Retreat of Niagara Falls Activity 11.6 Name: Course/Section: Date: The Niagara R

Question

Retreat of Niagara Falls Activity 11.6 Name: Course/Section: Date: The Niagara River flows north from Lake Erie to Lake Ontario and forms part of the border between the United States and Canada. Over this straight-line distance of approximately 43 km. the river drops approximately 100 m in elevation to Lake Ontario. Just over half of that total drop in elevation occurs al Niagara Falls, where the river flows over a hard dolostone caprock called the Lockport Formation (Fig. A11.6.1). A series of soft shales and handler dolostone beds are below the hard caprock. As the water tumbles over the edge, its turbulence is a potent agent of erosion that removes rock from the weaker formations and undercuts the stronger Lockport dolostone caprock. Blocks of the caprock break off, and the edge that marks the top of Niagara Falls migrates upstream (south), eroded block by block. South to Lake Ene North to Lake Ontano NIAGARA RIVER Lockport Dolostone NIAGARA GORGE Rochester Mudstone Dolostone Feet below edge of Niagara Falls Sandy mudstone NIAGARA RIVER Whirlpool Sandstone Mudstone Figure A11.6.1 Today, Niagara Falls is located about halfway between the two lakes at the head of Niagara Gorge (Fig. A11.6.2). Geologie evidence indicates that the Niagara River became the primary outlet river for the upper Creat Lakes into the Ontario Basin about 11,000 years ago, as an enormous continental ice sheet retreated north out of the area. At that time, the Niagara River cascaded over the edge of the Lockport Formation at the Niagara Escarpment near Lewistown, 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. About how long is the Niagara Gorge today? To measure the approximate length of the Niagara Gorge, manipulate a piece of string so that it follows the centerline of the Niagara Gorge as mapped in Fig. A11.6.2. 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 bar scale on the map, and calculate the true length of the gorge using proportions. Approximate length of Niagara Gorge: - — km. Based on this gcochronology and the length of Niagara Gorge as shown in Fig. A11.6.2, calculate the average rate at which Niagara Falls has migrated southward along the Niagara River course in cm/year. Show your calculations. Rate of retreat: - - cm/year. 319

Explanation / Answer

Ans A. The Niagara gorge extend from the falls for 11 km downstream to the foot of the escarpment at queenston.

AnsB. The rate at which niagara fall is retreating is .7 meter which is 70 centimeter per year .

Ans C. The flow of water , weak pieces of rock are broken and carried away .2 due to the cold weather in this part of the world the constant freezing and thawing of rocks lead to breakage. 3 racks falls which happen on and off weaken rock layers.4 minerals which strenghthen the rocks are also washed away constant erosion is the main reason for river retreating .

Ans D the water currently flows over limestone cap rock which is far resistent to erosion than other rocks. 2 the rate of water flow down the falls has decreases due to the development of hydroelectric generation plants. These have created through water diversion along niagara river's shoreline. 3engineers are also working on the regular basis to reduce the rate of erosion through various remedial meansures like repairing faults and constructing water weirs.

Ans E if in one year it retreat .0007km than to travel 35km it would take 50000 years

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