7995 - Laserfiche WebLink

Home Browse Search

channel. Bedrock forms the lower 6 to 8 feet of the terrace riser and it is overlain by coarse grained alluvial deposits. The strath terrace was not overtopped by the 1984 flood, and therefore, it serves the same confining function as the vertical sandstone walls. Between XS 12.1 and XS 4 at the downstream end of the bar, the left bank of the low flow channel (right chute channel) is comprised of the alluvium that forms the bar. With the exception of the subaerially exposed sand bar in the eddy at XS 4 the bank is composed of gravels and cobbles. There is an expected general fining trend in the channel margin sediments in the downstream direction. The left margin of the bar between XS 12.1 and XS 4 is comprised of both alluvial and colluvial sediments. From about XS 12.1 to XS 8 the left side of the bar is confined by a vegetated alluvial terrace that is composed primarily of sands. The terrace tread is about 10 to 12 feet above the bar surface. From XS 8 to XS 4 the bar is bordered by a vegetated surface that is comprised of a mixture of alluvial and colluvial sediments. On average the vegetated surface is about 8 feet above the bar surface, but the height varies considerably along its length. From about XS 9 to XS 5 there is a well defined chute channel along the left side of the bar. The bed of the channel is composed of gravels and cobbles. From just downstream of XS 7 to just upstream of XS 5 the left margin of the chute channel is comprised of a bench that is composed of cemented gravels and cobbles. The chute channel contained flow during 1993 and the cement did not slake. Laboratory analysis of the cemented sediments indicates that the cement is a carbonate-rich mud that has been sunbaked into a natural mortar that is not water soluble. Iron oxide and organic material staining (desert varnish) of the surface cobbles and gravels indicate that the surface has been stable for a considerable period of time. The downstream end of the chute channel is marked by a large scour hole. The highest portion of the bar is located between the left and right chute channels between XS 8 and XS 6. The highest part of the bar was not overtopped by the 1993 flows. The reduced flow depths over the higher portions of the bar have led to deposition of primarily sand sized sediment at this location. As flow depths increase away from the highest portions of the bar, the bar surface sediments become coarser. The upstream portion of the bar between XS 12.1 and about half way between XS 10 and XS 9 is composed of gravel and cobble size sediments with very little finer material evident among the coarser sediment. Similar sediments form the beds of the two chute channels. Many of the coarsest clasts on the surface of the bar are heavily varnished which indicates that they have not moved recently, whereas the smaller cobbles and gravels are unvarnished and were probably deposited during the 1993 runoff season. Between the coarse grained cobbles and gravels that form the bar head sediments and the sands that cover the upper elevations of the bar is a belt of sediments that have a bimodal distribution. The underlying cobbles and gravels have been covered to a greater or lesser extent by sands. The grain size fining towards the highest portion of the bar inundated during the 1993 flows confirms that backwater reduces the hydraulic energy over the bar as discharge increases. 3.5 Resource Consultants & Engineers, Inc.