Niagara FallsA case study of a waterfall

Location

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The Great Lakes

                                                                                                                                                   

                                                                                   

Niagara Falls are situated betweenLakes Erie and Ontario

Niagara Falls

Location

• Physically between lakes Erie and Ontario

• Geographically on the border between USA and Canada

• Situated on the Niagara River

• interactive map http://www.niagarafallslive.com/Niagara_Falls_Interactive_Map.htm

Niagara falls began here, at the Niagara Escarpment, about 12,000 years ago

Resistant limestone (Dolomite)

A resistant limestone

Starting point of the Falls

Position today

11.5 kilometres

Formation

• They originated on the Niagara escarpment 11.5 km to the north

• They fall over a resistant band of dolomite limestone

• The water from the Great lakes passes through the falls on its journey between lakes Erie and Ontario

• Today the Great Lakes hold twenty percent of the worlds fresh water supply. Ninety-nine (99%) percent of this water is of glacial origin

• The Niagara Escarpment was covered with a sheet of ice 2 - 3 kilometers thick (Wisconsin Glacier) 23,000 - 12,000 years ago.

• As the Glacier retreated, the water levels slowly lowered forming four lakes:

• As the late Wisconsin Glacier retreated northward, it created several outlets:

• The rising of the lands from glacier rebound finally cut off these outlets with the exception of the Niagara River.

What the Falls look like• They are

composed of two parts-

• The American Falls – straight and without a plunge pool, marked by rocks at the base

• 10% of the water from Lake Erie goes over these

• They are separated from the Horseshoe Falls on the Canadian side by Goat Island

A small part of the Falls is known as the Bridal Veil falls, which tourists can walk next to.

Features of the American Falls

• They do not have a plunge pool but are marked by a large number of jagged rocks at their base

• 56 metres high

Bridal veil Falls

• Note the tourists

Horseshoe Falls

• 90% of the water

• 54 metres high

• 675 metre long crest line

• Just above the falls is a rapids in which the River drops sixteen meters (fifty-two feet) in 1.6 kilometers (1 mile).

Facts

• In 1938, the deepest flow of water at the crest of the Horseshoe Falls was recorded at 7.6 meters (25 feet). Hydro diversion has reduced the water rate of flow.

• The plunge pool at the base of the Falls is 50 metres deep

• There was a single water fall 800 - 600 years ago. The separation of the water falls into the two current water falls: the American & Horseshoe Falls is remarkably new in terms of its 12,000 year history.

upstream • The UpperNiagara Rapids drop 15 m in the 800 m before the Falls. ...

Erosion

• Since the first recorded sighting of the falls in 1678 they have retreated 393 metres

• - a rate of approx 1.5 m a year

• This has been reduced to about 30cm every 10 years due to human intervention

The slowing of the erosion rate is caused by two major factors:

• 1) the erosion resistant limestone cap rock the Falls of Niagara have been flowing over. This limestone layer began approximately ½ kilometer north of the Rainbow Bridge. As the Falls erode southward, the erosion rate will increase when it reaches Navy Island where the Falls reach a softer layer of rock.

• 2) the development of hydro electric generating plants along the shoreline of the Niagara River have vastly reduced the rate of water flow through water diversion.

The US Army Corps of Engineers 1969 de-watered the American Falls.

• They found out that:

1. a complex system of joints in the Lockport Limestone cap rock was jointed increasing likelihood of undermining.

2.Ground water seepage into the numerous joints increased the hydrostatic pressure as did winter freezing forcing segments of the limestone to be forced further a part creating the possibility of further fracturing of this cap rock layer.i.e. Freeze and Thaw

• 3.Weathering and Erosion:

(a) falling and flowing water carry broken or weakened pieces of rock away.

(b) some of the mineral deposits of the Lockport Limestone are dissolved when water flushes through the fissures in the rock. These minerals which are normally used to maintain the strength and resilience when absent speed the fracturing process.

(c) wetting and drying of Rochester Shale - tests show that after approximately ten (10) such cycles, the Rochester shale will deteriorate naturally undermining the Lockport Limestone cap rock.

(d) freezing and thawing repeatedly forcing rock to separate and break away.

• e) rock falls that occur expose more weakened and vulnerable rock layers.

Interesting facts about Niagara Falls

The flow over the American Falls was stopped completely for several months 

in 1969. The idea was to determine the feasibility of removing the large 

amount of loose rock from the base of the falls to enhance it's appearance.  

In the end the final decision was that the expense would be too great.

                                                                       

                                                 

Accurate surveys of erosion of the Falls of Niagara began in 1842.

• From 1842 to 1905, the average rate of erosion of the Horseshoe Falls was 1.16 meters (3.8 feet) per year.

• From 1906 to 1927, this rate of erosion was reduced to .70 meters (2.3 feet) per year. This reduction coincided with the large quantity of water being diverted for hydro-electric generation.

• In 1896, Thomas Martin wrote that the annual recession of the American Falls was 7½ inches per year and the Horseshoe Falls was 2.18 feet per year.

• From 1842 to 1905, the average rate of erosion of the Horseshoe Falls was 1.16 meters (3.8 feet) per year.

• From 1906 to 1927, this rate of erosion was reduced to .70 meters (2.3 feet) per year. This reduction coincided with the large quantity of water being diverted for hydro-electric generation.

• The mean rate of erosion was 3.5 meters (5 feet) per year. Since 1942 the rate has been much slower.

• Today, through increased water diversion and anti-erosion remedial steps, the rate of recession at the Horseshoe Falls has been reduced to a fraction of what it used to be. Today it is estimated that erosion of the Horseshoe Falls is less than one foot per year.  In the future, through remedial efforts and further water diversion that the amount of erosion at the Horseshoe Falls has been projected to be reduced to approximately 1 foot every 10 years.

• Today, erosion of the American Falls is estimated at 3 - 4 inches every 10 years. The water flow which is regulated at a minimum level of 10% of the estimated 100,000 cubic feet per second during the summer (50,000 cubic feet per second during winter) is insufficient to cause major erosion.

Control structure

The control structure built in the 1950s has reduced erosionrates to 1 metre every 10 years from 1.50 metres every year.

Down stream from the falls there are more rapids and a gorge

Do the Falls Freeze over in the Winter?