Atkinson Adventures EarthCache

Atkinson Dam is a lovely place to stop on your drive if you are passing by the area. The reservoir is called Lake Atkinson.

HOW TO LOG: Send the owner a message with the answers to the questions listed below. Also include a pic if you would like to.

Farms need water and they need to get that water from somewhere. In some places this water comes from underwater aquifers and in others it comes from dams. You are the Atkinsons Dam and this dam provides water the much of the surrounding farm land.

If this was a natural formation, the process for ensuring that the water doesn't disappear into underground aquifers would have started millions of years ago - but in the case of a man made structure the process is similar except made to happen much faster. The first part of the process is the removal of the top soil. This is the layer of the earth which is normally between 5-20cm deep and contains much of the biological activity. Once the top soil is removed some impervious material must be used to cover the base of the dam in order for the water to no sink through. This is generally some mixture of soil, sand, clay or rocks.

It is interesting to know that there is a international standard covering grain sizes of rocks (ISO 14688-1:2002) which gives a structure for which soil types can be classified. So for the lining of the dam, soils in similar sizes as listed below would have been used. This would have been highly compressed to ensure it was impermeable to water.

Here are some of the smaller grain sizes:

Clay: ≤0.002mm
Silt: 0.002–0.063mm
Sand: 0.063-0.20mm
Gravel: 0.20-63mm

As a general guide you can tell the difference between the different soil types by feel: Sand, feels gritty. Silt, feels smooth or floury. Clay, feels sticky when wet or solid when dry.

In natural circumstances, millions of years and significant pressure causes the compacting of material in a similar way, leading to impermeable rock (among other types of material).

Many of the rocks used for this process were clastic rock (which is a rock made of fragments of minerals and other rocks). If you look at many of the rocks around the area you will notice they are made up of these fragments.

Question:

1. Out of the different types of soil mentioned above (clay, silt, sand and gravel) what ones can you see underfoot on the lakes edge?
2. Find a small clastic rock and estimate the size of the fragments in the rock.
3. In the piece of clastic rock you are looking at, what are the colours of fragment? Are they all the same colour?

Facts about the dam

1. The total surface area is 5.56 km²
2. Length: 4.2 km
3. Width: 2.5 km
4. Shore length: 14.4 km
5. It was built across the Buaraba Creek.
6. It is a naturally forming lagoon.
7. The primary purpose of the dam is for irrigation for nearby farming land. 
8. The dam wall is 9 metres high and 2,088 metres long.
9. The dam holds 30,500 megalitres of water when at full capacity.
10. The dam is connected to Seven Mile Lagoon via a 1.2-kilometre channel. 
11. In mid 2006 the dam was empty due to drought conditions in the area.