The Three Faces of the Iška River

The Three Faces of the Iška River

Conceptual design: Dr. Bojan Erhartič, Jernej Tiran

Authors: Dr. Bojan Erhartič, Dr. Aleš Smrekar, MSc. Mateja Šmid Hribar, Jernej Tiran

Difficulty level: 15 years and up

Estimated time: 10 to 20 minutes

Language: English (links are available only in Slovenian)

Translation: DEKS d.o.o.

eLecture is a part of educational trail

PROJECT IS PARTIALLY FINANCED BY EUROPEAN UNION MED PROGRAMME

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Photographs of the landscapes by the Iška river. (Photo: Bojan Erhartič)

The source of the Iška River is on the edge of the Bloke Plateau and it empties into the Ljubljanica River in the Ljubljana Marsh. Over its short 29 km course it has had a great impact on the terrain and has helped create very diverse landscape types.

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The Iška begins its path in the dolomite landscape of Slovenia’s Dinaric region, where water gathers in many upstream ravines. The river therefore has many sources, but they aren’t particularly big. Look for them at the Geopedia website.

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One of the springs of the Iška River. (Photo: Bojan Erhartič)


Do you know how to read a map? Click on the topographic symbol for a spring.

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Great, you are an excellent map-reader! Go on.

Continue

That’s not right. Try again.

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The beginnings of the Iška are modest trickles, but soon they’re joined by many tributaries that start giving it more and more power. The largest of these is Zala Creek, which has its source below the village of Rakitna.


The Iška watershed is distinctly asymmetrical. Check the map to see which tributaries are longer.

Check
















The Iška watershed. (Cartography: Manca Volk)

Right! That wasn’t hard, was it? Did you know that in the past people were good at using water power? Once there were more than fifty sawmills and grain mills set up along the Iška River and its tributaries.

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That’s not right. Try again.

That’s not right. Try again.

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The upper course of the Iška River is steeper, giving the river more erosive power, and so there are many picturesque stream pools, waterfalls, rapids, evorsion channels, and other features.

Some of the most interesting relief features are potholes. The photo shows an example from the Iška River. For more information, see the schematic drawing of how they form.


Pothole. (Photo: Bojan Erhartič)

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Immediately after its confluence with Zala Creek, the Iška enters an area with (nearly) vertical walls. This well-known gorge is the Iška Gorge.

It was created through the rapid sinking of the nearby Ljubljana Marsh. The big elevation difference caused the Iška River to cut through the terrain faster. During colder geological periods the slopes were less forested, and so the Iška was able to cut into the rock even more easily.

The illustrations show the different development stages of the Iška Gorge. Put them in order from oldest to most recent.

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Illustrations: Marijan Pečar

Check

That's right! The gorge was deepened through time mostly by the deep erosion.

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Hmmm, you didn't convince me. Try again!

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In Slovenia, only a few gorges are referred to as vintgar gorges.

Which of the following are named this way?

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Photographs of the vintgar gorges





That’s right! The first one with a name like this was the Bled Gorge; in addition to the Iška Gogrge, there’s also the Bistrica Gorge above Slovenska Bistrica.

Continue

Not quite. Try a little harder.

Not quite. Try a little harder.

Click on the photos to make them bigger.

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A vintgar gorge is only one of many types of valleys, which are distinguished by the shape of their cross-sections, the width of their floors, the slope of their rivers, and other features.


Match the names of types of valleys with their schematic cross-sections. You can use the Dijaski.net website for help (wait a few moments for the Word document to open).


U-shaped valley
V-shaped valley
Shallow valley
Vintgar gorge
Canyon

Check

That’s right! Shall we continue?

Continue

Uh oh, that’s not right. Try again!

Did you take a good look at the Dijaski.net website?

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You can also imagine Iška Gorge as a "hole" that’s 400 m deep and several kilometers long.

But where did all the material "go"?

Check

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Deposited rock. (Photo: Matija Zorn)

Right! The Iška is a flashy stream and when there’s lots of water it also carries away enormous amounts of gravel and sand, and even boulders as big as cars.

Continue

Your imagination is too wild! Think again.

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The material from the Iška Gorge was carried away by the Iška River. Did you ever wonder where the river put it?

The answer’s simple: like many other rivers, it deposited the material where it exits the narrow valley into a broad, level area. The result was a cone-shaped alluvial deposit known as a fan.

The material accumulated because of the reduced transport power of the river on the level area.

After leaving the Iška Gorge, the Iška River created the Iška Fan over thousands of years.

On the map, click on the place where the Iška Fan is located.











The Iška watershed.

(Cartography: Manca Volk)

That's right! Did you know there are rich supplies of drinking water in the Iška fan?

That's not right. Here is the Ljubljana Marsh.

That's not right. Here is the Iška Gorge.

That's not right. This is the Dinaric region.

That's not right. This is the Dinaric region.

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Now you know that a fan is created by a river when it comes out of a gorge onto a plain and its ability to transport material decreases. Gradually it deposits so much material that it fills up its own bed, and then it starts to flood and look for a new bed. While looking for the ideal bed, the Iška constantly changed its course.


Take a good look at the third picture to see where the Iška flows now.

How do you explain the fact that it has moved completely to the edge of the fan?

Pick the most likely reason.

Check





Illustrations: Marijan Pečar

That’s right! Over the past few centuries, people tried to tame the river, and so they presumably pushed the Iška below the foot of Mount Krim. The naturally twisting riverbed, filled with meanders, was replaced by a much straighter riverbed.

Continue

I don’t think so ... Think about it; it’s not hard!

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After the Iška leaves the fan, it flows about 4 km across the flat, wet Ljubljana Marsh to its confluence with the Ljubljanica. The depression of the Ljubljana Marsh is of tectonic origin; it formed through sinking of the surface, which is still ongoing. The resulting depression was filled with material from rivers, especially the Sava, Ljubljanica, Gradaščica, and Iška, so that thick layers of alluvium accumulated.

The depth of the alluvium has been determined near Črna Vas through drilling.

How deep do you think the layer of alluvium there is? The Ljubljana Marsh Nature Park website can help you.

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Right! Some parts of the Ljubljana Marsh have even deeper alluvium — so deep that it could cover the Nebotičnik high-rise in Ljubljana four times, stacked one on top the other.

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No. The alluvium is much deeper. Try again!

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Because of the insignificant slope, the flow of the Iška River in the Ljubljana Marsh is much slower, and therefore it only has enough power to carry small particles.

In order to facilitate drainage, people rerouted the course of the Iška, cutting off the meanders, deepening the bed, and "sending" its water into the Ljubljanica by a shorter route.


What made people change the Iška watershed so much? Mark all the possible right answers.

If you’re not sure, check the sign Into the Canal along The Iška River Meander Nature Trail.

Check


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Riverbed. (Photo: Bojan Erhartič)

Congratulations! Good thinking!

Continue

Not quite. Try a little harder.

I don’t think so ... Think about it; it’s not hard!

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While traveling along the Iška River, you’ve gotten to know the basic laws of water flow and river terrain. Because you’ve surely been following the lesson with enthusiasm, this last task will be a piece of cake for you.

Match the transformation processes on the right with the best word on the left:


Gorge
Fan
River
Marsh
taking away material, erosion
depositing material, accumulation
transport of material
flooding

Check

Right! Remember that a river always uses its power in the following order: first to flow, then to transport material, and, if there’s any power left, to cause erosion.

Continue

That’s not right! Think a little more.

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Look at the pictures below showing the land along the Iška. Click on them to make them bigger.

Photo: Bojan Erhartič



Now open the map of the Iška watershed and put the photos on it.

Iška watershed. (Cartography: Manca Volk)

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Put the photos of the land along the Iška on the map in order from source to mouth. Check

The Iška watershed. (Cartography: Manca Volk)

Congratulations! You succesfully completed the final task.

Continue

That's not right! Maybe this shematic display of the watershed will help you.

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On the exciting journey from the source to the mouth of the Iška you’ve gotten to know the three faces of the river: the wild upstream gorges with the Iška Gorge, the gigantic cone-shaped deposit of the Iška Fan, and the wet world of the still waters of the Ljubljana Marsh.

Parts of the landscape types that you’ve learned about in this e-lesson can be seen "live" on the Iška River Meander Nature Trail.

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