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U.S. Department of Interior U.S. Geological Survey

GEOMORPHIC HISTORY OF THE IN THE AREA, SOUTHWEST

U.S. GEOLOGICAL SURVEY OPEN-FILE REPORT 95-515

Prepared in Cooperation with the National Park Service

This report is preliminary and has not been edited for conformity with U.S. Geological Survey editorial standards or the North

American Code of Stratigraphic Nomenclature. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Digitized by the Internet Archive

in 2012 with funding from LYRASIS Members and Sloan Foundation

http://archive.org/details/geomorphichistorOOhere GEOMORPHIC HISTORY OF THE VIRGIN RIVER IN THE ZION NATIONAL PARK AREA, SOUTHWEST UTAH by

1 2 3 Richard Hereford , Gordon C. Jacoby , and V.A.S. McCord

U.S. GEOLOGICAL SURVEY OPEN-FILE REPORT 95-515

Prepared in Cooperation with the National Park Service

1 2255 North Gemini Drive Flagstaff, 86001 2Tree-Ring Laboratory Lamont-Doherty Earth Observatory

Palisades, New York 1 0964 3 Laboratory of Tree-Ring Research University of Arizona Tucson, Arizona 85721

. 1 1

CONTENTS

ABSTRACT 1 INTRODUCTION 2 Previous Studies 2 Methods 4 Generalized Hydrology of the Virgin River 5 GEOLOGIC SETTING OF THE ALLUVIAL VALLEYS 5 LATE SURFICIAL GEOLOGY AND GEOMORPHOLOGY 9 Mainstem and Tributary Deposits 10 Surficial Geology and Geomorphology of the Alluvial Valleys 10

East Fork Virgin River 1 North Fork Virgin River 17 Late Holocene Alluvial History 20 Regional Correlation of Alluvial Histories 21 FLOODS AND HISTORIC CHANGES IN THE CHANNEL OF THE VIRGIN RIVER 24

Historic Changes in the River Channel 24 Floods and Settlement 24 Historic Floods and the Beginning of Arroyo Cutting 26 HISTORIC PHOTOGRAPHS OF THE VIRGIN RIVER 28

North Fork in Springdale and Zion Areas 28 Virgin River in the Grafton and Rockville Areas 30 East Fork near Shunesburg 3 Discussion and Summary of Channel Conditions Inferred from Historic Photographs 32 STREAMFLOW HISTORY AND GEOMORPHIC CHANGE OF THE VIRGIN RIVER 54

Calibration and Verification of Reconstructed Streamflow 54

The Gaged Streamflow History, 1 9 1 0- 1 992 57 Annual and Seasonal Streamflow 57 Annual Flood Series 59 Streamflow and Geomorphic Change 61 Causes of Long-Term Streamflow Variation 63 Landuse 63 Climate Variation 64 Regulated Streamflow and Geomorphic Change 66 SUMMARY AND CONCLUSIONS 67 ACKNOWLEGMENTS 71 REFERENCES 71 FIGURES

1 Study area 3

2. Photograph of Crater Hill 6

3. Photograph showing alluvium in Virgin River valley 7 ..

CONTENTS (cont.)

4. Photograph of late gravel 8

5. Surficial geology of East Fork 12

6. Photograph of sediment-filled ditch 14

7. Photograph showing alluvial valley of East Fork 16

8. Photograph of modern alluvium 17

9. Surficial geology of North Fork 18

10. Correlation chart of upper Holocene alluvium 22

1 1 Photograph illustrating bedrock erosion 27 12-22. Photographs showing historic-channel development 34-56

12. North Fork and East Fork west of Springdale, 1873 and 1994 34

1 3 North Fork and , 1 873 and 1 994 36

14. North Fork and Great White Throne, 1873, 1929, and 1994 38

15. from , 1903, 1929, and 1993 41

16. , 1909 and 1994 44

17. North Fork from , 1909 46

1 8. Virgin River near Grafton, mid- 1920s and 1994 47

19. Virgin River near Rockville, 1937 and 1994 48

20. , 1873 and 1994 49

2 1 East Fork near Shunesburg, 1 926 and 1 994 51

22. East Fork at boundary fence, 1936 53

23. Average-annual daily discharge and ring-width index 55

24. Time series of actual and estimated average daily discharge 55

25. Reconstructed average daily discharge 56

26. Time series of average daily discharge 57

27. Time series of seasonal discharge 58

28. Time series of deviation from long-term discharge 59 29. Discharge of the annual flood 60

30. Reconstructed streamflow and geomorphology 62

3 1 Time series of smoothed ring-width indices 64 TABLES

1. Historic floods, Virgin River basin 25

2. Date, location, and source of historic photographs 29 GEOMORPHIC HISTORY OF THE VIRGIN RIVER IN THE ZION NATIONAL PARK AREA, SOUTHWEST UTAH by Richard Hereford, Gordon C. Jacoby, and V.A.S. McCord

ABSTRACT opment of the historic terrace was from after 1883 until 1926, deposition of the modern traces the geomorphic devel- This study alluvium was from 1940-1980, and develop- of the alluvial valley of the Virgin opment ment of the active channel and River in the Zion National Park region of was after about 1980. southwest Utah. The purpose is to identify, date, and interpret the patterns of erosion The principal deposits (prehistoric, set- and deposition that formed the alluvial val- tlement, and modern alluviums) are sepa- ley over the past 1,000 years. This informa- rated by two periods of entrenchment tion is a basis for understanding how the and channel widening referred to as the pre- geomorphology of the alluvial valley changes historic and historic arroyo cutting, respec- under essentially natural flow conditions. tively. Erosional activity of roughly similar age occurred in most southern Pla- Sediment in the alluvial valley is classi- teau . The early erosion is not well fied as mainstem or tributary in origin. dated in the study area, although regional Mainstem alluvium is the largest volumetri- relations suggest A.D. 1200-1400, if not cally; it is mainly light-colored sand derived earlier. Historic arroyo cutting from upstream sources that accumulated on somewhat began after 1883 in the study area and con- now abandoned (or terraces) by tinued until around 1940 when deposition of overbank deposition. Tributary sediment is the modern alluvium began. Both erosions typically dark colored, coarse-grained sand affected the human population of the region. or gravel transported to the alluvial valley The Anasazi abandoned the region during by streamflow or debris-flow. Tributary and cutting, partly because of mainstem sediment are interbedded near prehistoric arroyo adverse environmental conditions. Likewise, the margin of the valley, and the older depos- historic arroyo cutting caused major losses of its are truncated parallel to the river, sug- property and economic hardship gesting that in time the river removes both among Anglo settlers. its own deposits and those of tributaries. In the natural flow regimen, the river probably Erosion and deposition were largely con- maintains a balance between erosion and temporaneous with variations in streamflow. deposition of mainstem and tributary depos- Long-term streamflow of the Virgin River its. was estimated from calibration of annual Four terraces and the active channel and tree growth with measured streamflow. floodplain are widespread along the Virgin Results indicate that erosion was during River; from oldest to youngest these are the unusually high streamflow and that deposi- prehistoric, settlement, historic, and modern tion was during relatively low streamflow. terraces. Dating was done by archeologic These relations are best illustrated by his- context, tree-ring methods, historic docu- toric arroyo cutting and subsequent deposi- ments, relocation of early photographs, and tion of the modern alluvium. Precipitation correlation with other streams on the south- and runoff immediately before and during ern . Results indicate that historic arroyo cutting were the most prehistoric deposition ended by about A.D. unusual of the past 300 years; they varied 1100-1200, deposition of the settlement allu- from the driest immediately preceding ero- vium was from about A.D. 1400-1880, devel- sion to the wettest during erosion. Deposi- tion of the modern alluvium was during the alluvial valley is unknown, although relatively low runoff after 1940. streamflow is probably not decreased much High runoff destabilizes the channel, (fig. 1; Everitt, 1992). Rapid population enhancing the effectiveness of floods. Con- growth in the St. George and Hurricane area versely, relatively low runoff increases chan- (fig. 1) creates a demand for additional water nel stability, reducing the erosional effect of supplies. This demand could be met through floods and enhancing floodplain deposition. construction of reservoirs. Ninety two-reser- Adjustments in the width and depth of the voir sites are identified in the Virgin River channel are frequent, even after relatively basin (Utah Division of Water Resources, small, short-term variations of streamflow. 1988), and several large reservoirs are pro- The normal pattern of geomorphic change posed for construction in or near Zion requires streamflow that varies from high to National Park. Downstream of the reser- relatively low. Regulated streamflow would voirs, alluvial valleys in the park will quite likely alter this pattern through moderation likely change as the channels adjust to regu- of flow rates and reduced sediment loads. lated streamflow, as suggested by studies of other alluvial rivers (Williams and Wolman, 1984; Andrews, 1986). Flow regulation will INTRODUCTION eliminate the high flow rates that over time widen and deepen the channel, and reduced This study addresses the alluvial geo- sediment loads will eliminate or curtail morphology of the upper Virgin River in channel and floodplain deposition. In short, southwest Utah during the past 1,000 years, the magnitude of geomorphic change will the late Holocene of geologic time. The study probably decline from the unregulated regi- area lies in and near Zion National Park and men. Understanding the scope of geomorphic includes the North Fork (Virgin River) in change in the recent past is necessary to Zion Canyon below The Narrows, East Fork comprehend the effects of regulated stream- (Virgin River) in lower Parunuweap Canyon, flow on geomorphic development of alluvial and the Virgin River downstream of the park valleys in Zion National Park. to Virgin, Utah (fig. 1). The question of how natural variations of discharge and sediment load alter the alluvial valley and channel Previous Studies was the research topic. This information is The late Holocene geomorphic history of necessary to understand the role of dis- the Virgin River in the Zion National Park charge variability in the recent geomorphic area has not been studied. Gregory (1950, p. development of the alluvial valley, which in 44, 170-177) recognized that most valleys in turn effects riparian and aquatic resources. the Zion Park region and the southern Colo- These resources are linked to changes in the rado Plateau in general are filled with a con- alluvial valley, because alluvium is the sub- siderable thickness of alluvial fill of strate for riparian vegetation and the allu- Holocene age. He concluded that the surface vial channel is the habitat of six species of and upper portion of this fill are very young, native fish (Deacon, written commun., 1993). because historic documents and anecdotal Results of this study suggest that the allu- reports revealed that most streams, includ- vial valley has changed rapidly in response ing the Virgin River, flowed in fairly well- to natural discharge variations. defined channels with floodplains during the The Virgin River retains its presettle- 1700s to the late 1800s.

ment discharge regimen; the only impound- Streams in the Zion National Park region in ment the study area is Kolob Reservoir underwent a catastrophic change in the late 3 with capacity of 6.89 hm (5,586 ac-ft). The 1800s that Gregory (1950, p. 174) compared reservoir is located near the headwaters of with earthquakes and volcanic eruptions. North Fork, and its affect on streamflow and This destructive change of the alluvial val- 37°30'+

Kanab

Index Map

Figure 1. Study area in southwest Utah. leys involved downcutting of stream chan- tem unrelated to either climate or landuse nels up to 24 m (80 ft) followed by extensive are the main explanations (Graf, 1988, p. channel widening. This historic stream 220-224). The argument for overgrazing as entrenchment, or arroyo cutting, caused the the cause of increased floods and stream abandonment or movement of settlements, entrenchment in southwest Utah was aptly relocation of roads, and the destruction of made by Bailey (1935). He concluded that arable land, , reservoirs, and irrigation overgrazing during settlement of the region ditches. Indeed, the agricultural history of reduced and modified the plant cover on hill- the region was largely determined by floods slopes and valley bottoms, leading to and the resulting changes in stream chan- increased runoff and entrenchment. Expla- nels (Larson, 1961). Stream entrenchment of nations for arroyo cutting based on climate the type described by Gregory (1950) was are somewhat contradictory with entrench- widespread in the Southwest around the ment from increased precipitation, turn of the century, eventually affecting decreased precipitation, and from variation almost every stream (Bryan, 1925). of precipitation intensity.

The causes of historic arroyo cutting in The overgrazing explanation fails to the Southwest have been a topic of debate for account for arroyo cutting during prehistoric more than 90 years (Graf, 1983). Three time in the absence of domestic livestock. explanations are put forth in numerous pub- Furthermore, precipitation over the south- lications, these are summarized by Cooke ern Colorado Plateau during historic arroyo and Reeves (1976), Graf (1983, 1988), and cutting was the largest of the 20th century Webb (1985). Overgrazing, climate change, (Balling and Wells, 1990; Hereford and and internal adjustments of the channel sys- Webb, 1992), resulting in high streamflow and large floods (Webb, 1985; Graf and oth- cess until at least the early 1980s (Hereford, ers, 1991). Widespread instrumented rain- 1984; 1986; 1987a; Graf, 1987). This deposi- fall and streamflow records, however, begin tion since 1940 is widespread in the western after entrenchment, and association of high , as noted by Emmett (1974) rainfall with the initiation of regional arroyo and Leopold (1976). Older deposits, those cutting is conjectural. A local rainfall record predating arroyo cutting, were studied in the that predates arroyo cutting has a small Black Mesa region of northern Arizona by number of low-intensity events and a large Karlstrom (1988) and in Chaco Canyon and number of high-intensity events before and the eastern Colorado Plateau of during entrenchment (Leopold, 1951). These by Hall (1977). rainfall characteristics are thought to enhance runoff by destabilizing vegetation. Methods Briefly, historic arroyo cutting was associ- ated with settlement of the region, introduc- Several methods were used to study the tion of livestock, and unusually high late Holocene geomorphic history of the Vir- River. field precipitation and large floods. gin Geologic work was done by the senior author over three months in the The third explanation is that streams fall of 1992 and of 1993 and 1994. alternately erode fill as channel gradi- and This work consisted of mapping, dating, and ent adjusts to locally oversteepened seg- classifying the deposits of the alluvial val- ments. This is the semiarid cycle of erosion leys of North Fork and East Fork. The as developed by Schumm and Hadley (1957), sequence of terraces and associated deposits further elaborated by Patton and which was identified at these sites was then traced Schumm (1981). Although channels in small downstream to Virgin, Utah (fig. 1). The his- basins are affected by this process, large toric-age terraces were dated by V.A.S. basins drainage area greater per- with than McCord in a dendrogeomorphic study of haps 10 km (3.9 mi ) are probably not riparian trees, principally cottonwood, asso- altered by these adjustments of channel gra- ciated with the terraces at the two sites. A dient (Graf, 1988, p. 223). In addition, tem- long-term history of annual discharge of the poral correlation of modern deposits Virgin River was developed from a dendrohy- indicates they are synchronous both within a drologic investigation by G.C. Jacoby. This particular basin and among separate basins consisted of calibrating tree-ring chronolo- (Hereford, 1986; 1987a); regional correlation gies obtained at two semiarid sites to the is not expected if channels erode and fill epi- gaged streamflow record, resulting in a 303- sodically. Results of the present study sug- year streamflow reconstruction from A.D. gest that both historic and prehistoric arroyo 1690-1992. The reconstructed and measured cutting of the Virgin River were contempora- streamflow are used to interpret dated epi- neous with increased streamflow, although sodes of erosion and deposition. overgrazing could have increased the sever- Changes in the size and shape of the ity of historic arroyo cutting. channel of the Virgin River were studied The recent geomorphic history of streams using relocated historic photographs on the southern Colorado Plateau east of the obtained from a number of archival sources. Virgin River is addressed in several studies. The position of the original camera site was The timing and causes of the historic relocated and the scene was rephotographed entrenchment of nearby (fig. 1) in 1993-1994. The early photographs show and the were studied by the channel of the Virgin River in Zion Can- Webb and others (1991) and Webb (1985), yon as early as 1873. The photographs dis- respectively. Other studies showed that play a sequence of changes beginning with a entrenchment had largely ended by about relatively stable, unentrenched, and proba- 1940 and that deposition was the main pro- bly narrow channel in 1873. This early-set- tlement channel was followed by a wide, of the water year, is typically between July entrenched channel with a braided pattern and September. On average, 59 percent of in the early 1900s. Contemporary photo- the annual flood peaks of the Virgin River at graphs show a relatively narrow, stable, and Virgin are between July and September, a locally meandering channel. season with higher flood peaks than during spring. The annual flood, however, can hap- Generalized Hydrology of the Virgin pen in any month and can result from snow- melt, rain on snow, or rainfall. River Mean annual discharge of East Fork North East Fork, both with Fork and (which is only an estimate because of the drainage area of about 900 km (348 mi ), short record) and North Fork near Spring- head in the region at the south- 3 dale (fig. 1) is 1.4 and 2.8 m /s (50 and 100 ern boundary of the Markagunt Plateau in 3 ft /s), respectively. These average figures the high plateau section of the Colorado Pla- obscure the extreme variability of daily dis- teau physiographic province (Hunt, 1967, p. charge. Maximum daily flow rates range 278). The average elevation of the two basins from 4-70 times larger than the minimum is about 2,240 m (7,350 ft). North Fork flows flow rate for any given day. This variability through the central portion of the Kolob Ter- results from year-to-year fluctuations in pre- race in spectacular Zion Canyon. East Fork cipitation that increase either the depth of flows through Long Valley to Parunuweap the winter snowpack or the amount of sum- Canyon in Zion National Park. The two mer rainfall. Finally, streamflow depletions streams join downstream from Springdale. for irrigation and other purposes is esti- From there, the Virgin River flows west mated to account for only 3.7 and 6.6 percent across the where it enters of the mean annual flow of North Fork (96.9 the Basin-and-Range physiographic prov- 3 ince. Flowing to the south-southwest, the hm /yr, or 78,600 ac-ft/yr) and East Fork 3 river crosses southwest Utah, northwest Ari- (55.5 hm /yr, or 45,000 ac-ft/yr), respectively zona, and southeast to the junction (G.E. Diaz and W.R. Hansen, written com- with the at . mun., 1994). During July when demand is high and flow is low, depletion is 80-90 per- The Virgin River is a cent of monthly flow volume. Nevertheless, with baseflow from groundwater in the in terms of annual flow volume, the Virgin Sandstone (Jurassic), the dominant River in the study area is little affected by rock at the surface in Zion and Parunuweap agricultural or domestic water consumption. . Precipitation is biseasonal with rainfall during the warm season of summer and early fall and rain or snow in late fall GEOLOGIC SETTING OF THE through early spring. Most of the annual ALLUVIAL VALLEYS runoff volume above baseflow is during spring from March through early June. This The geologic setting of the Virgin River in runoff typically results from melting of the the Zion National Park region is relevant to winter snowpack. The snowpack forms in the understanding the late Holocene geomor- relatively high elevation of North Fork and phology of the alluvial valleys. The present East Fork basins from winter frontal-type elevation and course of the river result from precipitation. extensive bedrock erosion, or canyon cutting, A second runoff season results from rain- that occurred in the past 1-2 million years. fall in the summer and early fall seasons. Canyon cutting was controlled by movement Although the runoff volume is substantially along the Hurricane , which parallels less than the spring snowmelt, the annual the Hurricane Cliffs west of Virgin (fig. 1). flood, or the largest instantaneous discharge Pauses in canyon cutting during the late SJESara

Figure 2. Photograph of Crater Hill volcanic field and Virgin River valley. Dark hill in upper center is Crater Hill. Basaltic flows are dark, smooth, and flat areas above river. Base of youngest lava flow is 20-30 m (66-98 ft) above channel of Virgin River. Line indicates axis of ancestral Virgin River, dashed where covered by lava flows.

Pleistocene were accompanied by deposition power and the erosion of Zion and Parunu- of gravel that is preserved at several levels weap Canyon as well as the other canyons in along the margin of the alluvial valley. the region (Gregory and Williams, 1947). The fault has not caused surface rupture in the The Hurricane fault is usually consid- study area in historic times (Anderson, ered the physiographic boundary between 1978). Thus, historic arroyo cutting of the the Basin-and-Range province to the west Virgin River is not related to tectonic activ- and the Colorado Plateau to the east (Hunt, ity, a conclusion that quite probably applies 1967, p. 277-347). A system of faults, the to erosion and deposition during the late Hurricane fault extends north from Grand Holocene as well. Canyon through southwest Utah. In the Zion National Park region, the fault has normal The Virgin River was probably within 20- displacement of 600-850 m (2,000-2,800 ft); 30 m (65-100 ft) of its present elevation and movement on the fault was mainly in the the canyons were eroded to near their Quaternary (Anderson and Mehnert, 1979). present depths by at least 293 ka (which is The eastern block or footwall of the fault has 293,000 years before present). This interpre- moved up while the western block remained tation is based on the inferred age of basaltic

stationary (Hamblin, 1984). Repeated, epi- lava flows associated with Crater Hill (fig. 2), sodic uplift of the eastern block of the fault is a volcanic field on the north side of the valley. the principal cause of canyon cutting. This Two periods of volcanism and four lava flows uplift increased the gradient of the ancestral are recognized by Nielson (1977). The older Virgin River, resulting in increased stream flows are assigned to Stage-II relative geo- fc *

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Figure 3. Photograph (upstream) of alluvial valley of Virgin River in Rockville area. Alluvial valley is flat, vegetated area extending diagonally across mid-ground. Moenkopi Formation borders valley, resistant sandstone bed in right mid-ground is Shinarump Sandstone Member of Chinle Formation (Late Triassic). Late Pleistocene gravel deposits form terrace to right of road junction in foreground. morphic age and the younger flows are the Crater Hill area. assigned to Stage-Ill relative age by Nielson flows of the younger Stage-Ill epi- (1977). This classification system is used to sode are related to the present course of the establish relative age of basalt flows in Virgin River. Although the absolute age of southwest Utah based on geomorphic evi- the Stage-Ill flows at Crater Hill is dence of and erosion (Hamblin, unknown, a Stage-Ill flow near Hurricane 1970). Stage-II flows were deposited about was dated by the Potassium/Argon method 60-90 m (200-300 ft) above the present drain- at 293 ± 87 ka (Hamblin, 1984). The Stage- age, and Stage-Ill flows were deposited Ill flows at Crater Hill could also be this age, about 5-30 m (20-100 ft) above the present in which case the river was essentially in its drainage. Lava flows during the Stage-II present position by 293 ka. period of volcanic activity filled the ancestral valley of the Virgin River (fig. 2), eventually Figure 3 illustrates the alluvial valley of causing the bedrock channel to shift 1-2 km the Virgin River in the Rockville area. The (0.6-1.2 mi) south of the previous course, as width of the valley is controlled by the under- noted originally by Threet (1958). The abso- lying bedrock. In this area (fig. 3), the valley lute age of the Stage-II flows and the mini- is wide as the river flows across the non- mum age of the ancestral valley is uncertain, resistant, slope-forming Moenkopi Forma- although Hamilton (1987; 1992) reports a tion. Generally, the valley is relatively wide date of 500 ka for possible Stage-II flows in where the river crosses the less resistant \-+\r

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Figure 4. Photograph showing late Pleistocene gravel with interbedded sand in quarry south of Rockville. Note scale on left, 1.4 m (4.6 ft) long with 20 cm (5.08 in) divisions. This is topo- graphically the lowest and youngest gravel. bedrock of the Moenkopi, Chinle, Moenave, above the floor of the alluvial valley. The ter- and Kayenta formations. The age of these races are best developed in the reach down- formations is Early Triassic, Late Triassic, stream from the Shunesburg area to and Late Triassic to Early Jurassic, and Grafton. According to Dalness (1969), the Early Jurassic, respectively. The upper gravels consist of two units. The older gravel, Holocene alluvium occupies most of the val- informally termed the Parunuweap forma- ley from side-to-side in the wide reaches. tion, is well-consolidated fluvial gravel with Upstream in Zion Canyon, the valley is rela- locally derived, subrounded to rounded tively narrow where the bedrock is the resis- clasts. Large, angular boulders up to several tant, cliff-forming Navajo Sandstone meters on an edge are present locally and (Jurassic). In the slot-like canyons of The were derived from nearby rockslides. This Narrows (North Fork) and in upper Parunu- older gravel mostly predates the Crater Hill weap Canyon (East Fork; fig. 1), very little volcanics, as the gravel was deposited in the sediment accumulates and alluvial terraces ancestral valley that was subsequently filled are mostly absent because of the high water by the older Stage-II lava flows. velocities produced in the narrow, bedrock- The younger gravel unit, informally lined channels. referred to as the Orderville gravel, is gener- Gravel deposits of late Pleistocene age ally unconsolidated, although otherwise it are present locally in the wide reaches of the resembles the Parunuweap gravel. This Virgin River downstream from the forks and younger gravel post-dates the Crater Hill in East Fork (fig. 4). These deposits form sev- volcanics, therefore the gravel is probably eral discontinuous terraces rising 30-50 m younger than 293 ka. The younger gravel is well exposed in a quarry south of Rockville LATE HOLOCENE SURFICIAL (fig. 4). These deposits are substantially GEOLOGY AND thicker and much coarser-grained than the GEOMORPHOLOGY upper Holocene alluvium, which is mainly fine- to medium-grained sand with only In this section of the report, the geology minor gravel. The gravel deposits probably and geomorphology of East Fork and North result from increased competency of the Vir- Fork are discussed. The alluvial valleys were gin River during the late Pleistocene as well studied in lower Parunuweap Canyon as increased mechanical weathering that upstream from the abandoned, pioneer set- produced abundant gravel-size sediment tlement of Shunesburg and in Zion Canyon from the ledge-forming -age sand- downstream of The Narrows (fig. 1). The stone in the headwaters of the basin (Sable study consisted of mapping, classifying, and and Hereford, 1990). dating the various deposits and terraces at The geologic setting of the alluvial valley the two sites. Mapping was done in the field of East Fork is similar to that of the Virgin using low-altitude (approximate scale River downstream from the forks, as dis- 1:4,000) color-aerial photographs taken in cussed above, except that bedrock is the June 1992. Ground control surveyed by W.R. Chinle and Moenave Formations near river Hansen of the National Park Service was level. The North Fork in Zion Canyon, how- used to develop 1:2,000 scale planimetric ever, differs substantially in that the alluvial base maps of the two areas; these maps were valley is lacustrine deposits underlain by used to compile the surficial geology mapped that predate the upper Holocene alluvium. on the aerial photographs. Dating was done These deposits accumulated in a lake that by archeologic context in the case of the pre- formed upstream of a large that historic deposits and by tree-ring methods in originated on the west side of Zion Canyon the case of the historic deposits. The about 5 km (3.1 mi) upstream of Springdale; sequence of terraces at the two sites is the landslide measures about 2.4 km (1.5 mi) broadly similar, and the terraces were traced long and 1.2 km (0.75 mi) wide (Grater, as far downstream as Virgin, Utah (fig. 1). 1945). The landslide blocked the entire Therefore, the Virgin River downstream of width of the canyon near river level, and the the forks as well as East Fork and North resulting lake was up to 115 m (377 ft) deep Fork have similar geomorphic histories. and extended up the canyon at least 4.3 km (2.6 mi; Grater, 1945; Hamilton, 1992, p. 59- Generally, the streams have undergone 60). The minimum age of the lake is 3.6±0.4 several episodes of aggradation and erosion ka based on a radiocarbon date from plant in the past 1,000 years. Aggradation builds material associated with the lake deposits up the channel and alluvial valley, erosion (Hamilton, 1992, p. 59). lowers the channel and partly removes pre- The landslide and related lake deposits viously deposited alluvium and tributary lower the gradient of North Fork through deposits, producing terraces, or abandoned floodplain surfaces. timing Zion Canyon (Hamilton, 1992, p. 25). In The and number addition, the slide debris is a fixed baselevel of erosional and depositional events of the control that limits erosional downcutting. Virgin River is generally similar to other This baselevel control is probably why the streams on the southern Colorado Plateau. upper Holocene stream terraces in the can- This suggests that erosion and deposition yon have very little topographic separation. are triggered and maintained by regional Similar terraces in East Fork and the Virgin factors. These factors are climate over the River downstream of the forks are each sep- long term as well as landuse during the his- arated vertically by several meters. toric period. Mainstem and Tributary Deposits and because of the reddish-purple color of the clay-size component. Debris-flow depos- Sediment in the alluvial valleys comes its have a wide range of thickness, ranging from upstream sources as well as from from only a few centimeters where they are nearby, adjacent hillslopes. Mainstem depos- interbedded with mainstem alluvium to sev- its are transported by the Virgin River from eral meters at the valley margin. distant sources. Tributary deposits are Landslide deposits are mainly coherent transported to the alluvial valley by fluvial blocks of local bedrock that have slid downs- activity of tributary streams, gullies, and lope into the alluvial valley. The size of the rills, as well from debris flow and landslide. landslide blocks ranges from a few square Near the margin of the alluvial valley, main- meters, the typical case, to several square stem and tributary deposits are interbedded. kilometers (Grater, 1945). Rockfall deposits Mainstem deposits are primarily yellow- typically consist of isolated blocks of resis- ish gray, poorly to moderately well-sorted, tant bedrock that have fallen from steep very fine to medium-grained sand and minor ledges down the hillslopes into the alluvial subrounded to rounded gravel. The alluvium valley. is well bedded; beds range in thickness from The older mainstem and tributary depos- only several centimeters to as much as 50 cm its are confined to the margins of the alluvial (1.6 ft). These beds are primarily flood depos- valleys. Generally, the margin of the deposits its, the result of overbank floods at relatively is truncated parallel to the river, forming the high discharge levels. The exposed thickness older terraces. This truncation or erosion is of the mainstem alluvium ranges from 1-5 m evidence that in time the river removes both (3-16 ft). Mainstem deposits form several its own deposits and those of tributary ori- terraces and the active channel and flood- gin. In the natural flow regimen, the river plain. Surficial geologic maps, discussed in a maintains a balance between erosion and following section, show that mainstem allu- deposition of mainstem and tributary depos- vium is the principal deposit in the alluvial its. If tributaries were unaffected by main- valleys. stem erosion, sediment input from tributary Although of minor volumetric impor- sources would probably block the river form- tance, tributary deposits are conspicuous ing small lakes and ponds. along the margins of the alluvial valley and on the steep hillslopes leading to the valley. Surficial Geology and Geomorphology Tributary deposits result from streamflow, of the Alluvial Valleys debris flow, landslide, and rockfall. Tributary streamflow deposits are in the active chan- The two study sites differ in physical and nel and in terraces of the relatively small, floral setting. East Fork is a relatively open ephemeral, and typically unnamed tributar- valley formed in the slope-forming, easily ies of the Virgin River. These deposits consist eroded Moenave Formation (Jurassic). Vege- of sand and subrounded gravel of cobble to tation is mostly shrubs with subordinate boulder size. Debris-flow deposits are in trib- grass and a sparse woodland of cottonwood utary channels, in debris fans at the junction (Populus fremontii). On the other hand, with the Virgin River, and as levees extend- North Fork is a confined valley with nearly ing down steep hillslopes to the alluvial val- vertical cliffs of Navajo Sandstone (Jurassic). ley. Debris flow is a gravity induced mass Woodland with a grass understory is the movement of sediment with mechanical dominant vegetation. These differences in characteristics that differ from streamflow vegetation result mainly from aspect; North (Costa, 1984). The debris-flow deposits in Fork is well shaded because of the high cliffs, East Fork are distinctive because of the wide narrow canyon, and the north-south orienta- range of particle size ranging from clay to tion. Of the two streams, East Fork has the angular boulders 1-3 m (3-10 ft) on an edge most complete record of upper Holocene geol-

10 ogy and geomorphology. For this reason, the three isolated outcrops. These deposits are composition, age, and interpretation of the probably equivalent to the gravel at river deposits in East Fork are discussed first. level in the Rockville area (fig. 4). The gravel at Rockville post-dates the Crater Hill volca- East Fork Virgin River nics; therefore the gravel in Parunuweap Canyon is younger than 293 ka. The surficial geology of the East Fork site is shown in figure 5. The east boundary of The prehistoric terrace is the oldest Holocene terrace identified in Fork. the map is near Stevens Wash 2.6 km (1.6 East The terrace is of allu- miles) upstream from Shunesburg (fig. 1). confined to the margin the The surficial geology consists of five deposits vial valley and is situated 6-10 m (20-33 ft) of late Holocene age and a gravel deposit of above the modern terrace (cross-section A-A', Pleistocene age. Four terraces and the active fig. 5). The principal exposures are the channel and floodplain comprise the mouth of Stevens Wash, the unnamed wash Holocene deposits of East Fork. From oldest on the south side of the river, and the to youngest, the terraces are referred to as unnamed wagh near the west end of the the prehistoric, settlement, historic, and mapped area (fig. 5). At these localities, the modern terrace. plan-form of the terrace resembles an allu- vial fan that has been eroded along the distal Pleistocene gravel deposits crop out in margin. A highly eroded exposure of the pre- three mound-shaped hills that rise 30-40 m historic deposits is also on the north side of above the modern terrace (cross-section A-A', the river near survey control point 1205.17 fig. 5). The base of the gravel is not exposed; m. At this locality, the alluvium is overlain at least 30-40 m of gravel is exposed locally, by 1-2 m (3-7 ft) of historic-age debris-flow which is probably close to the maximum sediment that forms the surface. thickness. Although poorly exposed, the deposit is coarse-grained sand and granule- Alluvium underlying the prehistoric ter- to boulder-size gravel. The gravel is well race is poorly exposed, but it appears to con- rounded, and the clasts are derived from dis- sist mainly of light-colored very-fine to tant bedrock sources exposed near the head- medium-grained sand interbedded with waters. The sedimentary clasts are mainly numerous dark-colored beds of debris flow coarse-grained sandstone of Cretaceous age, origin. Gravelly debris-flow deposits are clasts of fine-grained Navajo Sandstone, and present on the surface at the outcrop near finely crystalline limestone of the Carmel the west boundary of the map and at the out- Formation (Jurassic). Well-rounded basalt crop on the south side of the river. The sur- clasts derived from lava flows in the Long face gravel is patinated with a brown- to Valley area are present sparingly. dark-brown rock varnish. Beneath the sur- face, clasts are coated with a very thin, The gravel once filled an ancient channel mostly continuous coating of calcium carbon- in Parunweap Canyon. The contact of the ate. This evidence of soil development indi- deposit with bedrock rises steeply from cates that the surface has been exposed to below the level of the prehistoric terrace to long-term weathering. the top of the deposit; this contact defines the margin of the ancient channel. Parunuweap The age of the prehistoric terrace and Canyon has widened and deepened substan- related deposits is not well constrained. The tially since deposition of the gravel. Erosion base of the deposit is not exposed and the deepened the canyon at least 40 m (130 ft) beginning of deposition could not be deter- and the canyon widened as much as 100-200 mined. However, dateable cultural remains m (330-660 ft), as shown by retreat of bed- of the Virgin branch of the Kayenta Anasazi rock that formed the margin of the channel. (Altschul and Fairley, 1989) are present near Subsequent widening and downcutting the top of the alluvium at two localities. On removed most of the gravel, leaving only the the south side of the river about 50 m (160 ft)

11 12 northwest of spot elevation 1210.2 m (fig. 5), ded with the alluvium. a small structure of upright slaps is present The alluvial valley upstream of Shunes- 1 (3 ft) beneath the surface. Ceramic mate- m burg was farmed extensively beginning in rial associated with the site is tentatively spring 1862 (DeMille, 1977). Evidence of this assigned to the early Pueblo I to middle and subsequent farming is found on both the Pueblo-II periods, which is A.D. 800-1100. surface of the prehistoric and settlement ter- On the north side of the river 0.6 km (0.4 mi) races, but is characteristic of the lower ter- downstream from the west boundary of the race. Pieces of cut and milled wood, small map, a midden containing dateable ceramics rock piles, steel wire, and other artifacts are lies 1 ft) below the surface of the prehis- m (3 on the settlement terrace. Grass and Rus- toric alluvium. The ceramic material dates to sian thistle on these terraces suggest that the Pueblo-I period, which is A.D. 800-1000. the area was farmed historically and aban- third site on the east bank of Stevens Wash A doned (L.D. Potter written commun., 1993). has several small structures and a midden Vegetation was probably removed from all of extending from the surface of the prehistoric the settlement terrace and large parts of the terrace to a depth of more than 2 (7 ft). m prehistoric terrace. The terrace rise separat- Ceramic material near the top of the site ing the terraces is rounded and subdued, dates to the late Pueblo II period, or A.D. whereas the terrace rise between younger 1100 to 1200 (L. P. Naylor, written commun, terraces is steep and sharp. This subdued 1994). topography probably resulted from plowing The stratigraphic context of these arche- and vegetation removal by the pioneer set- ologic sites indicates that the Anasazi were tlers. living on the floodplain of East Fork between Remnants of irrigation ditches excavated A.D. 800 to at least 1100-1200. The prehis- into the prehistoric terrace are on the south toric alluvium had aggraded to its present and north side of the river. The two rem- level by late Pueblo II time (A.D. 1100-1200). nants were traced as far downstream as In short, deposition of the prehistoric allu- Shunesburg, and the north-side ditch was vium was well underway by A.D. 800-1100 traced upstream of Stevens Wash for 0.8-1.5 and probably ended after A.D. 1100-1200, km (0.5-1 mile), where the inlet structure for although the beginning of deposition is not the ditch was probably located. The north- well known. side ditch is evidently not the same ditch portrayed in a 1903 map of arable land in the The settlement terrace is situated 2-4 m Virgin River basin (Adams, 1903, p. 208- (7-13 ft) below the prehistoric terrace (cross- 209). The inlet of the north-side ditch shown section A-A' fig. 5). The settlement terrace in the 1903 map was at the mouth of Stevens and related alluvium are named for historic Wash. Remains of the 1903 ditch could not be archeologic material and evidence of agricul- found downstream of Stevens Wash, proba- tural activity on the surface. This terrace is bly because it was constructed in the his- present on the north side of the river, except toric-age channel and was subsequently for two small outcrops on the south side of eroded or buried by younger sediment. the river west of the unnamed wash (fig. 5). The composition of the deposits resembles The north-side ditch is well exposed in a the prehistoric alluvium, except that coarse- gully near survey control point 1215.07 m grained debris-flow sediment is not abun- (fig. 5); figure 6 is a cross-section of the ditch dant. At the exposure on the south side of the exposed in the west side of the gully. The river, the alluvial sand is well bedded with light-colored sand beds outline the perimeter light-colored beds ranging in thickness from of the ditch, which extends vertically from 10-50 cm (0.3-1.6 ft). Thin beds of dark-col- the base of the collapsible shovel to the ored silty sand derived from the bedrock trowel. Deposits in the ditch are light-colored exposed on the nearby hillslope are interbed- mainstem alluvium interbedded with dark-

13 Figure 6. Photograph showing irrigation ditch filled with sediment and overlain by debris- flow deposits, id = cross-section of irrigation ditch; df = debris-flow deposits; p = prehistoric alluvium interbedded with debris-flow sediment. colored tributary deposits. The mainstem must have been the active channel at the deposits accumulated when streamflow was time the ditch was constructed. This level is diverted into the ditch; the tributary depos- several meters above the present channel of its resulted from sheetwash originating on East Fork, which at this locality has deep- the nearby bedrock hillslope. The present ened at least several meters since the late physical setting of the ditch demonstrates 1800s. the rapid and extensive changes that The age of the settlement terrace could affected the alluvial valley. Sometime after not be determined directly due to the lack of 1862 and before at least 1903, the top of the dateable material. The inset topographic ditch, which corresponds with the prehis- relation with the prehistoric terrace (cross- toric terrace, was covered by 2 m (7 ft) of section A-A', fig. 5) indicates that the settle- debris-flow sediment; enlargement of the ment terrace and related alluvial deposits tributary channel then eroded through the are younger than the prehistoric terrace. debris-flow sediment, irrigation ditch, and Moreover, Anasazi cultural remains are not terrace; this erosion was probably coincident present on the terrace nor have remains with entrenchment of East Fork between been found in the alluvium. Altschul and 1883 and 1893. Fairley (1989, p. 107) suggest that the Virgin Upstream from Stevens Wash to near the Anasazi abandoned the area inlet area, the older north-side ditch crosses around A.D. 1225. This is largely consistent the prehistoric terrace to the level of the set- with Larson and Michaelsen (1990) who esti- tlement terrace; in the inlet area, the ditch is mate that the area was abandoned by A.D. low on the settlement terrace near what 1200. Deposition of the alluvium forming the

14 settlement terrace, therefore, is younger Cottonwood and one Utah juniper (Juniperus than about A.D. 1200-1225, or simply A.D. osteosperma) growing on or just beneath the 1200. As previously discussed, Anglo settlers terrace were sampled and dated. The loca- utilized the terrace for farming and portions tion of the trees and results of this dating are of the pioneer settlements of Rockville, shown in figure 5. The germination dates Grafton, and Virgin City (fig. 1; Larson, range from 1893-1909. The dates show that 1961, p. 85-100) were located on the terrace historic downcutting of the East Fork allu- between 1859-1862. The upper part of the vial valley happened by 1893 and that the alluvium and terrace, therefore, are mostly floodplain and channel of the historic terrace older than to partly contemporaneous with were in place through 1909. early Anglo settlement of the region. Historic The modern terrace is present almost descriptions discussed in a following section continuously on both sides of the river of the report suggest that the settlement ter- throughout the mapped area (fig. 5). The ter- race was the active channel and floodplain of race forms two large point bars on the north the river in the early 1860s. In short, deposi- and south side of the river west of the south- tion of the alluvium forming the terrace was side unnamed wash (fig. 7). Elsewhere the after A.D. 1200 until at least about A.D. terrace is a recently abandoned floodplain on 1880. both sides of the active channel. The terrace The historic terrace is an abandoned is only 1-2 m (3-7 ft) above the active channel channel situated about 3 m (10 ft) below the and it shows little evidence of recent flood- settlement terrace and 1-2 m (3-7 ft) above ing. Modern flood debris is present on the the modern terrace (cross-section A-A', fig. terrace and consists of automobile tires dat- 5). The historic terrace is present mainly on ing from the 1960s to early 1970s, tin cans, the north side of the river where it occupies abundant cut driftwood, and short lengths of meander-like features eroded into the settle- steel cable. Plastic was not found on the ter- ment terrace (fig. 5). On the surface, the race, suggesting that exposure time was long deposits resemble mainstem alluvium enough for the plastic to deteriorate. A flood except that the sand is coarser grained and on August 29, 1993 with discharge calcu- 3 3 cobble to small-boulder size gravel deposits lated at 54.9 m /s (1,940 ft /s) and recur- are present locally, such as the near spot ele- rence interval of about 10 years flowed on vation 1207.6 m in the eastern part of the the modern terrace locally, wetting the base map. Although the deposits are poorly of cottonwood trees near the channel margin exposed, they are probably very thin and do (E.D. Andrews, written commun., 1994). not represent substantial deposition. Photo- Tree-ring dating of cottonwood was used graphs of the channel near this area from the to date the modern terrace. Cottonwood is early 1900s show that the channel was flat- widely distributed across the terrace as dis- floored without sites of overbank deposition. tinct, arcuate zones along the margin of the The historic terrace, therefore, is considered two point bars and in linear zones along the to be largely an erosional feature resulting near-channel margin of the floodplain (fig. from channel widening and deepening. 7). Although they were not excavated, these Archeologic material dating from early set- trees are growing beneath the terrace, as tlement of the area is absent on the terrace; indicated by the absence of root collars and evidence of agricultural activity and stone or the fence post-like appearance of the trunks. wood structures were not found on the sur- The distribution of sample sites and the tree- face in the mapped area. ring dates are shown in figure 5. Seven trees The date of downcutting from the settle- were sampled; germination dates range from ment terrace to the level of the historic ter- 1940-1963. These dates indicate that the race as well as the age of the historic terrace alluvium began to aggrade by at least 1940 was determined by tree-ring dating. Twelve and that deposition continued after 1963.

15 Figure 7. Photograph showing alluvial valley of East Fork and prominent point bar of modern terrace on south side of valley, a = active channel; m = modern terrace; p = prehistoric terrace.

Alluvium of the modern terrace is well rowed, disrupting the continuity of the strat- exposed in the cutbanks of the active channel ification. Nevertheless, six-mainstem beds and floodplain. Figure 8 shows the alluvium are present in the upper unit, indicating that on the south side of the river just upstream the unit accumulated from at least six-over- of a large rockfall clast southeast of spot ele- bank floods. vation 1202.0 m (fig. 5). The alluvium con- The active channel and floodplain are 1-2 sists of units, two a basal sand about 50 cm m (3-7 ft) below the level of the modern ter- (1.6 ft) thick and an upper stratified sand. race. A steep cutbank typically rises above The basal unit is massive without distinctive the floodplain and channel to the level of the stratification, largely the result of fluvial modern terrace. The channel and floodplain reworking. A cottonwood tree resembling are not subdivided in figure 5, because they those on the modern terrace that date are too small to show separately at the com- between 1940-1963 is rooted on the top of the pilation scale (1:2,000) of the map. Channel basal unit, suggesting that the unit predates deposits are mainly gravel of pebble to cobble 1940-1963. The upper unit in the right side size. The channel contains discharge up to and 3 3 upper portion of the exposure (fig. 8), bankfull, which is about 7 m /s (240 ft /s; consists of yellowish-gray beds of very fine to E.D. Andrews, 1993, written commun.). fine-grained sand of mainstem origin with Floodwater spreads across the floodplain nterbedded dark-reddish brown silty and above this discharge level. Floodplain depos- clayey sand derived from erosion of nearby its are mainly very fine to fine-grained sand lslopes. The deposits are extensively bur- that is poorly to moderately well sorted. Two

16 ..'-.*?

Figure 8. Photograph showing exposure of basal and upper unit of modern alluvium in East Fork. Vertical bar is 30 cm long. Light-colored beds are mainstem deposits; dark-colored beds are sheetwash deposits of clay, silt, and sand. saltcedar trees {Tamarisk chinensis Lour.) Park (Hamilton, 1987) shows upper growing beneath the surface of the floodplain Holocene lacustrine deposits in the mapped were dated by ring counts. The germination area (fig. 9). As previously discussed, these dates were 1980, indicating that the flood- deposits accumulated in a lake that formed plain began to aggrade after 1980 and that in Zion Canyon sometime before 3.6 ka the modern terrace had been incised by that upstream of a large landslide (Grater, 1945; date. Hamilton, 1992, p. 59). However, in the study area only fluvial deposits are present on the surface and in cutbank exposures. North Fork Virgin River Sampling with a 2 m (7 ft) boring tool shows that lacustrine deposits are not present in Figure 9 shows the surficial geology of the shallow subsurface, although lacustrine the North Fork study site in Zion Canyon. deposits could be present at depth. The northern boundary of the map is 0.37 km (0.23 mi) south of The Narrows parking Pleistocene gravel deposits are not area. Bedrock exposed at river level and in present in upper Zion Canyon, the result of the walls of Zion Canyon is the Navajo Sand- nondeposition, burial by lacustrine sedi- stone. This formation is well cemented and ment, or lack of preservation in the relatively lacks clay and silt, therefore, it resists slope narrow canyon. The valley consists entirely failure by debris-flow. Rockfall is an impor- of alluvial deposits and terraces that are tant process, however, and the slopes at the similar in age and number to those in East base of the cliffs are covered with rockfall Fork, although the upper Holocene terraces debris. The geologic map of Zion National of North Fork have significantly less topo-

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18 graphic separation than those of East Fork. suggest that this surface is equivalent to the In addition, some uncertainty is added to settlement terrace of East Fork. correlation of the terraces by flood deposits of As cross-section A-A' (fig. 9) shows, the the modern terrace. These deposits resulted historic terrace is present below the settle- from the largest flood of the gaged record on ment terrace in the north portion of the map December 1966. This flood carried sand 6, area. The woodland vegetation on the his- far out of the channel onto the historic ter- toric terrace differs from the settlement ter- race and locally onto the settlement terrace. race in that gambel oak is largely absent; the

A terrace about 3-4 m (10-13 ft) above the dominant tree of the historic terrace wood- active channel and floodplain is present on land is velvet ash (Fraxinus velutina) with the west side of North Fork. This terrace is subordinate boxelder and minor cottonwood probably equivalent to the prehistoric ter- near the river. When traced south-southwest race in Parunweap Canyon. The deposits are from the line of cross-section A-A' (fig. 9), the very poorly exposed, but they are mostly topographic separation between the settle- fine-grained sand. Sandstone clasts on the ment and historic terrace becomes indis- surface have a very thin coating of calcium tinct, indicating that the historic alluvium carbonate on their undersides. This degree of was locally deposited on the settlement ter- soil development is similar to the prehistoric race in this downstream location. terrace in Parunuweap Canyon. Archeologic Tree-ring dating of cottonwood and velvet remains were not found on the terrace in ash was used to date the settlement, historic, Zion Canyon; thus correlation with East and early modern terraces. The location of Fork is inconclusive based on archeologic the sampled trees and the dates are shown in context. Nevertheless, the high terrace in figure 9. The 13 dates fall into three catego- North Fork is probably correlative with the ries: 1867-1883, 1892-1906, and 1926-1937. prehistoric terrace in East Fork based on The older dates are from four cottonwood degree of soil development and position trees at the southernmost outcrop of the set- above the active channel and floodplain. tlement terrace. These trees appear to have on or very near the surface, The equivalent of the settlement terrace germinated although the exact germination position is is present on the west side of the river at the difficult to determine because of the large north end of the mapped area (fig. 9). Here size. It seems likely that the trees germi- the settlement terrace is about 3 m (10 ft) nated on the floodplain or high sand bar of above the active channel and 1-2 m (3-7 ft) between 1867-1883. Between above the historic terrace (cross-section A-A' North Fork 1883-1892, this floodplain was abandoned fig. 9) in the north part of the map area. Veg- and the channel was deepened, causing the etation is a woodland (L.D. Potter, written to flow at the lower level of the historic- commun., 1993) of Gambel oak (Quercus river gambelii) with subordinate boxelder (Acer age channel. nugundo). A photograph (Anderson, Number The intermediate-age category is from 249) taken from the saddle near The Pulpit five trees associated with the historic ter- in 1909 shows this terrace just upstream of race, although a velvet ash dated at 1903 is the map boundary. The photograph shows within the zone of the modern-terrace flood that the terrace was furrowed as if it had deposits of December 1966, which are dis- been plowed, and the river channel appears cussed below. Two cottonwood trees dated at adjacent to and immediately below the level 1892 and 1902 germinated on the surface of of the terrace. A map of agricultural areas in the historic terrace. Erosion of the settle- the Virgin River basin published in 1903 ment terrace and deepening of the channel to shows irrigated farmland in this area the level of the historic-age channel was (Adams, 1903, p. 208-209). This information probably from 1883 to 1892. Finally, dates and the position of the terrace in the valley from the early modern terrace indicate that a

19 floodplain had formed below the historic ter- saltcedar trees, which are partly buried in race between 1926-1937. floodplain alluvium, yielded germination The modern terrace has three topo- dates of 1983, 1985, and 1990. These dates graphic levels that are shown as two map indicate that the channel and floodplain attained their present position units in figure 9. The oldest and highest level by at least 1983. of the modern terrace is the elevated island or bar-like feature at the north end of the map area, this is mapped as the early mod- Late Holocene Alluvial History ern terrace. The lower level is on the east Deposition of the prehistoric alluvium side of the bar extending downstream to was ongoing at A.D. 800-1100. This deposi- south of survey control point 1334.61 m. tion continued for some time after A.D. 800- There is virtually no topographic separation 1100, because an additional 1-2 m (3-7 ft) of between the historic terrace and the modern sediment accumulated above the dated terrace at this location (cross-section A-A' archeologic horizons. Sometime after A.D. fig. 9). The deposits of this intermediate level 1100-1200, the prehistoric alluvium was are moderately sorted, light gray, fine- to entrenched. Entrenchment deepened and medium-grained sand, which is coarser widened the channel, which was subse- grained and lighter colored than the nearby quently filled by the settlement alluvium. historic terrace. Flood debris is sparse, con- The date of entrenchment is poorly known, sisting of a rubber hose and a piece of milled but deposition of the settlement alluvium wood. Woodland vegetation on the surface was after A.D. 1200, based on the absence of resembles the historic terrace, except the Anasazi cultural remains. Accumulation of trees are partly buried by the light-colored the settlement alluvium continued until at sand. These deposits are probably related to least A.D. 1880 on both tributaries. The allu- the December 6, 1966 flood with a discharge vium was entrenched after 1883 on North 3 3 of 260 m /s (9,150 ft /s). This is the largest Fork, and the river was flowing at the level of flood of the gaged record (1926-present) on the historic terrace by 1892. Entrenchment North Fork. The lowest level of the modern of East Fork happened after 1873 based on terrace is in the vicinity of spot elevation photographic evidence, and the river was at 1333.46 m (cross-section B-B* fig. 9). This the level of the historic terrace by 1893. Geo- level is about 1 m (3 ft) below the intermedi- morphic similarities between the two tribu- ate level of the terrace. Two saltcedar trees taries suggest that East Fork was also were sampled and dated from a grove along entrenched after 1883. the terrace rise between the historic terrace The historic terrace was the channel of and the lower level of the modern terrace the river beginning about 1892 or 1893, on south of spot elevation 1334.58 m. The ger- North Fork and East Fork, respectively. The mination dates were 1969 and 1971, suggest- river was near this level in 1906 on North ing that the lower level of the terrace Fork and in 1909 on East Fork. By about developed by at least 1969. 1926 the channel of North Fork was deep- The active channel and floodplain are ened to the early modern level and the his- typically separated from the modern terrace toric channel was abandoned. The basal unit by steep cutbanks or gently sloping banks of the modern alluvium at East Fork (fig. 8) that range in height from 0.5-1 m (1.6-3 ft). probably corresponds with the early modern Deposits in the channel consist mainly of terrace. This correlation is suggested by a pebble to cobble-size gravel. Floodplain tree of the 1940-1963 age group that is deposits are very fine to fine-grained sand rooted on the basal unit. Deposition of the with minor silt and clay. Bankfull discharge modern alluvium began by 1940 in East Fork 3 3 at this locality is 21.8 m /s (770 ft /s; E.D. and continued until as late as about 1980, Andrews, 1993, written commun.). Three when the channel lowered slightly to the

20 present level of the active channel and flood- showed that the depositional units are wide- plain. spread and of roughly similar age through- The modern alluvium at North Fork con- out the southern Colorado Plateau. sists mainly of the December 1966 flood The geomorphic relations among the dep- deposit. Early deposits of modern age were ositional units varies from valley-to-valley as either removed during the flood or were cov- well as along individual valleys, as demon- ered by the flood deposit. The lower level of strated by Karlstrom (1988, p. 46) in the the modern terrace dates to sometime before Black Mesa region and illustrated in general 1969, and the channel could have eroded by Leopold and others (1964, p. 460-461). slightly to this level during the 1966 flood. Three geomorphic and stratigraphic rela- Finally, the present position of the channel tions prevail that effect the number and and floodplain of North Fork were obtained height of terraces; these are superposed, by at least 1983. inset, and mixed superposed and inset. Allu- In summary, the alluvial history of the vial valleys with a single terrace have super- Virgin River alternates between episodes of posed stratigraphy in which the terrace is deposition and erosion. The sequence of ter- the abandoned floodplain of the most recent races is the result of this progressive down- deposition. This is similar to the arrange- cutting and backfilling. The erosional ment of units shown on the correlation chart episodes were after A.D. 1100-1200, 1883 to (fig. 10). Inset-terrace stratigraphy, or cut- 1926, 1937-1940, and 1980-1983. The effect and-fill stratigraphy, is perhaps the classic of the first and second episodes was substan- case of alluvial-valley geomorphology. This tially larger than the third and fourth epi- geomorphic relation is characterized by sode. Historic entrenchment and widening of younger deposits at successively lower topo- the Virgin River between A.D. 1883-1940 graphic positions in the valley. East Fork has coincides with regional arroyo cutting that inset-terrace stratigraphy (fig. 9, cross-sec- deepened and widened the channel of almost tion A-A'). The third type of stratigraphic every stream in the Southwest (Bryan, and geomorphic relation is a combination of 1925). The aggradational episodes on the inset and superposed stratigraphy. In this Virgin River were the prehistoric alluvium situation, a younger sequence of superposed from before A.D. 800-1100 to 1100-1200, the depositional units has an inset relation with settlement alluvium after A.D. 1200 until an older sequence of superposed units. North 1883, the early modern alluvium from 1926- Fork locally has elements of superposed and 1937, and the modern alluvium from 1940- inset stratigraphy (fig. 9, cross-section A-A') 1980. These geomorphic relations preclude cor- relation of the upper Holocene alluvial units Regional Correlation of Alluvial based on the number and height of terraces, Histories except in the case of inset terraces. Deposi- tional history, therefore, does not necessarily Broadly speaking, the timing and num- correspond with the number and height of ber of erosional and depositional events of terraces. This geomorphic variability led the upper Virgin River are similar to other and Patton and streams on the southern Colorado Plateau. Patton and Schumm (1981) Boison (1985) to conclude that, in general, Figure 10 shows the regional correlation of depositional histories need not be synchro- upper Holocene fluvial stratigraphic units. The correlation chart was compiled and nous from basin-to-basin or within a basin. Thus, geologic dating is necessary to estab- interpreted from recent or current work in lish regional synchroneity of deposition and the particular drainage or region. Details of the dating, stratigraphy, geomorphology, are erosion. in the cited references. For the most part, Results of dating shown in figure 10 sug- early work, summarized by Cooley (1962), gest a broad synchroneity of upper Holocene

21 Locality Terminology Black Mesa region- valley Chaco Black Canyon- Kanab Escalante Mesa Virgin eastern Creek River reqion River Colo. Plat. Virgin Paha Little Chaco River River Colorado Canyon River oo 1990 Modern P"S5 alluvium to S 1 8- 1950- 32_

3 1850H

3 o \ 1800 E.2 o > -C TO m ND a> [ 1600H I"

1400 Prehistoric arroyo cutting 1200

fl 1000 F r o 03 ND ND .»> Q. SZ -D A.D./B.C C 0) o HV) EXPLANATION

[::: Deposition Erosion ND Not dated

Sources of information

Virgin River- This study Kanab Creek - Webb and others (1991); Webb (personal commun., 1994) Paha River - Hereford (1986); (1987b) Escalante River - Webb (1985); Webb and Baker (1987); Little Colorado River -- Hereford (1984) Webb (personal commun., 1994) Chaco Canyon - Hall (1977); Love (1977) Black Mesa region - Dean (1988); Karlstrom (1988)

Figure 10. Correlation chart of upper Holocene fluvial erosion and deposition, southern Col- orado Plateau region. alluvial histories across the southern Colo- tified due to sedimentologic differences rado Plateau, although the timing of erosion caused by different source rocks and deposi- and deposition varies across the region, par- tional environments. Uncertainties in the ticularly for the older deposits. However, radiocarbon method of dating compounds some variability is expected for geologic rea- the problem further. These dates have an sons. Deposits are poorly preserved in the analytical error that may span several cen- turies, A.D. fluvial environment, and it is difficult to dates younger than about 1400 the know if the youngest beds of a particular have inherently poor resolution, and relation of the to the age of stratigraphic unit are present. Likewise, the dated material the alluvial deposit is usually not well under- vagaries of exposure make it difficult to iden- stood (Taylor, 1987, p. 108-143). Finally, each tify the base of older deposits. Some uncer- drainage probably has a lag or threshold tainty, therefore, pertains to dating either effect that delays the response of the channel the base or top of alluvial units. Generally, it system to the process causing change. is not possible to date exactly the same stratigraphic horizon, because either it is not As shown in figure 10, the nomenclature xposed, has been eroded, or cannot be iden- applied to the deposits varies across the

22 region. The prehistoric and settlement allu- 1900, except for the Escalante River. This viums of the upper Virgin River correspond erosion also affected the human population, to the Tsegi and Naha Formations, respec- causing widespread problems with erosion of tively, of the Black Mesa region and the Lit- farmland, disruption of irrigation struc- tle Colorado River valley (Karlstrom, 1988). tures, abandonment of pioneer settlements, Although in the Little Colorado River valley and a redirection of economic activity from and basin, the Naha Formation farming to livestock (Gregory, 1950; Graf, was informally referred to as the cottonwood 1983; Webb, 1985). Although widely recog- terrace (Hereford, 1984; 1986). In Chaco nized and studied, the causes of this recent Canyon and the eastern Colorado Plateau, entrenchment are topics of debate (Graf, deposits of similar age are referred to as the 1983). Chaco and post-Bonito deposits (Hall, 1987), The erosional episodes are characterized respectively. by a lack of deposits, the result of nondeposi- Deposits equivalent to the modern allu- tion and erosion. In the Escalante River vium of the upper Virgin River valley are (R.H. Webb, personal commun., 1994) and present in most valleys of the southern Colo- parts of the Paria River basin (Hereford, rado Plateau (Hereford, 1987a; Graf, 1987). 1987b), the older erosion is marked by a sub- Because the alluvium was recognized only surface contact having the shape of a chan- recently, it has not been formally named. In nel incised deeply into the prehistoric the Little Colorado River valley and Paria alluvium. This channel-like feature was an River basin, the modern alluvium was arroyo cut into the prehistoric alluvium dur- referred to as the floodplain alluvium (Here- ing prehistoric arroyo cutting. Deposition of ford, 1984; 1986). At Chaco Canyon and the the settlement alluvium eventually filled eastern Colorado Plateau, the alluvium is and overtopped this early arroyo. referred to as the historic deposits (Hall, The historic erosion also lacks significant 1977; Love, 1977). Similar deposits that sediment accumulation, although a thin, dis- mostly post-date 1940 are widespread in the continuous deposit is present locally. This channels of Southwest streams (Leopold, deposit has been identified at the base of the 1976). modern alluvium in several streams of the Plateau (Hereford, Two erosional events or periods of non- southern Colorado 1987a). Referred to as the older channel allu- deposition, referred to as the prehistoric and vium, these deposits were probably channel historic arroyo cutting, are widespread in bars other transient features that were the upper Holocene of the southern Colorado and reworked frequently (Hereford, 1986). The Plateau (fig. 10). Although detailed studies basal unit of the modern alluvium, the early reveal a number of erosions during the upper terrace, and the historic terrace in Holocene, these were of short duration and modern the upper Virgin River are probably equiva- had little geomorphic effect (Hall, 1977; lent to the older channel alluvium. Karlstrom, 1988). Prehistoric arroyo cutting was ongoing regionally around A.D. 1200- This regional correlation (fig. 10) sug- 1400, if not somewhat earlier and later, and gests a shared causal mechanism for erosion probably had severe environmental conse- and deposition. Variations in climate are quences. Major cultural adjustments broadly similar across the region, and these affected the Anasazi population at about this variations probably altered rainfall and run- time, including a general abandonment of off regionally, which in turn altered long- the region. These adjustments were at least term patterns of erosion and deposition. In partly related to the environmental changes short, the late Holocene alluvial history of brought about by widespread arroyo cutting the upper Virgin River and other southern (Plog and others, 1988, p. 259). Historic Colorado Plateau streams probably results arroyo cutting was widespread around A.D. to some extent from climate variations that

23 change rainfall and runoff patterns. In his- described as flowing through a narrow, toric times, landuse has probably altered meandering channel with abundant brush, runoff, perhaps leading to increased erosion grass, and timber that made cultivation dif- at least locally. ficult ( Wittwer, 1927). The reference to the deep channel of East Fork is inconsistent with the condition of North Fork in 1865- FLOODS AND HISTORIC 1866 and with a photograph taken in 1873 CHANGES IN THE CHANNEL OF that shows an unentrenched tributary of THE VIRGIN RIVER East Fork at Shunesburg. It is possible that deep refers to the canyon rather than the This section of the report addresses the channel of East Fork. Briefly, at the time of effect of floods on the geomorphology of the settlement in the early 1860s, the channels alluvial valleys as interpreted, for the most of Virgin River downstream of the forks, part, from the observations of pioneer set- North Fork, and East Fork were quite likely tlers. The earliest description of the Virgin meandering, narrow, and shallow with an River downstream of the forks in the study expansive floodplain. area was September 1858. The alluvial val- ley has changed substantially since then, Floods and Settlement and the changes are contemporaneous with a run of large floods and high runoff in the lat- Historically noted floods of the Virgin ter part of the 1800s and early 1900s. River and their effects as recorded by regional historians who cite church records, Historic Changes in the River diaries, and newspaper accounts are listed in Table 1. A large flood in 1862 caused exten- Channel sive damage only 1-3 years after settlement Before settlement, the river flowed near of the river valley This flood, referred to the surface in a relatively narrow, shallow, locally as "The great flood" or the "The great and possibly meandering channel from Vir- storm" of 1862 (Reid, 1931; Larson, 1961; gin to at least as far upstream as the conflu- Crampton, 1965), was exceptional. Rain ence of East Fork and North Fork (fig. 1). began on December 25, 1861 throughout the The river valley was explored for farmland area and continued almost continuously for and the potential for settlement by Jesse N. 40 days. The resulting flood literally Smith in September 1858 (Crampton, 1965, destroyed the pioneer communities of Virgin, p. 84-90). Smith reported that the river Duncan's Retreat (upstream from Virgin), meandered across a narrow valley, showed Grafton, and Adventure (moved and signs of large freshets, and that a minor flood renamed Rockville); the settlements were would cover the bottomland extensively. A subsequently relocated to higher and safer year later in 1859, the channel at Virgin was ground. Loss of farmland and damage to described as being narrow with banks cov- other settlements in the river basin was just ered by grass. The channel was evidently as thorough, forcing most of the new settlers shallow, as a constructed in 1859 was to relocate houses and irrigation ditches merely a cottonwood log laid across the river (Larson, 1961). Other areas in the western anchored by two slots cut into each bank United States were also hit hard by extreme (Larson, 1961). floods and cold weather in winter of 1861- When Shunesburg was settled in the 1862. Rivers from Oregon to the Mexican spring of 1862, the channel of East Fork was border were at flood stage and rainfall narrow and deep with farmland on both amounts in parts of were 300-400 sides of the valley, according to DeMille percent above normal (Roden, 1989). (1977). In 1865 or 1866, exactly which year is The 1880s and early 1900s were charac- uncertain, North Fork in Zion Canyon was terized by unusually large floods (Table 1).

24 Table 1. Historic floods of the Virgin River

Date Area and Effect Source

Extensive flood damage to all riverside settlements; channel of Virgin and Santa Clara Rivers deepened and widened for miles above junction; Grafton January-February 1862 1,3-4,6 (early site) and Rockville (Adventure) destroyed; serious dislocations among most settlers Rockville and Grafton, considerable damage to farmland; Millersburg (near Winter 1867-68 4,7 Littlefield on Dam Wash) destroyed June 1872 Shunesburg, crops destroyed, dam broken 7 June 28, 1880 Mesquite, small flood followed by two larger floods lasting 4-5 hours 10 August 1880 Orderville, community center inundated 4 1881 Orderville, homes inundated with muddy water 4 Spring 1883 Bunkerville, disastrous floods 4

1884 Bunkerville and St. George, dams washed out 4

Orderville, farmland destroyed, deep gullies cut at various parts of valley; August 1885 St. George, dams and irrigation ditches washed out; damage to fields and ditches from Shunesburg to Bunkerville

Bunkerville and St. George, dams destroyed; floodmark two feet higher than December 7, 1889 flood of 1862

December 15, 1889 ibid. Springdale, canyon floor covered with water which reached bottom of win- July 14, 1896 dows in church July 25, 1902 Bloomington, diversion dam partly destroyed, many washes running

St. George, largest flood in the history of St. George, cropland flooded, one August 24, 1905 dam swept away another partly destroyed Springdale, bridge washed away, man drowned attempting to cross river March 25, 1906 near La Verkin

August 20, 1906 St. George, two dams damaged

October 1906 Mesquite, about the largest flood in 64 (?) years 11 August 13, 1909 Springdale and Northrup 2

St. George, largest flood in history of , dam washed out farmland flooded, crops damaged by heavy rains; Virgin, farmland eroded, low parts September 1, 1909 8,9 of town flooded; Orderville, much farmland eroded, bridges and roads washed out, $3,000 damage Mesquite, one of the largest floods with much property damage along the 1910 12 river

July 20, 1911 St. George, roads and canals damaged, rain damage to crops 8,12

October 29, 1912 St. George, dam constructed in 1874-75 washed out 4, 12 Floods in Virgin River and North Fork; Zion National Park, bridge and road August 23, 1920 3,8,9 washed out isolating park, considerable damage to land and ditches September 1929 Zion National Park, flood with seven foot waves 3 March 1938 Zion National Park, flood following five inches of rain 3

St. George, extensive flood damage following four days of rain; road dam- December 6, 1966 age isolates Rockville and Springdale; Rockville, old riverbank overtopped, 5,9 livestock killed

(l)Crampton(1965) (2)Crawford and Fairbanks (1972)

(3) Culpin, Mary S. (written commun., 1993) (4) Larson (1961) (5) Lewis (1982) (6)Reid(1931) (7) Stout (1972) (8)Woolley(1946) (9) Washington County News August 19, 1909

September 2, 1909 August 26, 1920

December 8, 15,29, 1966 Mesquite Historical Museum (10) Francis Leavitt (11) William Abbott

(12) J. Abbott

25 Two floods in December 1889 and the flood of The difficulty of controlling the Virgin September 1909 are noteworthy because River at Grafton was recalled by Russel they were probably as large or larger than (1982). In the early 1900s, he reports that the flood of 1862. Damage, however, was evi- the river shifted course and occupied a differ- dently not as widespread or extensive. This ent channel after each flood, which was sev- apparent lack of widespread damage sug- eral times a year. A rise of only a few feet gests that people had gained experience with would necessitate repair of the dam; these floods and had adjusted to the flood hazard difficulties explain the frequent complaints by careful selection of construction sites and regarding maintenance of irrigation struc- location of farmland. tures (Larson, 1961). Early settlement and economic develop- The erosional effects of this repeated ment of the Virgin River basin were affected shifting of the channel are illustrated by the to a large extent by the floods of the late remnant of an irrigation ditch west of 1800s and early 1900s. Flood-related loss of Grafton. About 1 km (0.6 mi) of the ditch is farmland and damage to irrigation struc- preserved discontinuously on the south side tures, dams, and dwellings were continuing of the river beginning 1.1 km (0.7 mi) west of problems for early settlers of the region (Lar- Grafton. At the upstream point, the ditch son, 1961). The cost of replacing and main- was dug into the prehistoric terrace; the ter- taining dams, irrigation ditches, and race and ditch are now truncated by a 6-7 m property was an economic burden on every (20-23 ft) vertical cutbank of the active river community. channel. At the downstream point west of Grafton Wash, the ditch was excavated into a Population growth, moreover, was lim- steep slope of Pleistocene gravel and the ited by the progressive loss of farmland after water was directed to the fields below. Pres- settlement. Unused land suitable for farm- ently, the ditch ends 5-6 m (16-20 ft) above ing was simply unavailable for future, historic-age deposits, which are younger expanded agricultural development. From than early-day farming. Erosion removed 1862-1886, 50 percent of the original bottom- both the inlet of the ditch and the bottom- land at Virgin was lost to erosion (Slusser land under irrigation. and Olson, 1992); at Rockville 50 percent Just east of Grafton Wash, the ditch was was lost by 1890 (Larson, 1961). The popula- excavated into a steep slope of Moenkopi tion of Duncan's Retreat, Grafton, and Formation that was subsequently eroded as Shunesburg was declining and the settle- the channel widened. A 70 m (230 ft) seg- ments were almost deserted in the 1890s. ment of the ditch was eroded from the slope The remaining farmland could not support and the bedrock was steepened into a nearly earlier population levels, and the economic vertical scarp 10-15 m (33-49 ft) high. His- base shifted from agriculture to livestock as toric-age alluvium is present at the base of farming could no longer provide subsistence the scarp 5-10 (16-33 ft) inside the pro- and was unprofitable (Larson 1961; Cramp- m jected path of the ditch (fig. 11). At this local- ton, 1965). ity, bedrock as well as alluvium were eroded Springdale, interestingly, not as seri- was as the river shifted course during arroyo cut- ously affected by flood-related problems. ting. This is probably because much of the chan- nel and the banks of North Fork near Spring- Historic Floods and the Beginning of dale are relatively coarse gravel, and the Arroyo Cutting settlers were able to successfully divert the river and control erosion (Larson, 1961). Historic accounts interpreted by (Bailey, Nevertheless, floods have been troublesome 1935) suggest that arroyo cutting began with at Springdale and are noted in local history the flood of 1862 and that the initial incision (Crawford and Fairbanks, 1972). progressed headward into the upper basin by

26 Figure 11. Photograph illustrating erosion of bedrock (Moenkopi Formation) associated with historic arroyo cutting west of Grafton. Line shows trace of irrigation ditch. Dashes show eroded ditch and bedrock, h = historic terrace; m = modern terrace.

1870 (Table 1). Previously discussed geomor- the misfortune of experiencing one of the phic evidence, however, indicates that arroyo largest floods in 130 years. cutting in the study area began after 1883. The loss of property and hardships As they apply to the study area, historic caused by the flood were no doubt real. What accounts are somewhat ambiguous, because seems less tangible is the extent to which the they reflect a set of circumstances that are channel was widened and deepened, particu- probably unrelated to the beginning of larly in regard to arroyo cutting in the Zion arroyo cutting. National Park region. In the St. George area, of the Virgin Santa Clara The 1862 flood is noted for its severe con- the channels and Rivers were widened and deepened for sev- sequences, and it is probably one of the larg- est floods of the historic era, according to eral kilometers upstream of the junction although large flood will cause Enzel and others (1994), although four floods (Table 1), a erosion upstream of a major junction. At any were as large or larger (Table 1). The severe rate, the in the study area evi- problems caused by the flood of 1862 were channel was dently not altered extensively. Samuel Wit- probably exacerbated by the inexperience of twer's (Wittwer, 1927) description of North the pioneer settlers. These people were recent immigrants from Europe and New Fork in Zion Canyon indicates that the chan- nel was not entrenched in 1865-1866 and England; the desert environment of the photographs of North Fork and East Fork flood-prone Virgin River valley was unlike indicate the channels were not entrenched in anything in their past experience (Larson, 1873. 1961, p. 90). Settling a narrow valley with a river subject to large floods and a shifting Farmland was lost in the upper valley in channel would present numerous problems 1862, but this was probably flood-related under the best of conditions. Farmland and shifting of the channel across the floodplain housing were probably established and built rather than erosional lowering of the chan- on or very near the active floodplain. After nel and abandonment of the floodplain. only 1-3 years in the area, the settlers had Interestingly, there is no geomorphic evi-

27 dence of this flood in the study area, either HISTORIC PHOTOGRAPHS OF because evidence was not preserved or THE VIRGIN RIVER because the effect of the flood was not large. Likewise, the flood of December 1966, which Relocation of early photographs provides ranks with the 1862 flood (Enzel and others, a wealth of information about subtle changes 1994), had negligible geomorphic effect, in landscape that are otherwise difficult to although roads and bridges were damaged describe, map, or interpret (Malde, 1973). In (Table 1). It is possible that a single large this section of the report, changes in the allu- flood isolated in time is incapable of trigger- vial valley are interpreted by comparison of ing arroyo cutting on a basin-wide scale. early images with contemporary photo- Nonetheless, loss of farmland was a con- graphs made at relocated camera stations. tinuing problem for every settlement in the The date, location, and archival source of the study area except Springdale. Was this loss photographs are listed in Table 2. Over the the result of arroyo cutting? Possibly, but years, numerous photographs have been again no evidence of entrenchment was made of Zion Canyon, beginning with the early work of J.K. Hillers, photographed found before 1883. Although it is clear that who (Fowler, floods were a persistent problem for settlers the area in April or August of 1873 1989, p. 39). Although most photographs in the valley (Larson, 1961, p. 357-375), it is less certain that early flood-related problems illustrate the rugged walls of Zion Canyon, resulted from arroyo cutting. The loss of many of the photographs include the channel farmland from 1862 to the early 1880s prob- of North Fork. Landscape photographers ably resulted from normal shifting of the were evidently less interested in East Fork channel during aggradation of the settle- and the Virgin River downstream of the ment alluvium. forks, as only a few early photographs were found. The photographs are discussed by Historic accounts show that floods were area and interpreted in terms of the previ- particularly frequent and troublesome from ously discussed terraces. The photographs 1880 through the early 1900s (Table 1). This are on pages 34-53. corresponds with a time of unusually high streamflow that began in the early 1880s, based on dendrohydrological estimates. North Fork in Springdale and Zion Between 1883-1892 or 1893, the channel was Canyon Areas deepened at least several meters throughout The junction of Virgin River with East the area to the extent that it no longer and North Fork is shown in figure 12 (Table flooded at the level of the settlement flood- 2). The early photograph (fig. 12a) was taken plain. In short, arroyo cutting in the study in 1873 by J.K. Hillers. In 1873, the river area quite likely began around 1883 when flowed at the level of the settlement terrace, the channel was demonstrably altered, as and the river appeared to flood widely across inferred from geomorphic evidence. Geomor- the bottomland, as indicated by the streaks phic studies are needed in the lower basin to of high-albedo material parallel to the river determine whether the 1862 flood initiated (fig. 12a). The valley was relatively free of arroyo cutting. vegetation, probably the result of grazing and clearing for farming. The channel of East Fork in 1873 (fig. 12a) was located about where the dirt road crosses the settle- ment terrace in the contemporary photo- graph (fig. 12b). East Fork has shifted about 100 m (330 ft) south of its 1873 position, and the junction with North Fork shifted down- stream. Although not visible in the contem-

28 )

Table 2. Date, location, subject, and source of historic photographs

Figure Relocation Date Location and Source of Place Names ( Source No. 1 Date

North Fork in Springdale and Zion Canyon Areas 500 m south of North and East Fork junction, 12a, b 1873 J.K. Hillers No. 80, USGS April 1994 SE'4 map 1 240 m north of West Rim trailhead, on river 20 m 13a, b 1873 J.K. Hillers No. 73, USGS April 1994 east of trail, SW'4 map 2 250 m south of Great White Throne turnout Floor 14a, c 1873 J.K. Hillers No. 78, USGS April 1994 of the Valley Road, ibid. At turnout north side Floor of the Valley Road, 14b 8/12/1929 National Archives 79-2-142 ibid

15a, c 1903 Observation Point, SE',4 map 2 W.T. Lee No. 271 3, USGS April 1993 15b 9/12/1929 ibid. National Archives 79-2-101 780 m north of West Rim trailhead, 150 m west of 16a, b 1909 R. Anderson No. 245, USGS April 1994 trail, SWW map 2 17 1909 In saddle at The Pulpit, ibid. R. Anderson No. 249, USGS Virgin River in Grafton and Rockville Areas September 18a,b 1917-1939 On cliff 1 km southwest of Grafton, SWM map 1 Gregory (1939, fig. 12) 1994

1 .5 km east of Rockville and 1 00 m north of State HE. Gregory No. 9-2455, Univ. 19a, b 1937 April 1994 Route 9, SE14 map 1 Utah Special Collections East Fork near Shunesburg 300 m west-southwest of Shunesburg, SW'4 map September 20a, b 1873 J.K. Hillers No. 520 3 1994 H.E. Gregory No. 9-2242, Univ. 21a, b 1926 460 m east of Shunesburg, ibid. April 1994 Utah Special Collections

660 m east-northeast of Shunesburg at park H.E. Gregory No. 9-2428, Univ. 22 1936 boundary fence, ibid. Utah Special Collections

(1) Place names from indicated quadrant of U.S. Geological Survey 1:24,000 scale topographic maps.

Map 1 , Springdale West (1980); map 2, Temple of Sinawava (1980); map 3, Springdale East (1980)

porary photograph (fig. 12b), the river now least one cottonwood. Presently, the riparian flows 4 m (13 ft) below the adjacent settle- vegetation is dominated by saltcedar, a non- ment terrace. A woodland composed mainly native species that is widespread in many of cottonwood is now well established along streams in the western United States below the river. At this locality, the width of the about 1,830 m (6,000 ft) elevation (Robinson, channel does not appear to have increased, 1965). but the channel has shifted position and North Fork in the vicinity of the Great deepened 4 m (13 ft). White Throne is shown in figure 14 (Table 2). Figure 13 is North Fork in Zion Canyon The camera station is 1.6 km (1 mi) looking upstream to Angels Landing and The upstream of the station in figure 13. In 1873, Organ (Table 2). In 1873, alluvium on the the channel in the vicinity of Great White floor and banks of the channel appears to be Throne also appears to be composed of sand sand with silt and mud (fig. 13a). The area in with silt and mud, similar to figure 13a. A the mid-ground of the channel in the early low bank just below the right mid-ground photograph is probably the settlement-age (fig. 14a) is probably the settlement-age floodplain. The contemporary photograph floodplain. The lower-right portion of figure shows that the channel is mainly cobble to 14b shows the foreground of figure 14a from small-boulder gravel and only the modern a position several hundred meters north and terrace is present in this area now (fig. 13b). about 10-15 m (33-49 ft) above the early loca- Riparian vegetation in the mid-ground of the tion. By at least 1929, the channel had early photograph appears to be willow and at changed substantially; it is wider, the vege-

29 tation canopy is gone, and the channel and shown in figure 16 (Table 2). The early pho- banks are gravelly. Figure 14c is the contem- tograph (fig. 16a) shows the historic channel porary photograph taken from the camera extending across most of the valley in 1909. position of figure 14a. The channel appears The cutbank and relatively high terrace on to be substantially wider than in 1873, the right side of the river is probably the set- although the discharge level is higher in the tlement terrace. However, a terrace cannot contemporary photograph. The modern ter- be identified at this spot in the Observation race is present in the foreground, and depos- Point photograph (fig. 15a) taken only six its of the 1966 flood occupy most of the years earlier. It is possible that the cutbank alluvial valley behind the camera station. and terrace are related to the historic ter- The present vegetation is mainly saltcedar race. Gravel is present in the channel on the and cottonwood, whereas in 1873 it appears right side of the photograph in the upper to be mainly willow and cottonwood (fig. foreground. The contemporary photograph 14a). (fig. 16b) shows that the channel shifted south, placing the active channel against Zion Canyon and North Fork down- bedrock. This shift occurred by 1929, as stream from Big Bend are shown in figure shown in the Observation Point photograph 15, an oblique aerial view from Observation (fig. 15b). The height and diameter of cotton- Point (Table 2). The foreground of figure 12 is wood on the left side of the river identify this visible in the mid-ground of figure 15. In the surface as the early modern terrace. This 1903 photograph, the active channel of reach is largely unaffected by channel stabi- North Fork is indicated by the high-albedo lization, which was unnecessary because the material on the floor of the alluvial valley Zion Canyon Road is on bedrock above the (fig. 15a); the channel appears to occupy alluvial valley. The left bank downstream most of the valley floor. At least five meander from the bedrock cliff has been stabilized, as scars are eroded into the settlement terrace shown by the linear feature in the lower downstream of the mid-ground of the photo- right of figure 16b. graph. The meander scars resemble the Figure 17 (Table 2) is North Fork down- plan-form of the historic terrace at East Fork stream of The Pulpit in 1909. This area was (fig. 5). A plowed field or orchard is present because dense vegeta- on the settlement terrace on the right side of not rephotographed tion in the foreground blocks the view. The the valley. This field is not present in the settlement terrace is present in the left fore- 1929 photograph (fig. 15b); the terrace and ground. The furrows show that the terrace field were removed by a shift in the channel recently plowed for farming, as between 1903-1929. Vegetation had begun to was occupy channel bars by 1929, these bars described in the photographers' notes. The is terrace, by the probably correspond with the early modern channel below the as shown feature in the left foreground, terrace mapped upstream at the North Fork dark, linear which is probably the shadow of the terrace study site (fig. 9). The contemporary photo- and cutbank cast on the channel. graph (fig. 15c) shows a substantial decrease of channel width since 1929. In addition, a woodland of cottonwood is well developed. Virgin River in the Grafton and Extensive channelization and realignment of Rockville Areas North Fork were done in this reach to main- An oblique aerial view of the Virgin River tain the present all-weather road. Much of in the Grafton area taken from the cliffs of this conservation work was done in the Shinarump Sandstone is shown in figure 18 1930s, and the present position of the chan- (Table 2). The early photograph was quite nel in many places is partly a consequence of likely taken before 1939, as it appeared in a this stabilization work. publication of that date (Gregory, 1939, fig. The alluvial valley south of Big Bend is 12). The earliest possible date for the photo-

30 graph is suggested by the condition of the recently. The point bar is equivalent to the road (State Route 9) on the north side of the early modern terrace of North Fork and the river. The roadbed is wide, unpaved, well basal unit of the modern alluvium at East graded, and is aligned differently than the Fork. There is little evidence of channel sta- existing paved highway (fig. 7b). The road is bilization work in this reach, except on the probably contemporaneous with relatively right bank just downstream of the roadbed. heavy automotive traffic, as the pioneer Between 1937-1994, the active channel route followed the south side of the valley. In shifted south 270 m (750 ft), and a mobile 1917, $15,000 of Federal funds were appro- home was located near the center of the for- priated to construct 17 miles of an "—inter- merly active channel (fig. 19b). In addition, state wagon road or highway—" into Zion an open-canopy woodland of cottonwood National Park (Larson, 1982). National Park developed on the point bar and in the earlier Service brochures (U.S. Department of Inte- channel. The height and diameter of the cot- rior, 1926) show that the road in the park tonwood trees resemble those in East Fork was graded and improved as early in 1926 associated with the modern terrace. and the road was oiled in 1928 to reduce dust (Deseret News, June 4, 1928). This informa- East Fork near Shunesburg tion suggests that the photograph was prob- ably taken sometime after 1917 and before Although the river is not shown in figure 1939; the date is assumed to be the mid- 20 (Table 2), this early photograph of 19208, a time that includes the late historic- Johnson Mountain is important because of age terrace. evidence that tributaries of East Fork and the channel of East Fork itself were not Grafton is on the settlement terrace (fig. entrenched in 1873. The camera station is 18), which has a steep terrace rise extending located on the settlement terrace at the junc- 2-3 m (6-10 ft) above the active channel. In tion with a small, entrenched tributary the mid-1920s (fig. 18a), the river flooded about 50 m (160 ft) north of the remains of a freely across a channel extending from cut-stone dwelling at the west end of the Grafton to the north side of the valley at former site of Shunesburg. The mid-ground State Route 9. The historic-age channel dom- on the right side of the early photograph inates the scene, extending from near the shows that the tributary was only a shallow Grafton school to bedrock on the north side of swale in 1873 (fig. 20a). In the contemporary the valley. The active channel was near the photograph (fig. 20b), a well-formed arroyo school building in the early 1900s, when a with vertical walls 2-3 m (6-10 ft) high is rock wall was constructed (Russel, 1982) to present in this area. This modern channel is stabilize the cutbank of the settlement ter- a first-order tributary of East Fork on the race. This wall is still present just east of the Springdale East 1:24,000 scale topographic school building. The contemporary photo- map, and the arroyo extends about 800-900 graph (fig. 18b) shows that the channel nar- m (2,620-2,950 ft) upstream from the junc- rowed substantially, shifted course, and a tion. The west side of the arroyo is aligned woodland of cottonwood developed. north-south for 200-250 m (660-820 ft) A downstream view of the valley between upstream from the junction, suggesting that Rockville and Springdale is shown in figure the arroyo was initiated along a fence line. A 19 (Table 2). The settlement terrace is few meters east of the camera station, the present along the south and north margin of river presently occupies a large meander the channel in the early photograph. In 1937, scar entrenched 5-7 m (16-23 ft) below the a point bar extended north across the valley settlement terrace. The settlement of and the roadbed was the right bank of the Shunesburg was built on the terrace at the river (fig. 19a). The point bar was lightly veg- present location of the entrenched meander. etated and appears to have been active The absence of an arroyo in 1873 suggests

31 that East Fork was not entrenched at that about the same to much narrower than the time, otherwise the tributary would also be present channel (figs. 13 and 14). In addi- entrenched as the main channel is the tion, the banks were vegetated and appear baselevel control. stable. The artistic rendering of the 1873 (figs. 13a and 14a) photographs proba- A downstream view of East Fork in 1926 was bly intended to portray a sense of tranquility near Shunesburg is shown in figure 21 along the river, and the photographs are (Table 2). The stone house of Oliver DeMille probably somewhat biased in their portrayal dating from around 1880 (DeMille, 1987) is of channel present on the high terrace of Pleistocene conditions. Nevertheless, historic gravel in the upper-right quadrant of the accounts indicate that in 1865-1866 the channel photograph. The prehistoric and settlement was heavily vegetated and the river flowed in a narrow, terraces are present in the foreground above meandering channel (Wittwer, 1927). This description of the chan- a steep terrace rise that extends 3-4 m (10-13 nel in 1865-1866 is consistent with the condi- ft) above the late historic-age channel. Gul- tion of the channel in the 1873 photographs. lies graded to river level are eroded into the terraces. In flood stage, the river flowed By 1903, the channel had widened sub- across the entire channel, and riparian vege- stantially and the settlement-age floodplain tation was sparse. Figure 21b is an approxi- was incised and undergoing active erosion mate relocation of the 1926 photograph. (fig. 15). This erosion and widening of the Presently, the steep terrace rise present in channel continued until at least 1929 (figs. 1926 is subdued and is eroded farther into 15a and 15b). Gravel was definitely present the valley margin. An open-canopy woodland in the channel in 1929, although the change and the modern terrace are present in the from a sand to gravel-bed stream probably former channel. occurred by at least 1909 (figs. 16 and 17). The change from sand to gravel bed suggests The channel of East Fork in 1936 at the stream competency increased. The gravel is park boundary fence east of Shunesburg is probably a lag deposit, resulting from selec- shown in figure 22. The photographic station tive removal of sand-size sediment during could not be relocated, because subsequent the large floods of the late 1800s and early erosion removed the camera station which 1900s. was on the settlement terrace at the terrace rise. In 1936, the early modern floodplain is The river appeared to flood freely across present slightly above the active channel. the channel in 1903 and 1909 with little evi- The historic terrace is present in the right dence of an active floodplain. These photo- mid-ground just below a cottonwood tree on graphs suggest that the historic-age channel the settlement terrace. Presently, the chan- lacked a clearly defined floodplain. By 1929, nel is occupied almost entirely by the modern however, stabilized mid-channel bars were terrace and a dense woodland of cottonwood. present in the channel that now form the early modern terrace. Since 1929, channel Discussion and Summary of Channel width decreased substantially and the deposits and riparian woodland of the mod- Conditions Inferred from Historic ern terrace partly fill the former channel. Photographs The photographic record of East Fork Relocation of historic photographs and the Virgin River downstream of the forks reveals striking changes in the channel of only begins in 1926 and the mid-1920s, Virgin River during the past 120 years. respectively. These photographs show the North Fork in Zion Canyon is the longest and late-historic and early modern channel. The most complete sequence of photographs that earliest photographs (figs. 18a and 21a) begins in 1873. A sand-bed channel was show that the late-historic channel was rela- present in 1873 that varied in width from tively flat, largely unvegetated without a

32 FIGURES 12 TO 22 FOLLOW

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39 Figure 14c, Approximate relocation of 14a.

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53 well-formed floodplain, and occupied most of the alluvial valley. The later photographs Calibration and Verification of (figs. 19a and 22a) suggest that some relief Reconstructed Streamflow had developed in the channel through point- bar and channel-margin deposition. These The dendrohydrological model assumes deposits are probably equivalent to the basal that streamflow during the water year (Octo- unit of the modern alluvium at the East Fork ber through September) is directly related to study site (figs. 5 and 8). Contemporary pho- precipitation, which in turn controls to a tographs show that channel width is large extent the ring width of trees growing decreased substantially, deposits of the mod- in the river basin. Generally, precipitation ern terrace partly fill the former channel, from October through June accounts for and an open-canopy riparian woodland is about 80 percent of ring growth during the present. Finally, first-order tributaries of water year. Summer rainfall during the East Fork were probably not entrenched in water year effects only about 6 percent of 1873, implying that the channel of East Fork ring width variability (Fritts, 1976, p. 238, was not entrenched either, as it is the 412). Thus, calibration of ring width to mea- baselevel control of the low-order tributaries. sured streamflow establishes correlation between annual tree growth and October through June flow rates.

STREAMFLOW HISTORY AND The analysis is done in two steps, calibra- GEOMORPHIC CHANGE OF THE tion and verification. Calibration is done sta- VIRGIN RIVER tistically by regression of standardized ring width against average daily discharge for In this section of the report, long-term water years 1910-1992. Verification tests the variations of streamflow and the relation to reconstructed streamflow against indepen- geomorphic change are discussed. The pur- dent data such as historic accounts of floods pose is to examine the temporal relations and droughts and other dendrohydrologic between variation of streamflow and dated studies in nearby areas. geomorphic activity. The gaged record, which The independent variable of the calibra- begins in 1910 at the gage near Virgin (fig. 1; tion regression is standardized ring width gage 09406000), is used to develop a 303- averaged between the two tree-ring study year dendrohydrologic reconstruction of sites, and the dependent variable is log- streamflow from A.D. 1690-1992. Dendrohy- transformed average daily discharge for the drological reconstruction is thought to be an water year. Standardized ring widths, or effective and reliable means to study long- ring-width indices, were calculated from the term variability of streamflow (Loaiciga and raw-width measurements to remove system- others, 1993). The reconstruction is done by atic changes of ring width related to the age calibrating ring width of pinyon growing on of the sampled trees. The discharge data semiarid hillslopes to measured average flow were transformed to reduce the positive rate on a year-by-year basis. Annual stream- skewness caused by large values. Water flow before 1910 was then estimated using years 1972-1978 are missing from the gaged the calibration function and ring width. This record; these missing data were estimated by information is used to assess streamflow pat- using annual flow rates from the North Fork terns during historic arroyo cutting and inci- gage near Springdale (gage 09405501). Dis- sion of the settlement terrace, geomorphic charge of North Fork is highly correlated 9nts that predate measured streamflow. with the Virgin River downstream of the ;velopment of the historic, early modern, forks such that 95 percent of the annual vari- modern terraces, are explained using ation of Virgin River discharge is explained instructed and gaged discharge. by discharge of North Fork.

54 r r

charge is unrelated to ring width is much 6.0 - less than 0.01. Although far from perfect cor- relation, ring width accounts for 59 percent of the measured variability in annual - streamflow. © 5.5

CD A time series of the actual streamflow .co w and streamflow estimated from equation (1) Q. 5.0 is shown in figure 24. For the most part, the c estimated values track the measured data reasonably well; the peaks and troughs of - 4.5 the calibrated values are in phase with the data in 80 percent of the cases. The cali- brated values are distinctly out of phase from 1927-1930 and in 1960. In addition, dis- 3 3 Ring-width index (I) charge above about 5 m /s (180 ft /s) is underestimated in many cases, the result of Figure 23. Natural logarithm of average- biologic limitations on tree growth. Water annual daily discharge (Q) as a function of years 1922-1923, 1978-1980 and 1983 are ring-width index (I). examples of underestimation. Overestima- In the regression equation, the natural tion is less frequent, only 1914 and 1949 stand out. log of discharge (Q in ft /s) is a power func- tion of the ring-width index (I). Streamflow reconstructed with equation L5 (1) from the full tree-ring chronology is ln(Q) = 4.41 + 0.70(I) (1) shown in figure 25. This reconstruction is The regression line and data are plotted in qualitatively verified from the flood history figure 23. The F-test for the significance of listed in Table 1, from an independently the regression gives F(l, 81) = 120.2; this is derived reconstruction in the lower Virgin substantially larger than F = 7.0, which is River basin (Larson and Michaelsen, 1990), the 0.01 significance level with 81 degrees of and from a tree-ring chronology in Kanab freedom. Thus, the probability that dis- Creek basin (Webb and others, 1991).

500

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i i i—i—i— -\——— 1910 1920 1930 1940 1950 1960 1970 1980 1990

Water year

Figure 24. Time series of actual and estimated average daily discharge by water year of Virgin River near Virgin, Utah, 1910-1992.

55 12 -12 CO E

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Water year

Figure 25. Reconstructed average daily discharge by water year, 1690-1992. Heavy line is dis- charge smoothed with fourier transform algorithm, smoothing level 35 percent.

Twenty-five floods of historic significance has 21 peaks and troughs of about 5-10 years occurring in 21 years are listed in Table 1. duration between A.D. 1700-1950 (fig. 25). Fourteen of the floods, or 67 percent (of 21), These match 17 of the peaks and troughs in had estimated annual discharge signifi- the Larson and Michaelsen (1990) recon- cantly larger than the 95 percent confidence struction. In addition, they cite two histori- interval of the estimated long-term average cally recorded droughts, 1856-1857 and 3 3 discharge of 5.2-5.7 m /s (183-200 ft /s). 1863-1864, that have discharge well below Thus, in 67 percent of the cases, historically the long-term reconstructed average. Esti- significant floods were associated with mated average daily discharge in the study reconstructed discharge that was equal to or area for the four drought years is 2.5, 2.5, 3 3 larger than the long-term average. 4.3, and 2.6 m /s (88, 150, 92 ft /s), respec- tively. is the dri- Streamflow of the lower Virgin River was The earlier drought among reconstructed by Larson and Michaelsen est 10 years in the 1,000-year reconstruction, although the later drought is overestimated (1990, figs. 8 and 9) from A.D. 966-1965. substantially in the Michaelson They used a tree-ring chronology from bris- Larson and tlecone pine on Charleston Peak in the (1990) reconstruction. Spring Mountains near , Nevada, Finally, a tree-ring chronology in the 270-km southwest of the Virgin River gage. Kanab Creek basin, which adjoins the Virgin The data were calibrated to streamflow mea- River basin on the east (fig. 1), was devel- sured at the gage near Littlefield, Arizona oped from Ponderosa pine by Webb and oth- (gage 09415000), which is south of St. ers (1991, fig. 1-12). Although not calibrated George (fig. 1). The reconstruction of Larson to streamflow, the ring-width indices are and Michaelsen (1990) is broadly similar to similar to Virgin River reconstructed figure 25 in the overlapping period, even streamflow (fig. 25). The broad pattern of though the distance between sites is large wet-and-dry years is well matched, and spe- and the tree-ring chronology is from a differ- cific wet-and-dry years are matched in most ent species growing outside the Virgin River cases. The high streamflow of the early basin. 1880s to mid-1890s and from about 1910- smoothed reconstructed streamflow 1920 (fig. 25) is matched, respectively, by

56 r

slightly above average to well-above average 3 flow rates declined from about 7-4 m /s (250- ring-width indices in the Kanab Creek chro- 140 ft /s) over the period of record. Stream- nology. In addition, a 5-year drought flow was largest from 1910-1923, when even between 1896-1900 that seriously affected the driest years were wetter than most fol- the cattle industry of the region (Gregory, lowing years. After 1941, discharge was rel- 1950, p. 45) is well expressed in both chronol- atively low, except for 1952, 1958, 1969, ogies, having the lowest ring-width indices 1973, 1980-1981, and 1983. This result is in the two chronologies. compatible with the reconstructed stream- In short, verification shows that the flow, which was consistently low after about reconstructed streamflow is consistent with 1940 relative to pre-1940 streamflow (fig. historic accounts of heavy flood years in 67 25). percent of the cases. Moreover, the recon- The decline of annual discharge results struction is broadly consistent with the largely from variation of seasonal stream- streamflow history of the lower Virgin River flow. Figure 27, shows average daily dis- basin developed from tree-ring samples of charge by water year for fall (October- different species growing outside the basin. December), winter (January-March), spring Reconstructed streamflow of the upper Vir- (April-June), and summer (July-September). gin River (fig. 25), therefore, is related to Spring is clearly the dominant streamflow regional climate. season, having average daily discharge nearly twice as large as the other seasons. The Gaged Streamflow History, 1910- Over time, spring discharge has remained 1992 essentially constant, without a significant trend toward either higher or lower dis- charge (fig. 27c). Fall, winter, and summer Annual and Seasonal Streamflow discharge have decreased over time, how- Streamflow of the Virgin River declined ever, and the trend is statistically significant from 1910-1992. Figure 26 is a time series (fig. 27a-b,d). The large annual streamflow showing annual variation and long-term from 1910-1923 (fig. 26) resulted from high decline of average daily discharge. As esti- streamflow during the latter three seasons mated by the regression, average annual combined with spring streamflow.

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Water year

Figure 26. Time series of average daily discharge, 1910-1992. Regression line shows long- term, linear decline of annual discharge, dotted lines are 95 percent confidence interval of regression.

57 Water year

1910 1920 1930 1940 1950 1960 1970 1980 1990

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1910 1920 1930 1940 1950 1960 1970 1980 1990 EXPLANATION

P = Probability that slope of regression equals zero NS = Slope not significantly different from zero

Figure 27. Time series of A) fall, B) winter, C) spring, and D) summer average daily discharge, 1910-1992. Data for 1972-1978 are missing. Regression line shows long-term, linear decline of discharge, which is not statistically significant for spring (dashed line), dotted lines are 95 percent confidence interval of regression.

Variation of seasonal streamflow is fur- produced most of the annual streamflow ther illustrated in figure 28, a time series since about 1930. Only one year, 1980, showing annual deviation in percent from approached the earlier conditions of large the long-term seasonal average. Discharge of spring, winter, and summer flow rates (fig. all four seasons was typically above or near 28b-d). the long-term seasonal average for most of Long-term decline of seasonal discharge 1910-1923 (fig. 28), and summer discharge of the Paria River is similar to the Virgin remained at these elevated levels discontin- River, except the decline is in the fall and uously until 1939 (fig. 28d). After about winter partial-duration flood series (Graf 1923-1930, fall and winter How rates and others, 1991). Runoff in the Paria River remained at or below the long-term average basin is in fall, winter, and summer with fig. 28a-b). Spring streamflow was rela- only negligible spring runoff. During the tively constant, however, and this season has period 1923-1986, winter, summer, and fall

58 Water year

1910 1920 1930 1940 1950 1960 1970 1980 1990

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i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i [ | | | | | | | i 1910 1920 1930 1940 1950 1960 1970 1980 1990 Water year

Figure 28. Time series of deviation in percent from long-term average flow rates, 1910-1992. A) fall, B) winter, C) spring, and D) summer. Data for 1972-1978 are missing. constituted 42, 26, and 25 percent of the annual peak flood by season (fig. 29a) and a average annual flow volume, respectively. time series of the annual flood classified by Summer and fall floods are evenly distrib- season (fig. 29b). The annual peak flood uted in time and both decrease in magni- occurs in every season, but is most frequent tude. Winter floods are less frequent, in summer. These summer flood peaks have unevenly distributed in time, and are with- little affect on average summer streamflow out a long-term trend. Thus, the early period because of short duration, which is substan- of large fall and summer runoff in the Virgin tially less than average spring streamflow River basin (figs. 26-27) was also typical of (fig. 27). Annual flood peaks in spring are the Paria River basin, as suggested by infrequent, although the largest spring flood declining seasonal-flood magnitudes. is close to the largest winter and summer peak flood. The largest annual flood was in Annual Flood Series fall and was almost twice the size of the larg- Figure 29 shows the statistics of the est flood of the other seasons (fig. 29a).

59 24,000 - .11 - 600 - 20,000 m CO COJ 16,000 E 11 48 c - 400 : - (D g> 12,000 CO CO o w 8,000 co b - 200 4,000 - -

(A)

Fall Winter Spring Summer

24,000 Explanation - 600 20,000 - Fall 1 Spring co - co *= 16,000 Winter Summer E J - 400 _c - CD o) 12,000 E° CO CO .c - o y 8,000 CO b - 200 4,000

nnnlHOn 1 I I T I rnr ——IT (B) 1910 1920 1930 1940 1950 1960 1970 1980 1990 Water year

Figure 29. Discharge of the annual flood. A) Statistical summary of the annual flood by sea- son; box shows median and 25-75th percentile, vertical line is range of 10-90th percentile, and solid circles are values less than 10th and greater than 90th percentiles, respectively. Number of floods shown at top of upper vertical line. B) Times series of annual flood, 1910-1992. Data for 1972-1978 are missing.

The time series of annual floods shows 1912, 1920, 1929, 1938, and 1967 (calendar little if any change in either flood magnitude year 1966)—are among the historically noted or seasonality (fig. 29b). The annual flood floods listed in Table 1. Damage from these was in the summer from 1919-1925 and from six floods was substantial, even though four 1979-1992, with the exception of 1986 and floods of similar size (1953, 1955, 1961, and 1990. A mixed seasonality of low annual 1980) did not cause enough damage to war- floods during spring, fall, and summer was rant attention. typical of the period 1941-1949. The flood of The annual flood series has little obvious 3 record, with a flow rate of 646 /s (22,800 m relation to historic geomorphic change of the 3 ft /s), was on December 6, 1966. According to Virgin River. Changes in the morphology of Enzel and others (1994), this flood may have the river channel, therefore, result mainly been as large as the previously discussed from variation of seasonal streamflow rather flood of 1862 (Table 1). Although one of the than long-term variation of the annual flood. largest floods since 1862, the geomorphic This conclusion is supported by a study of effects are negligible in East Fork and North sediment load and channel adjustment by Fork. Six of the floods in figure 29b-1911, E.D. Andrews (written commun., 1993).

60 Eighty percent of the average annual sedi- entrenchment of the prehistoric alluvium ment load is transported during the 13 and (fig. 10). Conversely, a period of relatively 24 highest days of streamflow at the North low streamflow from about A.D. 1400-1880 Fork and East Fork study sites (fig. 1), coincides with deposition of at least the respectively. These long-duration high flows upper settlement alluvium, although dating transport large quantities of sediment and is necessary to determine when deposition of modify channel morphology. High-transport the settlement alluvium began. Figure 25 rates are typical during spring; however shows that streamflow between A.D. 1700- streamflow was also high during summer, 1839 was at times larger than any subse- fall, and winter during the early 1900s (fig. quent streamflow. However, these were iso- 27). Although high-magnitude streamflow lated, short-term occurrences that lasted has large instantaneous transport rates, the only 1-3 years. In any case, geomorphic evi- streamflow is of short duration and rela- dence suggesting that the settlement allu- tively infrequent. These large events typi- vium was eroded before A.D. 1883 was not cally carry only a small part of the long-term found. average sediment load. Reconstructed streamflow of the upper Nevertheless, infrequent and large floods Virgin River and dated geomorphic events are clearly associated with channel erosion from 1850 1992 are shown in figure 30. The during the late 1800s and early 1900s (Table smoothed streamflow curve is used for inter- 1). The high-seasonal flow rates of this era pretation because the large annual variabil- probably the channel, destabilized enhanc- ity of reconstructed and actual streamflow ing erosion by infrequent, short-duration obscures the general correspondence high-magnitude floods. Thus, at times of between streamflow and dated terraces. In high-seasonal streamflow, these large floods addition, the smoothed curve has resolution are competent to modify the channel erosion- that is roughly comparable with the uncer- ally; conversely, when seasonal streamflow is tainties in dating the terraces. Entrench- low, floods of similar size are not particularly ment of the settlement terrace was between erosive. 1883-1892 or 1893, as suggested by dendro- geomorphic evidence (figs. 5, 9). This initial Streamflow and Geomorphic Change entrenchment coincides with about 12 years streamflow between 1883- Geomorphic change in the alluvial val- of unusually high 1895 that peaked in 1890 (fig. 30). A second leys is broadly contemporaneous with dis- of streamflow lasting charge variations that result in deposition or episode unusually high 20-30 years between the early 1900s and erosion. The main depositional periods led to was to flow rates were larg- accumulation of the prehistoric, settlement, the mid-1920s 1930; and modern alluviums. Erosional periods est from 1906-1917. The wide, flat-floored of this (figs. 21) forms resulted in floodplain abandonment, down- channel era 15, now the historic terrace. Development of this ter- cutting to lower depositional levels, and for- mation of the prehistoric, settlement, race coincides with the latter part of the early high-flow episode as well as the second historic, and modern terraces. episode of high streamflow. Streamflow during erosion of the prehis- toric terrace is not known in the study area. The early modern terrace and basal unit In the lower Virgin River basin, however, of the modern alluvium developed in the flow streamflow in the period A.D. 1200-1400 was regimen of the mid-1920s to late-1930s. The consistently higher than the unusually high terrace and deposits apparently record incip- levels around A.D. 1880-1920, as inferred ient deposition in the historic-age channel from Larson and Michaelsen (1990, fig. 9). (figs. 19, 22) during a period of relatively low This early episode of consistently high streamflow (fig. 26). This flow regimen devel- streamflow probably coincides with oped after a drop in fall, winter, and summer

61 Dated geomorphology Inferred change of valley and channel Reconstructed streamflow and stratigraphy Elevation Width

1990 Active channel and floodplain

Modern alluvium

1950

modern ter- 03 Early . race and basal i_ CD unit of modern alluvium

_c Historic terrace

Settlement alluv- ium, upper part 1850

3 100 300 ft /s EXPLANATION Discharge "I Deposition Erosion 8 m 3/s

Figure 30. Reconstructed streamflow, dated geomorphology and stratigraphy, and inferred change in width and elevation of valley and channel, respectively, 1850-1992. Streamflow data smoothed with fast fourier transform algorithm, smoothing level 32 percent. runoff (fig. 27). High streamflow during the of historic arroyo cutting and perhaps sev- late 1930s and early 1940s (fig. 26) evidently eral centuries in the case of prehistoric reestablished the channel below the historic arroyo cutting. The gaged streamflow record and early modern terraces. Deposition of the indicates that discharge in the early part of modern alluvium beginning about 1940 was the 20th century was above average during coincident with low-flow conditions from the three of four seasons. This produced high- early 1940s until the late 1970s. Most of the annual flow rates, which in turn increased runoff during accumulation of the modern sediment transport rates and destabilized alluvium was during spring with little con- the channel. Conversely, a decline of stream- tribution from fall, winter, or summer runoff. flow in fall, winter, and summer reduced sed- Finally, during the early 1980s, chan- minor iment transport rates, leading to channel nel adjustment during of an episode stability and floodplain accretion. These increased streamflow (fig. 26) eroded the relations may apply to entrenchment of the modern alluvium, forming the active chan- prehistoric terrace and aggradation of the nel and floodplain. settlement alluvium, although the seasonal In summary, historic channel instability distribution of streamflow is unknown dur-

ressed by widening and deepening (fig. ing this time. Entrenchment of the prehis- uring two periods of high stream- toric terrace probably happened during a lasted several decades in the case 200-year episode of high streamflow from

62 about A.D. 1200-1400. Aggradation of the believed by regional historians to have pro- settlement alluvium was probably during duced progressively larger floods, eventually relatively low streamflow between A.D. leading to abandonment of Shunesburg 1400-1880. (Stromberg, 1992) and other communities A generalized model of fluvial erosion along the Virgin River. Erosion was contem- poraneous with large-scale cattle and sheep and deposition for the Virgin River is sug- gested by the association of geomorphic ranching that reached a maximum around this activity with streamflow variations. Arroyo 1900-1914 (Gregory, 1950, p. 44-45), coincidently was a time of unusually high cutting is evidently linked to periods of increased streamflow lasting at least several streamflow (figs. 25-26). decades. This increased streamflow supplies Likewise, conservation measures and excess transport capacity that eventually grazing control brought about by establish- destabilizes the channel resulting in widen- ment of the national forests and enactment ing and deepening. When the channel is of the Taylor Grazing Act of 1934 should unstable, large floods are particularly effec- result in channel stabilization and deposi- tive at eroding the channel margin. tion of the modern alluvium. Overgrazing Increased streamflow results from above has been controlled or at least reduced by normal runoff during fall, winter, and sum- limiting the number of cattle and sheep mer that augments the normally high spring allowed to graze on public lands. In addition, discharge. Conversely, deposition and rela- numerous stock tanks and small reservoirs tive channel stability are probably linked to have been constructed in the basin. These periods dominated by spring runoff lasting structures are designed to water cattle and several decades or even several centuries. As distribute grazing use, but the structures the channel stabilizes, large floods are rela- also reduce peak-flow rates and sediment tively ineffective at modifying the channel load. The descendants of pioneer families erosionally; instead floods probably spread believe that restricted grazing and other con- sediment across the floodplain, causing servation measures have resulted in fewer aggradation of the channel system. Stream- large floods (DeMille, 1982). flow during depositional episodes is rela- Nevertheless, landuse practices were tively low because fall, winter, and summer runoff probably do not contribute substan- quite likely not the single cause of historic stabilization, tially to annual or long-term streamflow. arroyo cutting or channel although the effects of overgrazing may have exacerbated the rate and extent of channel Causes of Long-Term Streamflow entrenchment and widening. The tree-ring Variation chronology strongly suggests that during the mid-1850s to about 1920-1930 precipitation Landuse was extremely variable. This period was Precipitation variability and human characterized by both the least and most pre- activity, particularly grazing, were probably cipitation in 300 years. This extreme vari- the main causes of streamflow variation and ability is illustrated in figure 31, a time geomorphic change in the Zion National series of the smoothed tree-ring indices used Park area. Landuse began to alter runoff to in the streamflow reconstruction. Smoothing some extent beginning with settlement of is necessary to remove the high year-to-year southwest Utah around 1860 (Bailey, 1935). annual variability that is characteristic of Extensive overgrazing alters the rainfall- ring-width data (Dean, 1988). For the 303- runoff relation by soil compaction and year record, the mid-1850s, mid-1870s, and destruction of vegetation, thereby reducing late 1890s were the second, third, and fourth infiltration and increasing runoff. Overgraz- driest periods, respectively; whereas the ing by large herds of sheep and cattle is wettest periods were the mid-1880s to mid-

63 related stabilization of the channel probably did not result entirely from control of graz- ing, increased water diversion, or construc- tion of water-retention structures. Diversion of water for agricultural and domestic pur- poses accounts for only a small amount of annual streamflow. Grazing control and water-retention structures probably improved range conditions and reduced

I I I I I I I I I I I I I I I I I I I I I

I I I I locally. effect I peak-flow rates However, the of 1700 1750 1800 1850 1900 1950 this activity was probably not large, because Water year it is not apparent in the annual flood series, which has not declined over time (fig. 29). Figure 31. Time series of smoothed ring- Indeed, annual peak floods after 1950 are width indices, 1690-1992. Pattern shows above and below normal growth indices. among the largest of the measurement Smoothing done with fourier transform algo- period. In short, historic arroyo cutting and rithm, smoothing level 32 percent. subsequent channel stabilization resulted mainly from variation of precipitation that 1890s and early 1900s to about 1920-1930, in turn controlled streamflow, although which tie for second wettest and wettest, human activity probably intensified erosion respectively. The latter wet period was the and might have enhanced stabilization of the longest episode of high precipitation since channel resulting in deposition. about A.D. 1400, according to the streamflow reconstruction of Larson and Michaelsen Climate Variation (1990). The late Holocene is a time of global cli-

Historic arroyo cutting followed settle- mate variability, beginning with the Little ment of the region closely, but erosion was Climatic Optimum, or Medieval Warm also contemporaneous with some of the wet- Period, from about A.D. 900-1300 (Lamb, test and driest climate since A.D. 1400. The 1977, p. 435-449), the Little Ice Age from combination of very dry conditions followed about A.D. 1400 to the mid-1800s (Bradley, by extreme moisture between the mid- 1850s 1985, p. 239; Grove, 1988, p. 241-262), and to mid-1890s (fig. 31) was probably sufficient global warming beginning in the late 1800s to initiate arroyo cutting, which was further (Lamb, 1982, p. 247-251). The Medieval enhanced during the second occurrence of Warm Period is recognized in Europe and is high precipitation and streamflow from 1900 noted for above average temperatures and to about 1920-1930. The two dry periods that generally favorable conditions. The Little Ice preceded widespread arroyo cutting proba- Age is defined largely on the basis of glacial bly weakened channel and hillslope vegeta- advances in mountainous regions. tion, thereby increasing runoff during the How or if Colorado Plateau climate was unusually wet climate beginning in the mid- affected during the Medieval Warm Period 18803. This information suggests that arroyo and Little Ice Age is not well known. The cutting resulted in part from the climate warm interval is based on European history variations of the mid- to late 1800s. Settle- and the Little Ice Age is recognized from gla- ment of the region, introduction of livestock, cial activity that has no immediate corollary overgrazing, and removal of vegetation prob- on the Colorado Plateau. Global warming, ably intensified erosion, but it seems however, is evident in historic temperature anlikely that these were the only causes of records from the southern Colorado Plateau arroyo cutting. (Hereford, 1984, fig. 15).

ition of the modern alluvium and Aggradation of the prehistoric and settle-

64 ment alluviums was partly contemporane- which he attributes to relatively cool and dry ous with the Medieval Warm Period and conditions. These conditions resulted from Little Ice Age, respectively. The effect of the less vigorous atmospheric circulation that warm interval on southern Colorado Plateau decreased summer rainfall and presumably climate is poorly understood. Dean (1994) lowered temperatures. There may be some found little correspondence between the 400- connection between deposition of the settle- year warm interval and temporal variations ment alluvium and the Little Ice Age climate of human adaptive behavior and hydrocli- described by Petersen (1994). Relatively dry matic indicators such as groundwater levels conditions in the Virgin River basin during and floodplain deposition and erosion. He this time are suggested by the reconstructed concluded that the Medieval Warm Period streamflow of Larson and Michaelsen (1990). could not be viewed as an interval of consis- Streamflow at this time was probably domi- tent climate; rather it was an interval of nated by spring runoff with little contribu- environmental variability that differed little tion from fall, winter, or summer. These are from the variability that is typical of the past the seasonal streamflow patterns associated 2,000 years. with deposition of the modern alluvium, which could also pertain to deposition of the A somewhat different conclusion was settlement alluvium if conditions persisted reached by Petersen (1994) for the eastern for at least several centuries. In short, there Colorado Plateau and the southern San Juan appears to be only a weak link to deposition Mountains based on pollen studies of pinyon. of the prehistoric and equivalent alluviums Summer rainfall is important for establish- with the climate of the Medieval Warm ment of pinyon. The abundance of pinyon Period. However, deposition of the settle- determined from pollen studies, therefore, is ment alluvium during an interval of rela- an indicator of the long-term strength of the tively low runoff is at least broadly summer (Petersen, 1994). Pinyon consistent with Little Ice Age conditions. were particularly abundant between A.D. 750-1150, suggesting that summer rainfall Generally, the erosion around A.D. 1200- was at least comparable to present levels. In 1400 and 1900 ensued during the transition addition, during the height of the Medieval from one climate regimen to another. The Warm Period from A.D. 1000-1100, the early erosional episode was between the end region was characterized by a more vigorous of the Medieval Warm Period and the begin- atmospheric circulation resulting in both ning of the Little Ice Age, and historic arroyo wetter summers and winters than at cutting happened near the end of the Little present. Thus, the Medieval Warm Period Ice Age. Little is known about climate during could have produced anomalous precipita- the early entrenchment, although stream- tion and temperature patterns that effected flow of the Virgin River was unusually high, deposition of the prehistoric alluvium of the implying that precipitation conditions study area as well as the Tsegi and Chaco enhanced runoff, thereby increasing erosion formations. However, initial deposition of and channel instability. the latter two alluviums and probably the Climate during historic arroyo cutting is prehistoric alluvium as well predates this somewhat better understood. The anoma- warm and wet interval. The relation lous climate beginning in the mid-1850s to between climate of the Medieval Warm mid-1860s (figs. 25, 31) may arise from the Period and fluvial activity is evidently not strong and frequent warm ENSO (El Nino clear, as Dean (1994) suggested. Southern Oscillation) conditions that pre- During the Little Ice Age from about A.D. vailed from 1864-1891 (Quinn and others, 1300-1850, as temporally placed by Petersen 1987). A complex system of global climate (1994), pinyon was relatively scarce at the fluctuations, ENSO typically increases rain- eastern margin of the Colorado Plateau, fall in the Southwest from April through

65 October (Ropelewski and Halpert, 1986), ter particularly during ENSO conditions although weather at other times of the year (Webb and others, 1991). is also affected. During ENSO conditions, a Meridional circulation and the high- warm pool of water is present in the eastern amplitude ENSO phase of the early 1900s Pacific Ocean near the equator that is a affected climate of the southern Colorado source of precipitable moisture for advection Plateau and Virgin River basin. The recon- into the Southwest (Cayan and Webb, 1992). structed streamflow and tree-ring chronol- In addition, the strength of the jet stream ogy (figs. 25, 31) show that annual increases during ENSO conditions, guiding precipitation and runoff were unusually high weather disturbances into the Southwest. from 1900 to about 1920-1930. The gaged record shows that seasonal runoff from 1910 Generally, runoff producing precipitation to about 1920-1930 was unusually high, in the Southwest results from four storm except for spring (fig. 27). Warm-season rain- types—frontal systems, monsoonal storms, fall over the southern Colorado Plateau was dissipating tropical cyclones, and cut-off low pressure systems (Webb and Betancourt, above normal from 1900 (when widespread instrumentation began) until about 1930- 1992, p. 6-10). ENSO conditions tend to 1940, this is the longest interval of above increase the severity and frequency of these normal warm-season rainfall during the storm systems, although the affect is com- 20th century (Hereford and Webb, 1992). plex, variable from one ENSO to another, Similar results were obtained by Balling and and poorly understood in most cases. The Wells (1990) in the basin of New amount of precipitation and severity of fron- tal-type winter storms increases during Mexico. ENSO, leading to large floods in high-eleva- The relatively low discharge levels of the tion basins such as the Virgin River (Cayan post-1940 era that led to deposition of the and Webb, 1992). Dissipating tropical modern alluvium resulted in part from a cyclones tend to occur during ENSO, and breakdown of the previous ENSO pattern precipitation is extremely high in some and a decrease of meridional circulation. The cases. However, monsoonal rainfall is appar- earlier high-amplitude ENSO phase ently little affected by ENSO (Hereford and decreased into a low-amplitude phase domi- Webb, 1992). Cut off-low pressure systems nated by weak ENSO after the early 1940s probably increase in number during ENSO, (Michaelsen, 1989). Weakened and reduced leading to flooding in the fall months. The ENSO would alter precipitation patterns unusual ENSO activity from 1864-1891 evi- through less frequent tropical cyclones and dently increased precipitation year-around, less vigorous atmospheric circulation. In resulting in frequent heavy floods and high addition, around 1930, circulation of the runoff. upper atmospheric shifted from meridional to dominantly zonal, a west-to-east upper The ENSO activity that increased pre- level wind that generally produces dry cipitation evidently continued in form some weather in the Southwest. through about 1930-1940. Michaelsen (1989) found that 1917-1941 was a high-amplitude Regulated Streamflow and ENSO phase, a pattern recurring every 80- Geomorphic Change 100 years on average that is dominated by strong to moderate ENSO activity. This pat- The recent alluvial history of the Virgin tern was also associated with a dominance of River indicates that alluvial valleys in the meridional atmospheric circulation from at Zion National Park region change frequently least 1899 to about 1930. This circulation and rapidly. Regulation of streamflow will nvolves large north-south excursions of the almost certainly alter this evolutionary pat- polar jet stream that increase the number tern. In the past 1,000 years, the channel has and severity of Pacific storms in fall and win- widened, deepened, and partly refilled sev-

66 eral times. Much of this change was only in pools in the flat stretches upstream of tribu- the past 100-120 years. Moreover, detectable taries separated by steep reaches where the bedrock erosion occurred at least locally dur- river crosses the alluvial fans. ing historic arroyo cutting, as illustrated in figure 11. In time, the cumulative effect of this short-term bedrock erosion might con- SUMMARY AND CONCLUSIONS tribute substantially to canyon cutting. Alluvial valleys of the study area are the alluvial history also indicates that The latest development in the geologic history of channel deepening and widening were asso- the Virgin River. Extensive erosion of Meso- ciated with periods of high streamflow. In zoic strata during the Quaternary estab- contrast, deposition resulting in of buildup lished the present course and elevation of the the channel alluvial valley during and was river. Termed canyon cutting, this erosion periods of relatively low streamflow. The nor- produced the major canyons in the Zion pattern of channel evolution evidently mal National Park area. Canyon cutting was con- requires streamflow that varies from high to trolled by movement along the Hurricane relatively low with an adequate supply of fault, a north-trending fault west of the sediment. study area that forms the physiographic Elimination of high-flow rates reduces boundary between the Basin and Range the scale of channel widening and deepen- province to the west and the Colorado Pla- ing. Short-term erosion of the bedrock chan- teau province to the east. The eastern block nel will also be curtailed, erosion that is a or footwall of the fault moved up episodically small-scale version of canyon cutting. Like- 600-850 m (2,000-2,800 ft) while the western wise, deposition will be limited by reduced block remained stationary. This uplift steep- sediment loads and peak-flow rates, depend- ened the gradient of the ancestral Virgin ing on the relative magnitude of change River, increasing its erosive power, eventu- between unregulated and regulated stream- ally cutting the canyons of Zion National flow. In short, the pattern of alternating ero- Park. The Hurricane fault has not been sion and deposition will be altered by active in the late Holocene, thus the recent regulated streamflow, reducing the magni- alluvial history of the Virgin River described tude of geomorphic change in the alluvial in this report is quite likely unrelated to tec- valleys. tonic activity.

The balance between tributary sediment Deposits of the alluvial valleys accumu- input and its removal by mainstem stream- lated during the late Holocene, or the past flow will be altered by regulated streamflow. 1,000 years. Although older Holocene depos- This balance is important in maintaining the its are probably present, they have not been channel in Parunuweap Canyon below the identified on or below the surface of the val- narrows and to a lesser extent in Zion Can- ley. The deposits are derived from upstream yon. Tributary sediment is transported to the sources as well as from nearby adjacent hill- alluvial valley by streamflow and debris flow, slopes and tributary streams. Mainstem and the deposits form alluvial fans in the deposits are transported by the Virgin River river channel. These deposits are readily from distant sources. The mainstem deposits removed by the river, and they are preserved are primarily light-colored very fine to only as eroded remnants along the valley medium-grained sand and minor sub- margin. Reduced streamflow will increase rounded to rounded pebble to cobble gravel; preservation of alluvial fans at the mouths of the fine-grained alluvium accumulated by tributaries, leading to low-gradient channel lateral and vertical floodplain accretion. reaches and partial ponding of the river. Tributary deposits are transported to the Thus, the relatively straight, uniform-gradi- alluvial valleys mainly by fluvial and debris- ent channel would likely become a series of flow processes. Relative to mainstem depos-

67 its, tributary deposits are typically darker occupation. The end of prehistoric deposition colored, coarser grained, and less rounded. was after A.D. 1100-1200, as suggested by Near the margin of the valley mainstem and the age and stratigraphic position of the tributary deposits are interbedded. archeologic material (fig. 10).

A study of the alluvial valleys of North The settlement terrace is named for his- Fork and East Fork (figs. 5, 9) shows that the toric archeologic material and evidence of two streams have similar alluvial and geo- Anglo agricultural activity on the surface. morphic histories. Moreover, the mapped Historic accounts suggest that the terrace sequence of terraces and deposits are and related alluvium were the active chan- present downstream of the forks at least to nel and floodplain of the Virgin River when Virgin. Four terraces and the active channel the area was settled in the late 1850s and and floodplain are present in the alluvial val- early 1860s. The terrace is situated 2-4m (7- leys. From oldest to youngest, the terraces 13 ft) below the prehistoric terrace on East are referred to as the prehistoric, settlement, Fork and less than 2 m (6 ft) below the ter- historic, and modern terraces. The terraces race on North Fork. The age of the terrace typically have inset relations with one could not be determined directly due to the another such that the oldest terrace is topo- lack of dateable material. The inset relation graphically the highest and closest to the indicates that the settlement terrace is valley margin. Except for the historic ter- younger than the prehistoric terrace; the race, the insets are the result of cut-and-fill absence of Anasazi remains in the settle- stratigraphy in which the alluvial channel is ment alluvium indicates that it post-dates repeatedly cut or eroded and then partly abandonment of the area by the Anasazi, refilled with alluvium. which was around A.D. 1200. The alluvium The main processes are erosion and dep- is younger than A.D. 1200, and deposition osition at the scale of this investigation. Ero- could have begun around A.D. 1400, based sion deepens and widens the channel, on correlation with alluvial deposits of the causing abandonment of the floodplain and Escalante River and Paria River basin (fig. truncation of earlier deposits; whereas depo- 10). Tree-ring dates from cottonwood grow- sition partly refills the channel to a lower ing on the surface of the alluvium at North topographic level than the preceding deposi- Fork indicate that the settlement alluvium tion. Two erosional events during the late probably pre-dates A.D. 1883 (fig. 9). Holocene removed most of the previously The historic terrace is mainly an ero- deposited sediment and locally eroded bed- sional feature that is not associated with rock as well. This prehistoric and historic aggradation. The terrace is situated 3 m (10 erosion entails substantial widening of the ft) below the settlement terrace on East Fork channel that in places includes the entire and less than 1 m (3 ft) below the terrace on width of the alluvial valley. North Fork. Typically, the historic terrace The prehistoric terrace is the oldest unit occupies meander scars cut into the settle- identified in the study area. The terrace is ment terrace. The historic terrace probably present at the margins of the alluvial valley represents an abandoned channel that was where it ranges from 6-10 m (20-33 ft) above active during incision of the settlement ter- the active channel on East Fork and 3-4 m race and during subsequent channel widen- (10-13 ft) above the channel on North Fork. ing. Ring-counts of cottonwood growing on or Near the top of the unit at East Fork, the just below the surface were used to date the deposits locally contain archeologic material terrace. The oldest dates indicate that the of the Virgin branch of the Kayenta Anasazi. historic-age channel formed in 1892 and This material dates to A.D. 800-1200, and 1893 at North Fork and East Fork, respec- the stratigraphic context suggests that depo- tively (figs. 5, 9). This channel was active ition of the alluvium was concurrent with until about 1926 when sediment began to

68 accumulate on point bars and along the ronmental consequences for the region. Pre- channel margin in straight reaches. historic arroyo cutting was probably one of the main factors leading to abandonment of Alluvium of the modern terrace is rela- southern Utah northern Arizona the tively thin, although the surface area of the and by Anasazi. Likewise, historic arroyo cutting terrace is large, as it occupies a substantial caused dislocations, loss of property, portion of the earlier historic-age channel major and economic problems for early settlers in (figs. 5, 9). The terrace forms point bars (fig. the Zion National Park area and elsewhere 7) in wide reaches of East Fork and Virgin in the southern Colorado Plateau. River downstream of the forks and flood- plain-like surfaces on both sides of the chan- Historic accounts and relocation of early nel in relatively narrow reaches. The photographs document the condition of the alluvium consists of two units: a basal, sub- alluvial channel shortly before, during, and surface unit of largely unstratified sand after historic arroyo cutting. In 1859-1862, present at East Fork (fig. 8) and a terrace, when the Zion National Park area was first referred to as the early modern terrace, on settled, the Virgin River downstream of the North Fork. The basal unit quite likely pre- forks flowed at the surface of the alluvial val- dates 1940 and the early modern terrace ley in a narrow, shallow, and possibly mean- formed between 1926-1937. Deposition of the dering channel. The valley was not upper unit began in 1940, based on tree-ring entrenched and the wide floodplain was dating of cottonwood partly buried in the overtopped with only a slight rise of water alluvium, and continued until at least the level. An unusual storm in the winter of 1862 late 1970s or early 1980s, when minor caused one of the five largest floods of the adjustment of the channel caused abandon- historic period (Table 1). The flood caused ment of the modern-age floodplain. major dislocations among the settlers, who had virtually no experience with alluvial- The active channel and floodplain lie 1-2 channel rivers. Farmland and housing were m (3-7 ft) and 0.5-1 m (1.6-3 ft) below the likely established and built on the active modern terrace at East Fork and North floodplain, a situation that probably contin- Fork, respectively. This topographic separa- ued to some extent after the flood. tion is defined by steep cutbanks at most localities. The floodplain and channel are too Photographs of the river suggest that the small to show separately on the 1:2,000 scale channels of East Fork and North Fork were maps used to compile the surficial geology. unentrenched in 1873 (figs. 12-14, 20). North Dating of vegetation associated with the Fork had a sand-bed channel without gravel floodplain suggest that it began to form by at and a floodplain vegetated with cottonwood least 1983 if not a few years earlier. Two peri- and willow. This condition of the channel ods of erosion, referred to as the prehistoric contrasts sharply with the historic-age chan- and historic arroyo cutting, separate the nel in 1903 and 1909 (figs. 15-17), only 20-26 principal depositional units. These erosions years after arroyo cutting began. In 1903- were widespread, affecting the entire drain- 1909, the historic-age channel was age basin of the Virgin River as well as many entrenched below the settlement terrace, other streams in the Southwest. Prehistoric gravel was present in the channel, and a arroyo cutting, which ended deposition of the steep terrace rise with meander scars was prehistoric alluvium, occurred after A.D. cut into the settlement terrace. These condi- 1100-1200 and between A.D. 1200-1400, as tions suggest that the channel had deepened suggested by regional relations. Historic and was then widening, removing portions of arroyo cutting ensued between 1883-1940 the settlement terrace. and ended deposition of the settlement allu- East Fork near Shunesburg and the Vir- vium. gin River near Grafton were photographed in These erosional changes had severe envi- 1926 and 1917-1939 (figs. 18, 21), respec-

69 tively. The exact year of the latter photo- tively low streamflow following A.D. 1400 graph is uncertain, although it probably until about 1880 coincides with deposition of dates to the mid-1920s. The channel in the the settlement alluvium. This interval of low mid-1920s was flat floored, vegetation was streamflow was punctuated by brief episodes sparse, and sites of overbank deposition of very high streamflow in the upper basin were largely absent. Photographs taken in that had no discernible geomorphic effect. the late 1930s (figs. 22) indicate that 19, Historic arroyo cutting was preceded by relief had developed in the channel some and concurrent with extremely variable pre- through deposition of channel bars and other cipitation, including the driest and wettest mostly transient depositional features. climate in 300 years. The second, third, and These were the incipient deposits of the mod- fourth driest intervals in at least 300 years ern alluvium. Contemporary photographs were the mid-1850s, mid-1870s, and late the channel has stabilized relative show that 1890s; the wettest intervals were the mid- to early conditions. The braid-like pattern of 18808 to mid-1890s and early 1900s to about the historic-age channel was replaced by a 1920-1930, which tie for second wettest and locally meandering channel having a wettest, respectively (figs. 30-31). Drought recently active floodplain, which is the mod- conditions that preceded arroyo cutting may ern terrace along with a well-developed have weakened hillslope and valley-floor woodland of cottonwood and other riparian vegetation. This and, to some extent, the vegetation. introduction of livestock increased runoff. These changes in the channel and allu- The first wet episode from the early 1880s to vial valley of the Virgin River were contem- the mid-1890s caused the initial entrench- poraneous with variations of annual ment and widening of the Virgin River; the streamflow. Long-term streamflow was esti- second episode from around the early 1900s mated by tree-ring chronologies from two to the mid-1920s widened the channel fur- sites in the study area. Ring width of pinyon ther. The end of arroyo cutting, which may growing on semiarid hillslopes is largely a only be temporary, began with deposition of function of October through June precipita- the modern alluvium in 1940, following a tion, which in turn is related to streamflow. brief resurgence of relatively high stream- Measured streamflow for 1910-1992 was cal- flow in the late 1930s. ibrated to ring width using regression analy- The gaged streamflow record shows that sis; ring width explains 59 percent of annual the high annual runoff of the early 1900s streamflow variability. In addition, esti- (fig. 26) resulted from mostly above average mated annual streamflow is in phase with streamflow during fall, winter, and summer actual streamflow in 80 percent of the cases that augmented normally high spring runoff (figs. 23-24). A 303-year record of streamflow (figs. 27-28). The early episode of high from 1690-1992 was developed from the full streamflow during the late 1800s could also tree-ring chronology (fig. 25), this was used result from increased seasonal runoff, along with the gaged record to interpret his- although the early episode predates stream- toric geomorphic activity. Prehistoric geo- flow measurements. These runoff variations morphology was interpreted using a are presumably related to climate; human streamflow reconstruction from the lower activity in historic times possibly increased Virgin River that begins in A.D. 966 (Larson runoff, but arroyo cutting would probably- and Michaelsen, 1990). have occurred without human interference.

Prehistoric arroyo cutting was probably In short, the recent alluvial history of the contemporaneous with a period of high Virgin River in the Zion National Park streamflow between about A.D. 1200-1400. region indicates that the geomorphic devel- Streamflow during this time was the largest opment of the alluvial valley and channel are since A.D. 1400. Conversely, a period of rela- linked to variations of streamflow (fig. 30).

70 Regulation of streamflow by upstream reser- E. Diaz, William R. Hansen, Marsha M. voirs will alter the geomorphic development Hilmes, Daniel J. McGlothlin, Laird P. Nay- of the Virgin River valley. In the recent past, lor II, and Charles S. Peterson improved the geomorphic development was rapid, and report. This work resulted from a coopera- changes in the alluvial valley are expected tive agreement between the National Park during the next several hundred years. For Service and the U.S. Geological Survey. example, in the late 20th century, two peri- ods of increased streamflow lasting less than five years caused readily detectable adjust- REFERENCES ment of channel width and depth. Longer Adams, Frank, 1903, Agriculture under irri- periods of unusually high streamflow, lasting gation in the basin of the Virgin River: several decades in historic times and several U.S. Department of Agriculture Bulletin centuries in prehistoric times, caused major 124, p. 207-265, plate 14. changes of width and depth of the channel Altschul, J.H., and Fairley, H.C., 1989, Man, and alluvial valley. Conversely, periods of models and management: An overview of relatively low streamflow resulted in aggra- the archaeology of the Arizona Strip and dation of the channel and increased the ele- the management of its cultural resources: vation of the alluvial valley. Upstream U.S. Government Printing Office, Report reservoirs and regulated streamflow will prepared for USDA Forest Service and quite likely change the magnitude and pat- USDI Bureau of Land Management, Con- tern of channel development, unless existing tract Number 53-8371-6-0054, 410 p. flow conditions can be maintained. Anderson, R.E., 1978, Quaternary tectonics along the intermountain seismic belt ACKNOWLEGMENTS south of Provo, Utah: Brigham Young University Geology Studies, v. 25, p. 1-10. Access to the facilities and back country Anderson, R.E., and Mehnert, H.H., 1979, of Zion National Park was expedited by Reinterpretation of the history of the Cheri Fedorchak and Vic Vierra; Laird P. Hurricane fault in Utah: Rocky Mountain Naylor II and Jack Burns shared knowledge Association of Geologists-Utah Geological of archeologic sites in East Fork. Denny Association 1979 Basin and Range Sym-

Davies provided access to park archives and posium, p. 145-165. special collections. Robert H. Webb kindly Andrews, E.D., 1986, Downstream effects of furnished the historic photographs of J.K. on the Green Hillers and those from the National Archives River, Colorado and Utah: Geological (Table 2). He also supplied unpublished Society of America Bulletin, v. 97, p. 1012- radiocarbon dates and stratigraphic infor- 1023. mation about the Escalante River used in Bailey, R.W, 1935, Epicycles of erosion in the figure 10. Edmund D. Andrews provided valleys of the Colorado Plateau Province: information about sediment transport; Journal of Geology, v. 43, p. 337-355. Gustavo E. Diaz and William R. Hansen shared information and data about the Balling, R.C., and Wells, S.G., 1990, Histori- hydrology of the Virgin River basin. James cal rainfall patterns and arroyo activity E. Deacon provided information regarding within the Zuni River , native fish of the Virgin River. Loren D. Pot- New Mexico: Annals of the American ter identified and interpreted the flora of the Association of Geographers, v. 80, p. 603- study sites. Marsha M. Hilmes shared addi- 617. tional flood information from the Mesquite Boison, P.J., and Patton, PC, 1985, Sedi- Historical Museum. Critical reviews and ment storage and terrace formation in comments by Edmund D. Andrews, Gustavo basin, south-central Utah:

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75