Geology topics
John G. Elliott
Publications and source records attributed to John G. Elliott.
Lava Falls Rapid in Grand Canyon: Effects of Late Holocene debris flows on the Colorado River
Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows in 1939, 1954, 1955, 1966, and 1995, as well as prehistoric events, completely changed flow through the rapid. Floods cleared out much of the increased constrictions, but releases from Glen Canyon Dam, including the 1996 controlled flood, are now required to remove the boulders deposited by the debris flows.
Geomorphic and sedimentologic characteristics of alluvial reaches in the Black Canyon of the Gunnison National Monument, Colorado
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Channel-pattern adjustments and geomorphic characteristics of Elkhead Creek, Colorado, 1937-97
Onsite channel surveys and sediment measurements made in 1997, aerial photographs taken from 1937 through 1993, and streamflow-gaging- station record from 1954 to 1996 were used to determine the probable cause of accelerated streambed and streambank erosion in the lower reaches of Elkhead Creek, a perennial, meandering tributary of the Yampa River. Concern about the possible effects of Elkhead Reservoir, constructed in 1974, has been expressed by landowners living downstream. Evidence cited as an indication of reservoir related effects include the trapping of bedload transported sediment in the reservoir, vertical incision of the streambed, and lateral erosion causing loss of agricultural land. A large deltaic deposit composed of approximately 163 acre-ft of bedload-transported sediment formed in Elkhead Reservoir between 1974 and 1993, the contemporary bankfull stage of Elkhead Creek is several feet below the elevation of a broad terrace that previously was the flood plain, and lateral erosion at meander bends occurs at a higher rate than in previous periods at some locations. Elkhead Creek meander migration rates were used as a measure of lateral instability in the study reaches. Meander migration rates based on changes in channel centerline position were calculated for three periods from five sets of rectified aerial photographs for reaches upstream and downstream from the reservoir. The creek upstream from Elkhead Reservoir was unaffected by impoundment and was used as the control reach. Mean meander migration rates in the downstream study reach were 1.2 ft/yr from 1938 to 1953, 2.5 ft/yr from 1954 to 1970, and 4.8 ft/yr from 1978 to 1993, compared to rates of 0.5 ft/yr, 1.6 ft/yr, and 6.6 ft/yr for the same periods in the upstream study reach. Sediment and channel-geometry measurements and estimated hydraulic conditions at eight cross sections indicate that most of the sediment sizes represented in the streambed are mobile at frequently occurring streamflows; those streamflows are less than or equal to the bankfull discharge of approximately 1,800 to 2,200 cubic feet per second. Discharge data from 1954 through 1996 recorded at a site upstream from the reservoir were examined to determine the effect of hydrology on meander migration rates. The discharge data were assumed to be representative of the total streamflow and flood hydrology of both the upstream and downstream reaches because Elkhead Reservoir normally has a full pool. Mean annual streamflow increased 122 percent, and the mean annual flood increased 130 percent from the pre-regulation period (1954 to 1970) to the post-regulation period (1978 to 1993), a possible explanation for much of the increase observed in meander migration rate in both the upstream and downstream reaches in the period after reservoir construction. Channel instability, quantified by meander migration rates, has increased throughout Elkhead Creek since 1977. The most probable cause is a combination of external factors affecting the 2 Channel-Pattern Adjustments and Geomorphic Characteristics of Elkhead Creek, Colorado, 1937-97 entire watershed, such as changes in annual runoff and flood magnitude and sedimentation in Elkhead Reservoir. Local land-use practices, such as intentional meander cutoff and riparian vegetation removal, also can decrease channel stability, but these factors were not addressed in this study.
Reconfigured-Channel Monitoring and Assessment Program
A U.S. Geological Survey program to monitor and assess channel reconfiguration activity is described. A data base available on the world wide web will enable land-management agencies and other interested parties to evaluate the long-term success of various channel reconfiguration projects. A demonstration project on the Lake Fork of the Gunnison River, Colorado, illustrates the program objectives and approach.
"The great cataract" - Effects of late Holocene debris flows on Lava Falls Rapid, Grand Canyon National National Park, Arizona
Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Although the rapid was once thought to be controlled by the remnants of lava dams of Pleistocene age, Lava Falls was created and is maintained by frequent debris flows from Prospect Canyon. We used 232 historical photographs, of which 121 were replicated, and 14 C and 3 He dating methods to reconstruct the ages and, in some cases, the magnitudes of late Holocene debris flows. We quantified the interaction between Prospect Canyon debris flows and the Colorado River using image processing of the historical photographs. The highest and oldest debris-flow deposits on the debris fan yielded a 3 He date of 2.9±0.6 ka (950 BC), which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate, although probably short, period. We mapped depositional surfaces of 6 debris flows that occurred after 950 BC. The most recent prehistoric debris flow occurred no more than 500 years ago (AD 1434). From April 1872 to July 1939, no debris flows occurred in Prospect Canyon. Debris flows in 1939, 1954, 1955, 1963, 1966, and 1995 constricted the Colorado River between 35 and 80 percent and completely changed the pattern of flow through the rapid. The debris flows had discharges estimated between about 290 and 1,000 m 3 /s and transported boulders as heavy as 30 Mg. The recurrence interval of these debris flows, calculated from the volume of the aggraded debris fan, ranged from 35 to 200 yrs. The 1939 debris flow in Prospect Canyon appears to have been the largest debris flow in Grand Canyon during the last 125 years. Debris flows in Prospect Canyon are initiated by streamflow pouring over a 325-m waterfall onto unconsolidated colluvium, a process called the firehose effect. Floods in Prospect Valley above the waterfall are generated during regional winter storms, localized summer thunderstorms, and occasional tropical cyclones. Winter precipitation has increased in the Grand Canyon region since the early 1960s, and the most recent debris flows have occurred during winter storms. Summer rainfall has declined in the same period, decreasing the potential for debris flows in the summer months. The history of river reworking of the Prospect Canyon debris fan illustrates the interrelation between tributary debris fans and mainstem floods in bedrock canyons. Lava Falls Rapid did not change despite Colorado River floods of 8,500 m 3 /s in 1884 and 6,230 m3/s in 1921. Floods up to 3,540 m 3 /s that occurred after the historical, pre-dam debris flows removed most of the deposits within 3 years. Releases in 1965 from Glen Canyon Dam that were above powerplant capacity but less than 1,640 m 3 /s removed most of the debris fan deposited in 1963, and the combination of dam releases and a 1973 flood on the Little Colorado River removed the 1966 aggradation. About 4,800 m 3 of the 1995 deposit was reworked on the day of the 1995 debris flow, dam releases of less than 570 m 3 /s had not reworked the remainder of the aggraded debris fan. Lava Falls Rapid has been the most unstable reach of whitewater in Grand Canyon during the late Holocene and particularly during the last 120 years. Rapids in bedrock canyons controlled by tributary deposition in the main channel are aggradational features that reflect the net effect of tributary-mainstem interactions. Boulders that form the core of rapids in Grand Canyon are essentially immobile by both regulated and unregulated Colorado River flows. Historical operation of Glen Canyon Dam, which was completed in 1963, has reduced the potential for reworking of debris fans, and has accelerated the rate of net aggradation at the mouths of tributary canyons. Because debris fans that formed after 196
Sediment Transport in the Lower Yampa River, Northwestern Colorado
Discharge measurements and sediment samples were taken at streamflow-gaging station 09260050 Yampa River at Deerlodge Park in 1982 and 1983 to determine the annual sediment supply to the Yampa Canyon in Dinosaur National Monument. Forty-three years of discharge records at two tributary sites were combined to determine the historic discharge of the Yampa River at Deerlodge Park. A mean annual hydrograph and flow-duration curve were derived from these data. Sediment-transport equations were derived for total sediment discharge, suspended-sediment discharge, bedload dischagre, and the discharge of sediment in several particle-sizes. Annual sediment discharge were determined by the flow-duration, sediment-rating-curve method and indicated annual total sediment discharge was approximately 2.0 million tons per year of which 0.8 million tons per year was sand-sized material. Bedload was almost entirely sand, and annual bedload discharge was 0.1 million tons per year. Development of water resources in the Yampa River basin could effect the geomorphic character of the Yampa River at Deerlodge Park and the Yampa Canyon. Several scenarios of altered streamflow frequency distribution, reduced streamflow volume, and reduced sediment supply are examined to estimate the effect on the sediment budget at Deerlodge Park. (USGS)