Instructor: Dawn Sumner
It is a very large river with a large watershed and a substantial sediment supply. The high concentration of bedload sediment causes it to have a braided river morphology when the valley is wide enough. Sediment is supplied by the many tributary creeks as well as erosion of the valley walls. Here are some examples of the sediment supplied by erosion of the valley walls:
Some of this sediment falls directly into the river. There is a large difference in the rounding of sediment transported by the river and that recently supplied from the valley walls:
Newly introduced sediment joins that in the river. The lab samples were collected a few hundred meters downstream of the area shown in the last two images.
Samples LW-10-1 and LW-10-2 were collected on a river bar in an area with shallow flow of variable speed. White arrows show the flow direction.
Sample LW-10-1 was collected in an area with current ripples. The sand was barely being transported over the crests of the ripples. Pen for scale.
Sample LW-10-2 was collected in an area with a flat bed and higher flow speed than LW-10-1. Sand was being transported by the flow, and it was accumulating in shallower water areas with slower flow such as right at the pen. It may also have been accumulating in the downstream flow shadows of larger grains. Larger grains moved occassionally in the flow, but not frequently. Pen for scale.
Sample LW-10-3 was collected in an area with even higher flow speeds, between the two arrows indicating flow direction. Here, sand and finer grains were in suspension in the flow. Note how brown the water looks due to that suspended sediment. In several minutes of watching the flow, I saw only 2 cobbles move, so I do not think the flow was fast enough to have transported these grains into this location. Rather, they were probably deposited when the local flow speed was higher. Pen for scale.
Sample LW-10-4 was collected in an old channel that was partially blocked by migration of a coarse-grained bar, seen in the background of the next photo. The old channel had ponded water in it when the sample was collected. However, the grain size of the sample is too coarse to have been deposited in this environment. Thus, I interpret it to have been transported to this location when the water was flowing, with the possible addition of silt and clay-sized grains after the water stopped flowing. Pen for scale.
Return to Lab 1: Sedimentary Grains Web Content
Sed Strat Home Page
UCD Geology Home Page