Recent Alluvial History of the Southern Colorado Plateau

by Richard Hereford, USGS, Flagstaff, AZ

Alluvial valley of the Lonely Dell reach of the Paria River near Lees Ferry, Arizona, looking upstream. The light-colored material near the river is Late Holocene alluvium deposited since about A.D. 1200. The most extensive deposit is the modern alluvium deposited since 1940. Historic-age arroyo cutting deepened and widened the channel beginning between 1870 and 1884. The low gray hills and slopes just above the river are Pleistocene gravels. A surficial geologic map of this reach (scale 1:5000) was recently completed and is in preparation for digital publication.

INTRODUCTION

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 GEOMORPHOLOGY

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.
 
This partial refilling may represent only a brief reversal of the cycle of erosion or the beginnings of a long-term shift to relatively stable conditions. One component of the Southwest Project is to evaluate the relative influence of climate and landuse on these recent changes in alluvial activity. In addition, through understanding the longer term (or late Holocene) alluvial history of the region we may be able to predict future alluvial activity under various climate change scenarios.

THE LONG TERM - LATE HOLOCENE ALLUVIAL HISTORY

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.
 
A preliminary analysis, using an existing tree-ring chronology, suggests that arroyo cutting occurs during extended periods of unusually high runoff. This high runoff is evidently associated with frequent large floods capable of deepening and widening alluvial channels. Valley-fill alluviation, on the other hand, is apparently related to extended periods of relatively low runoff when large, destructive floods are infrequent. This situation enhances alluviation and sediment storage in alluvial valleys. The modern alluvium, deposited contemporaneously with systematic discharge measurements, accumulated during a period of infrequent large floods and relatively low runoff, lending support to the notion that alluviation results from relatively dry conditions. The alluvial history component of the Southwest Project is developing a tree-ring chronology of the Paria River basin in cooperation with the Laboratory of Tree-Ring Research, University of Arizona. With this chronology, the long-term discharge history of the Paria River can be reconstructed. This information will help identify past episodes of climate variation and their relation to alluvial history in the Paria River basin.
 

SELECTED REFERENCES RELEVANT TO LATE HOLOCENE ALLUVIAL HISTORY

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