Highland Creek EarthCache

The Hydrologic Cycle

Groundwater

Groundwater is an essential component of the hydrologic cycle, and hence of the natural heritage system. As groundwater moves through one or more geologic layers, it can eventually discharge, or seep out, into valleys, streams, lakes or wetlands. In this way, groundwater provides the baseflow of many streams and can regulate factors such as water quantity, quality, and temperature. It is important to note that similar to surface water, groundwater typically flows downhill following the slope of the water table. Groundwater also flows toward and eventually drains into watercourses and lakes.

Highland Creek Watershed and Soil Composition

The Highland Creek watershed is underlain by layers of soil deposited over many thousands of years. Some of these layers are permeable and allow water to flow through them. These are known as aquifers. Others are less permeable and slow down the movement of water. These are known as aquitards. The Highland Creek watershed is underlain by three major aquifers, known as the Upper, Middle, and Lower Aquifers.

The Aquifers

1. The Upper Aquifer is intermittent across the watershed. It is confined to a thin layer of soil on the South Slope less than five metres in thickness, and to the Lake Iroquois sand plain situated south of the Lake Iroquois shoreline. It is through these layers that surface waters infiltrate to recharge local groundwater.
   
   
2. The Middle Aquifer is confined to the layer of sands, silt and clays of the Thorncliffe Formation, which were deposited in the area about 30-45,000 years ago. North of the Highland Creek watershed, up to the Oak Ridges Moraine, the Middle Aquifer is recharged by the Upper Aquifer as ground water leaks through the Northern Till which separates them. Within the Highland Creek watershed, recharge to the Middle Aquifer occurs through the overlying Northern Till and from the Lower Aquifer below it. Groundwater flow in this aquifer within the watershed is generally in an easterly direction. Discharge areas from the Middle Aquifer into Highland Creek exist in a number of locations, especially in the deeply defined valley system of the lower reaches of the creek where the aquifer is exposed. These discharges are a natural asset, contributing a baseflow of cold water to the creek throughout the year.
   
   
3. The Lower Aquifer is made up of the sands, silt and clays of the Scarborough and Don Formations, which were deposited between 135,000 and 60,000 years ago. Flow within this aquifer follows the contours of the bedrock, running in a southerly direction from the north. The designated Rouge River bedrock channel runs through the Rouge River watershed and into the lower portion of the Highland Creek watershed in the vicinity of Centennial Creek, exiting in the area of the current mouth at Lake Ontario. This aquifer also discharges directly into Highland Creek (contributing to baseflow), as well as directly into Lake Ontario. Evidence shows that in many locations the Lower Aquifer is linked to the Middle Aquifer through sand lenses in the intervening Middle Aquitard, permitting flow between the two aquifers.

Stream Channeling and its Effect on Erosion

In a healthy, undeveloped watershed, water flows are relatively constant and even. Erosion is gradual, kept in check by healthy riparian vegetation. Watercourses move slowly across the flood plain, which minimizes the amount of sediment entering the stream. The maintenance of a constant, stable, and natural hydrologic cycle is extremely important to the health of the watershed. When a watershed is developed, however, high streamflow events become much more frequent due to an increase in impervious surfaces and in the corresponding stormwater runoff. This throws the system out of balance and may result in excessive stream bank erosion, loss of vegetation, and more frequent and severe flooding. In this situation the response has often been to channelize sections of the watercourse to prevent bank erosion and to allow water to pass through the system more quickly.

Highland Creek central branch channel at Middlefield Road.

As urbanization spread into the upper portions of the Highland Creek watershed in the late 1960s and 1970s, the headwaters of the creek were channelized to facilitate development and to carry away stormwater as efficiently as possible. As development proceeded, the flow of water in the creek became more uneven, ranging from high flows during wet weather events to low flows during the dry summer months. The unnatural higher flows of water were quickly funnelled through the system via concrete channels. This has resulted in severe erosion downstream, especially along the sandy banks of the lower Highland on the Lake Iroquois plain.

Erosion

There are three main types of stream erosion that are vital as geological causes and these consist of, abrasion, solution and quarrying.

1. Abrasion is what a stream does to the sides of bedrock and boulders that are inside of it. The speed that abrasion occurs at in any stream is determined by its load, which consists of the complete amount of material that is carried by the stream at any given time.
   
   
2. Solution is a chemical compound that has dissolved in the stream and reacts chemically on the rocks or minerals that are in the subsurface. One example of this is carbonic acid and this can be caused due to the water, air and vegetation that decay. Rocks that have been faced with these solutions dissolved in stream water include dolomite and limestone.
   
   
3. Another essential erosional quality of a stream is known as quarrying. This signifies that the power of the water actually plucks weak cemented areas from the bedrock or banks and starts to carry it down stream. Sometimes this action leaves an evident undercut in the bank or leaves a crack in the bedrock.
   
   

Erosion has and continues to occur throughout the Highland Creek watercourse particularly where the large volume of water has resulted in an unnaturally wide channel causing the watercourse to flow as a thin sheet over a broad area. Thus, Highland Creek is currently characterized by highly variable streamflows, bank erosion, some risk of flooding, and extensive areas of artificial stream channel.

Erosion along Highland Creek

The largest erosion sites in the Highland Creek watershed are found along the sandy banks of Colonel Danforth Park. Evidence of past slumping and slope failures can be found there. Several sections along the bank have been eroded as a result of undercutting. These erosion scars extend up the valley wall, measuring from 30 to 70 metres in height.

Logging Requirements:

1. Go to Reference Point #1. Describe what you see on the west side of Highland Creek. Why is this happening? If erosion is taking place at this location, what type is it - abrasion, solution or quarrying?
   
   
2. Go to Reference Point #2. What do you see in the creek bed? How did they get here? How did they get their shape?
   
   
3. Go to Reference Point #3. What is happening on the east side of Highland Creek? Why is it happening on the east side and not on the west side at this particular location?