7.2.3: Samples 8 and 9

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Lake Wanaka Sample Descriptions for LW-10-8 and LW-10-9

Samples LW-10-8 and LW-10-9 were collected on a bar in the Matukituki River (GoogleEarth kmz file for LW-10-8 and 9 as shown here).

Both samples were down stream of a pile of boulders that created a significant flow shadow. Arrows show the flow direction. Pen for scale, but hard to see in the first two images. It is in the same place in the third image.

Sample LW-10-8 was collected immediately downstream of the boulders, above and to the left of the pen. I interpret the grains to have been transported over the boulders by a very fast flow and deposited as the flow slowed down immediately after it passed the boulders. Sample LW-10-9 was scraped off the tops of the cobbles that look coated and wet downsteam of the boulders. This sediment is present only downstream of these boulder piles, and its grainsize requires a very low flow speed for deposition.

Based on its grain size and distribution, I interpret LW-10-9 to represent sediment that is normally transported in suspension, but the flow was so slow in the shadow of the boulders that some of it could settle out on top of the cobbles. Other interpretations might be possible, such as water was somehow ponded here.

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