Humber River State of the Watershed Report – Surface Water Quantity
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Humber River State of the Watershed Report – Surface Water Quantity 2008 Humber River State of the Watershed Report – Surface Water Quantity EXECUTIVE SUMMARY • As of 2002, only one quarter (25%) of the total urban area in the Humber River watershed had stormwater management controls in place to treat urban run-off prior to it being released to receiving watercourses. • The majority of annual stream flow in both the Main and East Humber subwatersheds is generated by groundwater discharge (baseflow) due to the permeable soils and hummocky terrain of the Oak Ridges Moraine area, predominantly rural land uses and presence of aquifers. • The Oak Ridges Moraine, particularly its southernmost extent, and the Iroquois Sand Plain are major influences on the distribution of baseflow in the watershed. Both of these physiographic regions are characterized by highly permeable soils and underlying geology (sand and gravel) which produce high rates of infiltration and groundwater discharge to streams. • Half (50%) of the total stream flow in the Humber River during baseflow conditions originates from within the Main Humber and East Humber subwatersheds. Secondary subwatersheds observed to be major contributors to baseflow are Upper Main, Centreville Creek and Purpleville Creek. • Baseflows in the West Humber subwatershed are low with large tributaries becoming dry during summer months. The majority of stream flow is generated by surface run-off due to low permeability soils, impervious surfaces, and in some areas no aquifers being present. • The majority of baseflow in the West Humber originates from the west branch. • Seasonal variations in baseflow and minimum sustained baseflow rates at long term stream gauge sites have not changed significantly since continuous monitoring began in the late 1950s. August is typically the time of year when stream flow and water levels are at their lowest. • Risk of flooding remains an important issue in portions of Bolton, Woodbridge, Oak Ridges (Richmond Hill) and Toronto. • The Black Creek and Lower Humber subwatersheds have been almost entirely developed prior to the adoption of stormwater quantity and quality control measures. As a result, flooding is an issue of concern in some areas. Some reaches have been transformed into concrete channels to increase the conveyance capacity of the system. Stream flow tends to increase rapidly during storm events due to high rates of run-off from impervious surfaces and lack of stormwater controls. • Significant increasing trends in average annual and seasonal stream flow volumes have been observed in the East Humber and in urbanizing watersheds like Highland Creek and the Rouge River, which are indicative of the hydrologic impacts of urban development (i.e., impervious surfaces) and not climate variability. Humber_SW_Quantity_FINAL_062508F.doc ii Humber River State of the Watershed Report – Surface Water Quantity • Increases in annual and seasonal stream flow caused by urbanization may be creating long term problems in some watercourses as they adjust to accommodate the changed pattern of flow. If nothing is done, excessive channel migration could impair sensitive aquatic ecosystems and expose municipal infrastructure in valley lands, leading to costly repairs and maintenance. • Overall, water use in the Humber is predominantly for agricultural irrigation which accounts for 42% of the total annual withdrawals. Withdrawals for commercial use and water supply are also major water use sectors, comprising 30% and 18% respectively. • Reaches of the West Humber are most at risk of negative downstream impacts on aquatic habitat and water availability from permitted surface water takings, with more than 18% of average annual baseflow allocated for withdrawal. • Many water users in the Greater Toronto Area are switching from surface sources to groundwater sources as indicated by trends in MOE permits issued over the past four decades. Humber_SW_Quantity_FINAL_062508F.doc iii Humber River State of the Watershed Report – Surface Water Quantity TABLE OF CONTENTS 1.0 INTRODUCTION............................................................................................................ 1 2.0 UNDERSTANDING SURFACE WATER QUANTITY...................................................... 1 3.0 SURFACE WATER IN THE RURAL AND URBAN LANDSCAPE.................................. 3 4.0 MEASURING SURFACE WATER QUANTITY............................................................... 4 4.1 Stream Gauges............................................................................................................... 4 4.2 Field Measurements ....................................................................................................... 8 4.3 Assessment of Water Use............................................................................................... 9 4.4 Hydrologic and Hydraulic Models................................................................................ 11 5.0 EXISTING CONDITIONS............................................................................................. 11 5.1 Overview of Humber River Subwatersheds ................................................................. 11 5.2 Stream Flow.................................................................................................................. 15 5.2.1 Annual and Seasonal Stream Flow ..................................................................... 15 5.2.2 Baseflow Index...................................................................................................... 22 5.2.3 Minimum In-stream Flow Requirements .............................................................. 23 5.2.4 Seasonal Baseflow ............................................................................................... 25 5.2.5 Hydrologic Models................................................................................................ 28 5.2.6 Field Measurements of Baseflow ......................................................................... 31 5.3 Water Use...................................................................................................................... 39 5.4 Flooding ........................................................................................................................ 44 6.0 MANAGEMENT CONSIDERATIONS .......................................................................... 48 6.1 General.......................................................................................................................... 48 6.2 Baseflow........................................................................................................................ 49 6.3 Surface Water Use........................................................................................................ 49 6.4 Flooding ........................................................................................................................ 50 7.0 WATERSHED REPORT CARD RATINGS................................................................... 51 7.1 Stream Flow.................................................................................................................. 51 7.2 Flooding ........................................................................................................................ 53 7.3 Surface Water Use........................................................................................................ 54 8.0 REFERENCES............................................................................................................. 57 LIST OF FIGURES Figure 1: Planned levels of stormwater controls – Humber River Watershed............................... 6 Figure 2: Historical and current stream flow and precipitation gauges ........................................ 7 Figure 3: Locations of Baseflow Sampling................................................................................... 10 Figure 4: Primary and secondary Humber River subwatersheds ............................................... 13 Figure 5: Physiographic Regions in the Humber River Watershed............................................. 14 Figure 6: Historical Trends in Mean Annual Stream Flow at Selected Stream Gauges ............. 18 Figure 7: Historical Trends in Mean Summer Stream Flow at Selected Stream Gauges........... 21 Figure 8: Percentage of Total Annual Flow Volume Occurring in May to October at Selected Gauges.................................................................................................................................. 22 Figure 9: Historical Mean Monthly Baseflow Rates Showing Seasonal Fluctuations (East Humber, Stream Gauge #02HC009)– ................................................................................. 26 Figure 10: Mean Monthly Baseflow Rates (spring-summer) For Selected Humber River Stream Gauges.................................................................................................................................. 27 Figure 11: Baseflow Contributions by Primary Humber River Subwatersheds, 2004 ................ 32 Humber_SW_Quantity_FINAL_062508F.doc iv Humber River State of the Watershed Report – Surface Water Quantity Figure 12: Baseflow Normalized to Stream Length – Humber River Watershed, 2004 .............. 35 Figure 13: Geologic Model Cross-Section (West Humber)………………………………………...37 Figure 14: Geologic Model Cross-Section (East Humber)…………………………………………37 Figure 15: Water Abstractions by Purpose – Humber River Watershed ....................................