Stormwater Management. Prepared for Town of Markham.
Aquafor Beech Limited. 1999. Updated from 1996. Development Charges Study for Stormwater Management . Prepared for Town of Markham.
Aquafor Beech Ltd. 2001. Updated from 1999. Development Charges Study for Stormwater Management – Update . Prepared for Town of Markham.
Aquafor Beech Limited. 2003. City of Toronto Wet Weather Flow Management Master Plan, Study Area 5, Highland Creek, Rouge River, and Waterfront Area . Submitted to City of Toronto.
Aquafor Beech Ltd. 2005. draft final report in progress . Burndenet (Eckhardt) Creek Erosion Control Optimization Study. Prepared for Town of Markham.
Aquafor Beech Ltd. 2007. Erosion Restoration Plan – Environmental Assessment. Prepared for Town of Markham.
Aquafor Beech Limited. 2007. Stormwater Management and Watercourse Impacts: The Need for a Water Balance Approach . Prepared for Toronto and Region Conservation Authority.
Badelt, B. C. 1999. Change in Channel Morphology Due to Urbanization in Morningside Creek, Ontario. University of Guelph School of Engineering Master’s Thesis. Guelph, Ontario.
Bengtsson, L. and G. Westerstrom. 1992. Urban Snowmelt and Runoff in Northern Sweden Hydrological Sciences Journal . Vol. 37 (3) pp. 263-275.
Booth, D.B., 1990. Stream channel incision following drainage basin urbanization. Water Resources Bulletin. Vol. 26, pp. 407-17
Booth, D. B. and C. R. Jackson. 1997. Urbanization of Aquatic Systems - Degradation Thresholds, Stormwater Detention, and the Limits of Mitigation. Journal of the American Water Resources Association. Vol. 22 (5).
Foster, G., 1999. Pools, riffles and channel morphology of erosional streams in Southern Ontario. In Stream Corridors, Adaptive Management and Design ; Proceedings of the Second International Conference on Natural Channel Systems. Niagara Falls, Ontario.
Fuerstenberg, R. R. 1997. The impacts of urbanization on streams in King County,
Washington. In Workshop proceedings urban stream protection, restoration and stewardship in the Pacific Northwest: are we achieving desired results? Fraser River Action Plan, Department of Fisheries and Oceans, New Westminster B. C.
Hammer, T. R. 1972. Stream Channel Enlargement Due to Urbanization. Water Resources Research . Vol. 8, pp. 1530-40.
Knighton, D. 1998. Fluvial Forms and Processes , Arnold, London.
Metropolitan Toronto and Region Conservation Authority. 1987. Rouge River Urban Drainage Study.