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