Intensive monitoring from 2000-2002 to evaluate the efficiency of the system saw 81% removal of suspended solids, 77% removal of phosphorus, 73% removal of lead, 85% removal of copper, 89% removal of Zinc, and 75% removal of E. coli.

From here we can see the key keeping the bluffs from collapsing: roots. These roots are holding together the soil at the top of the bluffs and protecting much of the soft sediment underneath from rainfall.

Dunkers Basin has 5 cells. Cells 1, 2, and 3, store water pumped in from Lake Ontario. When there is a storm, this water gets pushed out from one cell to the next and replaced by the runoff. The collected runoff is pumped into the treatment system consisting of cells 4 and 5. This is cell 1.

These artifical ponds are the Dunker's Flow system. It's a series of 5 basins designed to clean city wastewaters before they reach Lake Ontario.

This is cell 1 of the Dunkers flow system. It is here that at least 60% of suspended sediments fall out of suspension, simply because the water slows down upon entering the basin.

This outlet is for a combined sewer system. Normally, storm water and sewage water are both transported to a treatment plant, but during very high rainfalls, the excess combined waters overflow into Lake Ontario. This outlet for exxample experienced 32 combined sewer overflow events between 2000-2002, though this represented only 1.6% of total runoff volume.

Figure: US EPA

Do you notice how the water is brown and opaque here, but clear just a little ways away from the cliff? Trace this plume along the shore. This is an excellent example of longshore drift. Sediment becomes suspended by wave agitation, but those waves also continually push the sediment back towards land, smearing it along the shore.

When city runoff is low, clean water is pumped from Lake Ontario through the system. The idea is that when a storm hits, the dirty water will first push out the clean water, thus reducing harm.

There are no actual primary glacial sediments here. Glacial till would have once capped this sequence, but it was removed by coastal erosion during Lake Iroquois times.

Waves from the east tend to be most damaging, because that is the direction of longest fetch (about 200 km) across Lake Ontario. However, prevailing winds are from the southwest, so longterm, waves in that direction push sediment along the coast.

Silts, clays, and sands of the Thorncliffe Formation were deposited in a glaciofluvial delta. Between about 45,000 and 20,000 years ago the ice front may have fluctuated to put Lake Ontario into and out of ice contact repeatedly.

This is the outlet for the runoff from a 171 hectare drainage area. Approximately 60% of land use within the catchment is residential, and the remaining 40% is a combination of industrial, institutional, commercial and open space.

From here, the importance of slope failure in creating this specatular scenary is particularly clear. Cliff faces like this one once extended for kilometers, but these have been lost in the name of protecting infrastructure and creating shoreline 'diversity'. Artificial bolstering measures starting in the 70's have caused extensive revegetation and muting of the cliff face.

The sediment in the lowest part of the cliff was deposited about 60 000 years ago. At that time, ice sheets were still restricted to northern Ontario, but were discharging large volumes of water and sediment through rivers that fed into Lake Ontario. The sediment built up into a large delta.

From the 1830's to the 1910's, the area below Scarbourough Bluffs was subjected to "stone hooking”, which is the raking of rock slabs off of the lakebed for use in construction. This process is often blamed for accelerating shoreline erosion along the Bluffs, and the practice was banned within 15 m of the shore as early as 1857.

At the very top of the cliffs is a wave cut platform of Lake Iroquois - cut some 12000 years ago with a thin layer of sand collected at the bottom of the lake. Just like the bluffs are eroding today, this earlier lake cut a bluff through about 30 m of of stratigraphy, which can now only be seen further eastwards at Cathedral Bluffs.

The Scarborough Bluffs were named in 1793 by the wife of Lietenant-Governor John Graves Simocoe for their resemblance to the chalk cliffs of Scarborough in Yorkshire, England.

Where we are right now is actually just outside the main area of the Dunker Flow System, during a period of very high lake levels. Normally this area would be dry, but as you can see, it has flooded and is currently acting as an improptu wetland.

This dark line going across the cliff face is caused by groundwater seepage These horizons tend to support plant life.

The reason that the Lake Iroquois water levels were so high was because ice was effectively blocking the St. Lawrence River outlet. This figure shows the outline of Lake Iroquois at it's maximum levels.

Geologist A.P Colemann suggested as early as 1933 that deforestation of the tableland atop the bluffs was instrumental in the creation of large ravines.

Because the basins are collecting sediment, they will need to be periodically dredged, probably around every 4-6 years.

The Sunybrook Drift formed over 45,000 years ago when the Laurentide Ice Sheet came into contact with the Lake Ontario Basin. Ice dammed the lake, raising the water levels substantially. The Sunnybrook Drift is what's known as a rain-out diamict, formed when floating ice drops larger particles of debris amidst background sedimentation of glacially-derived silts.

This flattish area is called a 'toe berm'. It bolters the cliffside and protects it from wave erosion.

Pinnacles like these are a dissappearing feature along the bluffs. Landscape engineering to prevent erosion is stabilizing the slopes and allowing vegetation to take over.

During colder months, water in the soil freezes and expands by about 9% in volume. This can This can help to wedge off parts of the cliff face.

If you look at the ground arround you, you'll notice that there is a lot of anthropogenic debris like crushed bricks, concrete, and asphalt. This is 'fill' and was placed here to prevent waves from undercutting the bluffs.

The result of all of this engineering is 'hardening' of the shoreline. The goal is to prevent erosion - a completely natural and inevitable process. This hardened shoreline prevents longshore drift from redistributing sand to Toronto Islands. The islands are at constant risk from wave erosion because they no longer naturally recieve sediment.

Here we are standing about 40 m above Lake Ontario. However, we are still several meters below the maximum height of Lake Iroquois which occupied this area a mere 12,000 years ago. This flat area we're standing on is a wave-cut platform formed at the edge of Glacial Lake Iroquois.

Several homes built in the early 1950's are perched precariously at the top of the bluffs. These are at risk from slope failures which take place regularly. This photo shows the state of the bluffs in 1947 and again in 1953.

This artificial promontory was constructed in the 70's extends and extends 585 m into Lake Ontario.

Lake Ontario is the uppermost Great Lake, closest to the outlet to the Atlantic Ocean. A number of high-density population centres line the shores. The lake basin was carved out by the Ontario Lobe during the last ice age.

Debris accumulates at the foot of the slope , later to be removed by wave activity

Stratigraphy is the study of layers. This figure provides a useful summary of the stratigraphy of the entire Scarborough Bluffs coastline.

Dunker's Flow basin is a series of 5 basins designed to clean city wastewaters before they reach Lake Ontario. It works by forcing the water to go through a series of screen filters to mechanically filter out particulate debris, before entering an engineered wetland in basin 5 for biological filtration. During very heavy storms, some water may bypass the wetland and go from basin 3 in to Lake Ontario.

Examine this topographic map to imagine what the landscape would have looked like 12,000 years ago. We would be standing offshore on the shoreface in shallow water. There was probably a small bluff at the shoreline. Several canyons cut down towards the lake.

Large blocks of limestone are acting as 'armourstones' laid along the shoreline to dissipate incoming wave energy.

A review of erosion rates between 1952 and 1976 were much higher than during the preceeding 30 years. This corresponds tot he greatest period of construction and urbanization of the catchment areas of Scarborough. Hardening of the watershed has lead to uncontrolled 'flashy' storm runoff from paved areas into the ravines cutting through the bluffs. At the same time, construction of the Bluffers Park Marina and other breakwater structures has starved this area of sediment that used to prevent cliff undercutting.

Based on historical maps since 1793, the rate of retreat averages to between 0.31-0.76 m per year. This average rate has been fairly consistent over the past 8500 years based on bathymetric observation of the location of the low-water level for Lake Ontario. Since 8500 years ago, Lake Ontario has been slowly rising, cutting the bluffs northwards. Although the average rate is slow, the range of retreat of the cliffs at any given location can vary from 0-15 m per year.

Sloping structures made of riprap, armourstone or concrete absorb the energy of incoming waves and increase the stability of the slopes.

The long cracks in the cliff surface are called joints, and they provide a fundamental control on cliff erosion and sediment failure. They are a conduit for water which softens and erodes the sediment.

The Bluffers Park embayment receives stormwater and combined sewer overflows (CSOs). Stormwater runoff associated with this growth has been identified as a major contributor to the degradation of water quality and the destruction of fish habitats.