Geosciences and Environmental Change Science Center
Recent Alluvial History of the Southern Colorado Plateau
by Richard Hereford, USGS, Flagstaff, AZ
INTRODUCTIONThe southern Colorado Plateau lies in the Four Corners region of the southwest United States between Flagstaff, Arizona on the southwest and Durango, Colorado on the northeast. Streams in the area are tributaries of the Colorado River. The Virgin River, Kanab Creek, Escalante River, Paria River, Colorado River in Grand Canyon, Little Colorado River, streams of the Black Mesa region, those of the northern San Juan basin, and the Chaco River in Chaco Canyon have reasonably well known alluvial histories.
In this region, excluding the Grand Canyon, broad alluvial valleys are filled to a depth of several tens of meters with fine-grained sandy alluvium. These valley-fill deposits are Holocene; mostly late Holocene to be specific. Early and mid-Holocene deposits are rare, probably owing to burial or a lack of preservation. The surface of many of these valleys overlies a late Holocene valley fill that dates from only about A.D. 1400. Deposition of this alluvium was interrupted by historic-age arroyo cutting that began in the late 1800s. These very young deposits and those of the modern era are the main topics of ongoing research in the Southwest Project of the Global Change Climate History Program.
THE SHORT TERM - HISTORIC-AGE ALLUVIAL GEOMORPHOLOGYWhen Anglo settlers arrived in the southern Colorado Plateau in the mid- to late 1800s they found streams flowing at or near the surface of the alluvial valleys. Within only a few decades of 1880, most alluvial valleys in the southern Colorado Plateau underwent catastrophic change. This destructive alteration of the alluvial valleys involved downcutting of stream channels up to 80 feet followed by extensive channel widening. Downcutting, or arroyo cutting, resulted in movement or complete abandonment of settlements and relocation of roads; arable land was destroyed along with dams, reservoirs, and irrigation ditches. The consequences were so severe that scientists working in the area during arroyo cutting compared it with the effects of earthquakes and volcanic eruptions.
Channels remained in this deep and wide state until about 1940. From then until around 1980, channels began to refill with sediment. Channel refilling was coincident with a substantial decrease of sediment load of the Colorado River. The reduced sediment load resulted from storage of sediment in the tributary channels.
THE LONG TERM - LATE HOLOCENE ALLUVIAL HISTORYAlluvial valleys of the southern Colorado Plateau contain several distinctive deposits separated by erosional discontinuities related to arroyo cutting. These deposits result from cut-and-fill alluvial processes, which produce a distinctive geomorphic pattern of inset terraces. An important objective of the Southwest Project is to better understand cut-and-fill and related alluvial processes. To accomplish this, we are preparing large-scale surficial geologic maps of selected rivers in the southern Colorado Plateau. These maps provide information about the age, distribution, thickness, and depositional environment of these deposits.
The oldest deposit typically exposed in the alluvial valleys is the prehistoric alluvium, deposited until about A.D. 1200. Deposition of this alluvium ended with prehistoric arroyo cutting, which is generally associated with abandonment of the region by the Anasazi people around A.D. 1200. This deposit usually contains remains of that culture, which clearly and firmly date the alluvium. Evidence suggests that the prehistoric erosion culminated around A.D. 1400. This date varies regionally by as much as plus or minus 100 years. The youngest valley fill, termed the settlement alluvium began to accumulate sometime after A.D. 1400. In many valleys, the alluvium eventually overtopped the prehistoric arroyo. These deposits lack Anasazi cultural remains, and the upper part, actually the floodplain at that time, formed the occupation surface used by Anglo settlers in the mid-1800s, hence the term "settlement alluvium" in southern Utah. This alluviation continued until around 1880 A.D., when erosion related to historic arroyo cutting entrenched the channel systems. This erosion is well dated from historic accounts at 1880 plus or minus about 20 years. Historic arroyo cutting was interrupted around 1940 when the channels began to aggrade and partly refill. These youngest deposits are termed the modern alluvium in the southwest Colorado Plateau. Viewed from a regional perspective, this record of late Holocene alluvial activity is broadly synchronous in the region. A situation that is evidently true of the most of the Southwest. The terminology applied to the deposits varies across the region, depending on the interests or purposes of individual workers. Nonetheless, within the limitations of the various methods used to date the alluvium, the deposits are about the same age. This broad synchroneity of alluvial activity suggests a common causal mechanism. Climate variation, specifically variation of precipitation and temperature, effects runoff, erosion, and weathering over a large area.
The late Holocene climate of the Colorado Plateau is not well known. Reliable measurements of precipitation and temperature began only in 1900. Using tree-ring chronologies, we can develop a much longer record of climate as expressed by the width of growth rings. Streamflow is reconstructed by correlating ring-width with measured annual streamflow, which in several streams is available from the early 1900s. The mathematical relation developed this way is then used to reconstruct past streamflow. This long record of streamflow is compared with the alluvial record to examine the relation between streamflow and alluvial history.
SELECTED REFERENCES RELEVANT TO LATE HOLOCENE ALLUVIAL HISTORYAndrews, E.D., 1991, Sediment transport in the Colorado River basin, in, Colorado River ecology and dam management: Washington, D.C., National Academy of Science Press, p. 54-74.
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Hereford, Richard, and Webb, R.H., 1992, Historic variation of warm-season rainfall, southern Colorado Plateau, southwestern U.S.A.: Climatic Change, v. 22, p. 235-256.
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