Retreat of Niagara Falls ara Falls Activity Name: The Niagara River flows north

Question

Retreat of Niagara Falls ara Falls Activity Name: The Niagara River flows north frm Lake Erie to Lake Ostario and formas part of he order between the Unised Staes Canada. Over this straighs-line distance of appeosimanely 43 am, t Ontario Just over half of that toal drop in elevation occurs at Niagara Falls, where the river fows wer a aprock called the Lockpon Formation (Fig A1141). A series of sofh shales and hardler dolostone bedh are capreck As the water tumbles over the edge, its turbulence is a potent agent of erosion thas removes rock from the weaker formatioes and undercuts the stronger Lockport do capeok Blocks of the caprock break off, and the edge that marks the rver drops approsimately 100 m in elevation to Lake a haed dolostone South to o- Dolasione 150-Dolestone RIVER 250 300- Mudstone Figure A11.6.1 Today, Niagara Falls is located about halfvay between the two lakes at the head ol Niagara Gorge (Fig. A11.6.2). Geologic evidence indicates that the Niagara River became the primary outlet river for the upper Great Lakes into the Ontario Basin about 11,00 acaded over the edge od the Lockport Formation at the Nugara Escarpment nar ?nisown, New York, and Queenston, Canada, near 43.16 N, 79.05W. Since that time, retreat of the waterfall has resulted in the excavation of Niagara Gorge down- stream (north) of the falls 0 years ago, as an enormous continental ice shees retreased north out of the area. At that time, the Niagara River About how long is the Niagara Gorge todan? To measure the approximate length of the Niagara Corge, manipulate a piece of string so that it follons the centerline of the Niagara Corge as mapped in Fig. A1162 Mark the beginning and end of the gorge on the string. Then pull the string straight and measure the distance between the two marked points to give you the map distance along the channel in the gorge. Then compare that map distance with the calculate the true length of the gorge using proportions. Approximate Ingth of Niagara Go ge:- bar scale on the map, and km. Based on this geochronology and the length of Niagara Gorge as shown in Fig. A11,6.2 calculate the anerage rate at which Niagara Falls has migrated southward along the Niagara River course in can/year. Show your calculations. Rate of retreatcm/year. 319

Explanation / Answer

A. Following the periphery (colored in brown) on the left-hand side of the Niagara River, we get a distance of 12 km, i.e the length of the Niagara Gorge.

B. In 11,000 years, the gorge has grown 12,00,000 cms. So, the rate of migration of the falls has been

1200000 / 11000 = 109.09 cm per year.

C. Niagara Falls will retreat at a faster rate if

1. it encounters a soft layer of cap rock on its path of retreat, as softer rocks are eroded quickly with time.

2. the hydro-electric power generation along the periphery of the Niagara River is minimized. This will increase the rate at which water flows into the falls.

D. Niagara Falls will retreat at a slower rate if

1. it encounters a hard layer of cap rock on its path of retreat, as it is now over Lockport Dolostone which is resistant to erosion.

2. the hydro-electric power generation along the periphery of the Niagara River is maximized. This will decrease the rate at which water flows into the falls.

E. Since the Lake Erie is 35 km south of the Niagara Falls, the time taken for the falls to reach the lake at a rate of 109 cm or 0.00109 km per year will be:

35 / 0.00109 = 32,110 years

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