Late Holocene alluvial geomorphology of the Virgin River in the Zion National Park area, southwest Utah

Abstract

This study traces the geomorphic development of the alluvial valley of the Virgin River in the Zion National Park region of southwest Utah. The purpose is to identify, date, and interpret the patterns of erosion and deposition that formed the alluvial valley over the past 1,000 years. This information is a basis for understanding how the geomorphology of the alluvial valley changes under essentially natural flow conditions. Sediment in the alluvial valley is classified as mainstem or tributary in origin. Mainstem alluvium is the largest volumetrically; it is mainly light-colored sand derived from upstream sources that accumulated on now abandoned flood plains (or terraces) by overbank deposition. Tributary sediment is typically dark colored, coarse-grained sand or gravel transported to the alluvial valley by streamflow or debris flow. Tributary and mainstem sediment are interbedded near the margin of the valley, and the older deposits are truncated parallel to the river, suggesting that in time the river removes both its own deposits and those of tributaries. In the natural flow regimen, the river probably maintains a balance between erosion and deposition of mainstem and tributary deposits. Four terraces and the active channel and flood plain are widespread along the Virgin River; from oldest to youngest these are the prehistoric, settlement, historic, and modern terraces. Dating was done by archeologic context, tree-ring methods, historic documents, relocation of early photographs, and correlation with other streams on the southern Colorado Plateau. Results indicate that prehistoric deposition ended by about A.D. 1100–1200, deposition of the settlement alluvium was from about A.D. 1400–1880, development of the historic terrace was from after 1883 until 1926, deposition of the modern alluvium was from 1940–1980, and development of the active channel and flood plain was after about 1980. The principal deposits (prehistoric, settlement, and modern alluviums) are separated by two periods of stream entrenchment and channel widening referred to as the prehistoric and historic arroyo cutting, respectively. Erosional activity of roughly similar age occurred in most southern Colorado Plateau streams. The early erosion is not well dated in the study area, although regional relations suggest A.D. 1200–1400, if not somewhat earlier. Historic arroyo cutting began after 1883 in the study area and continued until around 1940 when deposition of the modern alluvium began. Both erosions affected the human population of the region. Although the dating is imprecise, the Anasazi probably abandoned the region during prehistoric arroyo cutting, partly because of adverse environmental conditions. Likewise, historic arroyo cutting caused major losses of property and economic hardship among Anglo settlers. Erosion and deposition were largely contemporaneous with variations in streamflow. Long-term streamflow of the Virgin River was estimated from calibration of annual tree growth with measured streamflow. Results indicate that erosion was during periods of unusually high streamflow and that deposition was during periods of relatively low streamflow. These relations are best illustrated by historic arroyo cutting and subsequent deposition of the modern alluvium. Precipitation and runoff immediately before and during historic arroyo cutting were the most unusual of the past 300 years; they varied from the driest immediately preceding erosion to the wettest during erosion. Deposition of the modern alluvium was during relatively low runoff after 1940. High runoff destabilizes the channel, enhancing flood erosion. Conversely, relatively low runoff increases channel stability, reducing the erosional effect of floods and enhancing flood plain deposition. Adjustments in the width and depth of the channel are frequent, even after relatively small, short-term variations of streamflow.