The formation of a waterfall

Name that Waterfall

1. Gullfoss Iceland 2. Yosemite USA 3. Angel Venezuela 4. Niagara USA/Canada 5. Victoria Falls Zambia/Zimbabwe 6. Iguazu Argentina/Brazil 7. Plitvice Croatia 8. Baatara Lebanon

Harder rock Softer rock 1. Waterfalls are often formed where a layer of harder rock overlays a layer of softer rock.

Harder rock Softer rock 2. As the river passes over the softer rock, it is able to erode it at a faster rate, forming a step in the river bed. Harder rock Softer rock Harder rock Softer rock Harder rock Softer rock

Harder rock Softer rock i. deepens the step to form a plunge pool at the base of the waterfall ii. it undercuts the harder rock to form a notch Further erosion makes the plunge pool and notch bigger over time. 3. The force of hydraulic action does two things:

Harder rock Softer rock As the notch grows, eventually there isn’t enough support under the harder rock and so it collapses into the plunge pool.

Harder rock Softer rock This adds rocks and boulders to the plunge pool, and so the process of abrasion works with hydraulic action to further erode the plunge pool and notch.

The processes of erosion continue, further eroding out the notch and plunge pool. Eventually the harder rock above will collapse again, meaning that the waterfall will retreat upstream over time. Harder rock Softer rock

As it retreats, it leaves behind a steep sided gorge.

The formation of a waterfall notch Side of gorge

Waterfalls

Looking at a side view, the process becomes clearer. Splash back from the falling water erodes away the softer rock faster, leaving the hard rock without support and it collapses into the plunge pool. The process then repeats itself, with the waterfall further towards the source of the river. Draw a step by step series of 4 diagrams to show the formation and retreat of a waterfall.

Niagara Falls Triumph Dolomite Dolomite (Shea Stadium) Shale

Gorges In the Upper Course, the river is not only eroding vertically (down) but towards its source (HEADWARD EROSION). This means the feature shown in the diagram above is created. The river erodes the softer rock underneath the harder rock on top faster, and this means the level of the land along the river's course becomes lower over time and the waterfall retreats back towards the source. Successive positions of the waterfall are shown on the diagram. The movement backwards leaves a second feature called a GORGE.

Homework Describe the formation of a gorge (6 marks)

4 Mark Questions (and sometimes 6 mark questions) These are marked in 2 levels If you make 20 points that are judged to be at level 1, you will only score 2 out of 4 Level 1 – Basic Level 2 – Clear (data, evidence, place names, case studies etc)

Basic Knowledge of basic information Simple Understanding Little organisation with few or no links Little detail Few specialist terms used Reasonable accuracy in spelling etc Legible Might be a list of points

Clear Knowledge of accurate information Clear Understanding Organsied and some linkage Some details and examples (where appropriate) Considerable accuracy in spelling etc

6/8/9 Mark Questions These are marked in 3 levels. Level 1 and 2 same as the 4 markers Level 3 – Detailed Knowledge of accurate info, appropriately used at correct scale Well organised, supported by relevant examples Links and inter-relationships are clearly shown Clear and fluent expression of ideas in a logical form Specialist terms are widely used A perfect answer is not needed to get full marks Don’t be scared by 8/9 markers. They are just like 6 markers, only requiring a few extra comments but in no greater depth

Key to a good answer Lots of links Detail/facts/figures Examples Sequence