River Sediments. Sediment Types Are Not Randomly Distributed in a River s Sediment

Transcription

1 River Sediments Sediment Types Are Not Randomly Distributed in a River s Sediment

2 I ll just sit down on this bank of sand and watch the river flow. (Bob Dylan Watching the River Flow, 1971)

3 A river has different parts, and each of its parts have different parts. This might look like all one thing, but there are pretty intense differences between what s happening in different parts of this picture.

4 The photographs of the Alapaha River, including the one on the previous page, were taken at the GA 135 crossing south of Willacoochee, GA, just south of Corridor Z between Tifton and Waycross.

5 The opening photograph was taken in the direction shown by the arrow on the satellite photograph and on it we can see three interesting parts of the river any lowland river like the Alapaha that meanders. The Alapaha is a very small stream but it behaves in all the same ways that the Mississippi does.

6 The cutbank of the stream is erosional. Cutbanks always occur on the outside of meander bends because the water is forced to flow toward and alongside them as it enters the bend. You can see that trees are undercut and will fall, or have fallen, into the water along that far bank.

7 The deepest water in the stream is called the thalweg a German word that means river path or valley path. The thalweg is always where the fastest water is beside the cutbank in a bend but in the middle on a straight stretch of river.

8 A point bar (e.g., where Bob sat to watch the river flow) is a depositional feature on the inside of a bend, opposite the cutbank. Because the fastest (and deepest) water is on the opposite bank the inside has slower water and sediment is deposited, shallowing the stream here.

9 The white arrows point to obvious point bars from the side of the stream the bar is on. Look the picture over and satisfy yourself that they are all on the inside of a bend. The opposite side, in every case, is a cutbank. A

10 Here s another bend, just upstream from the first (labeled A on the previous slide). Which side is the pointbar and which side is the cutbank? Look back at the map. Can you tell which way the camera was pointing?

11 Since the pointbar was on the right side of the picture and the cutbank on the left the photo must have been taken looking west, as the red arrow shows. A

12 Sampling the pointbar for sediment is easy just scoop some up in a baggie.

13 Sampling the thalweg is a little more trouble, but still do-able in a small stream like the Alapaha (at least at low water).

14 If we are interested in sediments there s not much point in sampling the cutbank. Why not?

15 The pointbar sediment is all fine sand. The water moves only very slowly across the pointbar when it covers the bar. Consequently it cannot bring anything coarser onto the bar and cannot transport even sand this fine past it. (Finer material mud and possibly even finer sand get carried beyond the bar.) The thalweg sediment is mostly gravel. The water moves faster here and so sediment finer than this is taken farther along perhaps to the next pointbar, perhaps all the way to the mouth of the river. Only gravel is coarse (and heavy) enough to settle here. Scale: The thickness (not diameter) of the ring is ~2mm the cutoff between sand and gravel.

16 On a bigger stream, like the Flint River near Americus, the thalweg can only be sampled with scuba equipment. However, on a bigger stream there are additional interesting grain size patterns on the pointbar alone.

17 The previous composite photo was taken on this bar, not far above the boat ramp at Reeve s Landing looking southward, as the red arrow indicates.

18 Three sediment samples were taken across the bar one ~2m from the water ( toe of bar ), one ~10m from the water line( mid bar ), and one at the crest of the pointbar right where it begins to slope away from the stream into the floodplain ( edge of floodplain ) Toe of Bar Mid Bar Edge of Floodplain

19 At the toe of the pointbar the sediment is very coarse sand and gravel some of it coarser than anything in the thalweg of the smaller Alapaha River. At mid bar we see mostly medium to coarse sand with a little gravel. And at the edge of the floodplain the sediment is mostly mud no sand or gravel to speak of. The water never moves fast enough into this area to bring anything but mud. Scale: Bars on cross on Swiss army knife are ~2mm cutoff of sand and gravel.

20 You ll see these sediments in a lab and spend some time thinking about how the river sorts the sediment into different size classes in different parts of its environment.