LAND

CHAPTER 5 The Highland Creek watershed is approximately 102 km2 in size, which is small compared to the other watersheds within the Greater Area. The creek flows through two physiographic regions on its way to Lake : the South Slope, and the Lake Iroquois sand plain (see Map 8: Physiographic Regions). The surficial deposits and soil characteristics of these regions play an important part in the natural heritage system of the watershed.

5.1 GEOLOGIC HISTORY

Over the last 120,000 years, numerous ice ages have affected the lands that eventually became North America (Theberge, 1989). About 13,000 years ago the last glacier flowed along the St. Lawrence valley into present day . As this thick lobe of ice moved over the landscape, it scoured the surface and carried billions of metric tons of rocks and soil bound up within it. North of Toronto it collided with another glacier moving south from northern Ontario, depositing tons of soil and rock that had been scraped off of many kilometres of land by both glaciers. This created the Oak Ridges Moraine. The moraine sets the northerly limit of the Greater Toronto Bioregion, and forms the headwaters of many Greater Toronto Area (RCFTW,1992).

The last glaciers covering southern Ontario began to melt about 12,500 years ago, but a remaining ice jam blocked the St. Lawrence valley, forcing the water to flow over higher ground across present day New York State and down the Hudson valley. The backup of water created a lake larger than present day Lake Ontario, called Lake Iroquois (Archaeological Services Inc. et al., 1994).

The ice jam in the St. Lawrence River valley melted away approximately 11,500 years ago, Water which had previously flowed down the Hudson River valley took a new, lower route along the St. Lawrence to the Atlantic Ocean, resulting in dramatically lower levels in Lake Ontario (Archaeological Services Inc. et al., 1994). By about 11,400 years ago, the lake hit its lowest level and was about 80 metres lower than today (Anderson et al.). Over the next several thousand years the water levels of the lake slowly began to increase as the St. Lawrence Valley, no longer covered by a heavy ice sheet several miles thick, began to rebound and increased in elevation. By about 4,000 years ago, present day lake levels were reached (Archaeological Services Inc. et al., 1994).

47 STATE OF THE WATERSHED REPORT: HIGHLAND CREEK WATERSHED

5.2 PHYSIOGRAPHY OF HIGHLAND CREEK The following describes the different physiographic regions found within the Highland Creek watershed. Figure 3: Conceptual Groundwater Flow Model for the -Highland Creek Watershed, depicts these regions diagrammatically.

South Slope The headwaters of Highland Creek begin in the area defined as the South Slope (see Map 8: Physiographic Regions). The South Slope lies to the south of the Oak Ridges Moraine and to the north of the Lake Iroquois sand plain. In the Highland Creek watershed, the South Slope is actually made up of a number of layers of material laid down by successive glaciers over the last 150,000 years (Karrow, 1967).

The oldest layer is the sands, silt and clays of the Scarborough and Don Formations, which were deposited between 135,000 and 60,000 years ago. This deposit has been designated as the ‘Lower Aquifer’ under the Highland Creek watershed, and lies on top of weathered shale bedrock (Eyles et al., 1998).

Above the Scarborough Formation lies the Sunnybrook Formation, which was laid down about 50,000 years ago. This layer is less permeable and acts as a barrier to the movement of ground water (Eyles et al., 1998).

The layer above this, known as the Thorncliffe Formation, is comprised of layers of sands, silt and clays laid down about 45,000 years ago. This is known as the ‘Middle Aquifer’ (Eyles et al., 1998).

Over the Thorncliffe Formation lies the Northern Till, another less permeable layer, laid down between 20,000 and 45,000 years ago that acts as a barrier to the movement of ground water (Eyles et al., 1998).

Above this layer lies patches of the Halton Till/Mackinaw Interstadial Formations (sand and gravel). These were laid down approximately 13,300 years ago. For the most part these soils are relatively impermeable. However, some areas are comprised of more permeable sands and gravel, while other areas are extensively weathered allowing water to percolate beneath the surface. These soils act as the ‘Upper Aquifer’ in this region of the Highland Creek watershed (Eyles et al., 1998).

Some interesting features created by the glaciers are a series of little hills called drumlins. These features are usually found in groups and “point in the direction of movement of the glacier” (Chapman & Putnam, 1984, pg. 16). In the watershed, groups of drumlins are found in the Wexford and Malvern areas, as well as northwest of Agincourt (Chapman & Putnam, 1984). The soils of the South Slope are relatively impermeable so any precipitation that falls here quickly runs off to the local watercourses.

48 CHAPTER 5: LAND

FIGURE 3: Conceptual Groundwater Flow Model for the Rouge River - Highland Watershed

Source: M. Meriano, 1999

49 STATE OF THE WATERSHED REPORT: HIGHLAND CREEK WATERSHED

Lake Iroquois Plain Comprised mostly of permeable sands, the Lake Iroquois Plain was created about 12,500 years ago along the shores of glacial Lake Iroquois. This gravel and sand shoreline forms the southern boundary of the South Slope, cutting across the bottom of the Highland Creek watershed (see Map 9: Surficial Geology). This ancient shoreline, itself a distinctive ridge feature, ran roughly along present day , east of Markham Road, before turning in a north-east direction toward the Rouge River. It is comprised largely of sand and gravel, and was used by many sand and gravel companies (see Map 9: Surficial Geology). The area south of the Lake Iroquois beaches is made from sediments that were carried down from the South Slope by the ancient Highland Creek thousands of years ago and deposited offshore (Chapman & Putnam, 1984). When the ice dam in the St. Lawrence melted and the waters of Lake Iroquois fell to much lower levels, these beaches were left high and dry.

Known as the Iroquois sand plain, this area is composed of permeable sandy soils which act as the shallow, or ‘Upper Aquifer’ in this section of the Highland Creek watershed (Eyles et al., 1998). They allow precipitation to readily infiltrate the ground, resulting in ground water discharge to the creek. Groundwater discharge plays an important role in the hydrology of the creek and thus will play a prominent role in any future restoration plans for the creek.

Valley and Stream Corridors Valley and stream corridors are formed by natural processes which continue to influence the landforms, features, and functions, today. These corridors are the conduits for the collection and movement of water through the landscape. They are dynamic systems that perform important ecological functions such as the:

C transport of nutrients and sediments C provision of habitat and routes for the movement of fish and wildlife C improvement of air quality (forest cover) C attenuation of noise C creation of microclimates C maintenance of a genetic pool for native flora and fauna C hydrologic regulation

Valley and stream corridors are important biological and physical linkages which both contain and link many of the provincially, regionally, and locally designated significant natural areas. They are valued landscape features providing diversity and contributing to environmental quality and the provision of open space and recreational opportunities, and contain rich archaeological resources.

In many ways, the network of valley and stream corridors in a watershed can be viewed as a tree, with the main branch of the watercourse seen as the trunk and the radiating tributaries acting as the branches. Valley corridors are distinctive features which have been carved in the landscape by the movement of water over

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STATE OF THE WATERSHED REPORT: HIGHLAND CREEK WATERSHED

time. Such distinct valley features are most apparent in the lower portions of the Highland Creek watershed south of Highway 401 (see Map 10: Valley and Stream Corridors). Stream corridors, on the other hand, are less distinct than valley features, and generally consist of a watercourse and associated floodplain. In the Highland Creek watershed this includes most of the watercourses above Highway 401.

The function and form of valley and stream corridors are influenced by a number of factors including natural processes such as precipitation and land use practices in the watershed.

SUMMARY

The landform features discussed in this chapter (the south slope, the Lake Iroquois sand plain, and the valley and stream corridors) are important components of the natural heritage system. Of particular interest is the impact of these features on the hydrologic cycle, or movement of water, within the watershed. The valley and stream corridors provide wonderful opportunities for outdoor recreation. However, settlement and, more recently, extensive urban development, has impacted the functioning of these landform features. For example, filling, burying, piping and other watercourse alterations have resulted in a loss of these corridors and along with it a reduction in terrestrial, riparian, and aquatic habitat.

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