Papers by Richard Hereford
Open-file report /, 1989
Decreased runoff beginning in the early 1940s was probably the main cause of extensive sediment s... more Decreased runoff beginning in the early 1940s was probably the main cause of extensive sediment storage in alluvial-channel tributaries of the Colorado River. Most sediment is transported in these tributaries during summer and early fall, and a long-term change of runoff during the warm season (June 15 to October 15) would alter channel equilibrium. Runoff occurs in direct response to rainfall from subtropical moisture transported from the Gulf of Mexico or tropical Pacific Ocean onto the normally dry plateau through monsoon or tropical cyclone circulation. Analysis of average daily rainfall at 13 weather stations suggests that after the early 1940s the rainy season ended earlier, decreasing the length of the season as much as 40 percent. Moreover, regionally averaged rainfall frequency and amount for the season also decreased in the early 1940s. Total seasonal rainfall is closely related to rainfall frequency (r == 0.95), which in turn is correlated with runoff frequency (r = 0.79). In the Little Colorado River basin, measured runoff was unusually high from the beginning of the record in 1925 until the early 1940s. The high runoff, however, began before 1910, based on a multiple linear regression with rainfall frequency as the predictor variable. A long-term change of either monsoon strength or tropical-cyclone frequency or a change of both would reduce rainfall and runoff. Evidence for a long-term change in monsoon strength before 1945 is unavailable due to a lack of upper air pressure data. The number of tropical cyclones, however, decreased substantially after the early 1940s, which reduced average daily rainfall in the latter part of the season. In addition, decreased rainfall in late summer and early fall might be related to relatively low frequency of well developed El Nino-Southern Oscillation between about 1940-1985. v
Abstracts with programs, 2018
Open-file report /, 1995
FIGURES tion of the modern alluvium was during relatively low runoff after 1940. High runoff dest... more FIGURES tion of the modern alluvium was during relatively low runoff after 1940. High runoff destabilizes the channel, enhancing the effectiveness of floods. Conversely, relatively low runoff increases channel stability, reducing the erosional effect of floods and enhancing floodplain deposition. Adjustments in the width and depth of the channel are frequent, even after relatively small, short-term variations of streamflow. The normal pattern of geomorphic change requires streamflow that varies from high to relatively low. Regulated streamflow would likely alter this pattern through moderation of flow rates and reduced sediment loads.
Geological Society of America eBooks, 1993
is a north-flowing tributary of the Gila River. The area of the drainage basin upstream of the He... more is a north-flowing tributary of the Gila River. The area of the drainage basin upstream of the Headcut retreat in a semiarid watershed in the southwestern United. Channel Entrenchment in the Redington Reach of the San Pedro River. 18 conducted by the Arizona Geological Survey for the Arizona State Land Department channel changes along the upper San Pedro River, there is very little published information. widening or narrowing, sediment storage and episodic transport,. Pima
Quaternary Research, May 1, 1986
Stream channels in the Paria River basin were eroded and partially refilled between 1883 and 1980... more Stream channels in the Paria River basin were eroded and partially refilled between 1883 and 1980. Basin-wide erosion began in 1883; channels were fully entrenched and widened by 1890. This erosion occurred during the well-documented period of arroyo cutting in the Southwest. Photographs of the Paria River channel taken between 1918 and 1940 show that the channel did not have a floodplain and remained wide and deep until the early 1940s. A thin bar (<50 cm), now reworked and locally preserved, was deposited at that time. Basin-wide aggradation, which began in the early 1940s, developed floodplains by vertical accretion. The floodplain alluvium, 1.3–3 m thick. consists of two units recognizable throughout the studied area. An older unit was deposited during a time of low flow and sediment yield whereas the younger unit was deposited during times of high flow, sediment yield, and precipitation. Tree-ring dating suggests that the older unit was deposited between the early 1940s and 1956, and the younger between 1956 and 1980. The units are not time transgressive, suggesting that deposition by knickpoint recession was not an important process. High peak-flood discharges were associated with crosion and low flood discharges with aggradation. The erosional or aggradational mode of the streams was determined principally by peak-flood discharge, which in turn was controlled by precipitation.
Geological Society of America Bulletin, 1984
Fact sheet /, 2004
These storms build snowpacks that melt in the late spring, providing runoff to the Colorado River... more These storms build snowpacks that melt in the late spring, providing runoff to the Colorado River. Warm winter storms, which originate in the tropical Pacific Ocean, may cause rainfall on snowpacks, resulting in high runoff and floods on major rivers. The frequency and moisture content of frontal systems are strongly affected by atmospheric circulation patterns (particularly their strength) and sea-surface temperature (SSTs) of the tropical and North Pacific Oceans. Moisture delivered to the Colorado River basin during summer is typically a mixture of moist air from the Gulf of Mexico, the Gulf of California, and the eastern Pacific Ocean. Known as the "Arizona monsoon," this moisture arrives in July and August at low levels in the atmosphere. The moist air rises rapidly over the desert landscape, spawning thunderstorms that deliver high-intensity rainfall to elevations less than 7,000 feet and lower-intensity rainfall at higher elevations. Thunderstorms tend to be of small spatial extent, and although they spawn severe flash flooding locally, few floods are generated
Burke, K.J., Fairley, H.C., Hereford, Richard, and Thompson, K.S., 2003, Holocene terraces, sand dunes, and debris fans along the Colorado River in Grand Canyon, in, Beus, S.S., and Morales, Michael, eds., Grand Canyon Geology: London, Oxford University Press, p. 352-370
Hereford, Richard, Jacoby, G.C., and McCord, V.A.S., 1996, Late Holocene alluvial geomorphology of the Virgin River in the Zion National Park area, southwest Utah: Geological Society of America Special Paper 310, 46 p., 31 figs., 2 tables
Geology, 1987
Measured section-Contact-Approximately located Figure 1. A: Study area in southern Utah. B: Topog... more Measured section-Contact-Approximately located Figure 1. A: Study area in southern Utah. B: Topography of drainage basin. Contour interval 12 m (40 ft); pattern shows vegetated ground. P = ponded area deposits; D = delta deposits. From U.S. Geological Survey Telegraph Flat 15' quadrangle (1954). C: Map of reservoir showing relative elevation and distribution of deposits.
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Papers by Richard Hereford