Laserfiche WebLink
<br />w <br />..... <br />Cl' <br />C,..) <br /> <br />the margins of the pediments are being dissected to form narrow belts of <br />badlands. Also, some of the pediment surfaces are partially dissected <br />by headward eroding channels. The channels erode headward when <br />sheetwash from a thunderstorm cascades from the pediment surface into <br />the channel. <br /> <br />Alluvial Valley Floors <br /> <br />~ <br /> <br />Alluvial valley floors are the least abundant landform in the study <br />area. All upland, cliff, and piedmont channels contains alluvial <br />deposits, as do most tributaries. At present, all channels are incised. <br />All of the measured channels, with the exception of those in Basins A <br />and U, which contain no alluvium, are incised into alluvium near their <br />mouths. The alluvial valley floors, measured along the channels, have <br />gradients ranging from 2.0 to 3.3 percent. The alluvium contains <br />abundant im~ricated chips of shale and/or sandstone, depending on the <br />local bedrock. <br />The gradients of the alluvial valley floors cannot be explained <br />merely by the high sediment load of the runoff. For sediment to be <br />deposited at such steep gradients, a sudden change in channel gradient <br />and associated loss of energy are required. The alluvium can be best <br />explained by backfilling following a decrease in channel gradient <br />downstream. . <br />Schneider (1975) studied the subsurface geology and Quaternary <br />deposits of' Grand Valley near Fruita, Colorado. He concluded that the <br />Colorado River gradually shifted south without appreciable downcutting, <br />and this significantly reduced the gradients of the tributary channels <br />in this area. The high sediment loads could not be transported at lower <br />gradients. As a result, fan-like deposits formed along the Book Cliffs <br />in the present Grand Valley agricultural area, and there was backfilling <br />of the tributary channels in the badlands. The channels then became <br />graded to the slope determined by the present position of the Colorado <br />River. <br />The suggested sequence of events regarding the evolution of alluvial <br />valley floor surfaces is as follows. 1) all channels were once bedrock <br />Channels, 2) the channels were aggraded during a long, continuous period <br />of backfilling after the Colorado River shifted to the south, and 3) the <br /> <br />51 <br /> <br />~ <br /> <br />A <br />1 <br />.'2 <br />,I <br />.~ <br />" <br />, <br />':~ <br />.~ <br />'J <br />~ <br />':1 <br />, <br /> <br />,; <br />, <br />','; <br />. <br />, <br />j <br />, <br />" <br /> <br />-:'. --."'"' <br /> <br />