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J. C. Schmidt

Publications and source records attributed to J. C. Schmidt.

10 recordsLinked to original sources
USGS2008

Is there enough sand? Evaluating the fate of Grand Canyon sandbars

Large dams have the potential to dramatically alter the flow regime, geomorphology, and aquatic ecosystem of downstream river reaches. Development of flow release regimes in order to meet multiple objectives is a challenge facing dam operators, resource managers, and scientists. Herein, we review previous work and present new analyses related to the effects of Glen Canyon Dam on the downstream reach of the Colorado River in Marble and Grand Canyons. The dam traps the entire incoming sediment load in Lake Powell and modulates the hydrologic regime by, for example, eliminating spring snowmelt floods, resulting in changes in the geomorphology of the river downstream. The primary geomorphic impact has been the erosion of sandbars along the banks of the river. Recognition of this impact has led to many scientific studies and a variety of experimental operations of Glen Canyon Dam with the goal of rebuilding the eroding sandbars. These efforts have thus far been generally unsuccessful and the question remains as to whether or not the dam can be operated such that sandbars can be rebuilt and maintained over extended periods with the existing sediment supply. We attempt to answer this question by evaluating a dam operation that may be considered a "best-case scenario" for rebuilding and maintaining eroded sandbars. Our analysis suggests that this best-case scenario may indeed have viability for rebuilding sandbars, and that the initial rate at which sandbars could be rebuilt is comparable to the rate at which sandbars have been eroded since dam construction. The question remains open as to the viability of operations that deviate from the best-case scenario that we have defined.

Grand Canyon↗
USGS2007

The rate and pattern of bed incision and bank adjustment on the Colorado River in Glen Canyon downstream from Glen Canyon Dam, 1956-2000

Closure of Glen Canyon Dam in 1963 transformed the Colorado River by reducing the magnitude and duration of spring floods, increasing the magnitude of base flows, and trapping fine sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the first cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffles decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized. Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sandbars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage-discharge relations. In the downstream part of the study area where riffles controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species. Models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average flows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2) the potential importance of exceptional flows resulting from management decisions. ?? 2007 Geological Society of America.

Geological Society of America Bulletin↗
USGS2006

Influence of a dam on fine-sediment storage in a canyon river

Glen Canyon Dam has caused a fundamental change in the distribution of fine sediment storage in the 99-km reach of the Colorado River in Marble Canyon, Grand Canyon National Park, Arizona. The two major storage sites for fine sediment (i.e., sand and finer material) in this canyon river are lateral recirculation eddies and the main-channel bed. We use a combination of methods, including direct measurement of sediment storage change, measurements of sediment flux, and comparison of the grain size of sediment found in different storage sites relative to the supply and that in transport, in order to evaluate the change in both the volume and location of sediment storage. The analysis shows that the bed of the main channel was an important storage environment for fine sediment in the predam era. In years of large seasonal accumulation, approximately 50% of the fine sediment supplied to the reach from upstream sources was stored on the main-channel bed. In contrast, sediment budgets constructed for two short-duration, high experimental releases from Glen Canyon Dam indicate that approximately 90% of the sediment discharge from the reach during each release was derived from eddy storage, rather than from sandy deposits on the main-channel bed. These results indicate that the majority of the fine sediment in Marble Canyon is now stored in eddies, even though they occupy a small percentage (???17%) of the total river area. Because of a 95% reduction in the supply of fine sediment to Marble Canyon, future high releases without significant input of tributary sediment will potentially erode sediment from long-term eddy storage, resulting in continued degradation in Marble Canyon. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Measurements of bed load transport on Pacific Creek, Buffalo Fork and The Snake River in Grand Teton National Park, Wyoming

Dams disrupt the flow of both of water and sediment through a watershed. Channel morphology is a function of discharge and sediment load, and perturbations caused by dams often alter channel form, causing significant geomorphic and, potentially, ecological changes (e.g. Petts and Gurnell, 2005). At the first order, dams often produce a flow regime that is profoundly altered in the timing, magnitude, and frequency of flows (Magilligan and Nislow, 2005). Yet, the nature of channel adjustments will be specific to both the physical setting, size of the river, dam characteristics, and nature and severity of the flow regulation (Church 1995; Knighton, 1998).

Wyoming↗
USGS2005

A map overlay error model based on boundary geometry

An error model for quantifying the magnitudes and variability of errors generated in the areas of polygons during spatial overlay of vector geographic information system layers is presented. Numerical simulation of polygon boundary displacements was used to propagate coordinate errors to spatial overlays. The model departs from most previous error models in that it incorporates spatial dependence of coordinate errors at the scale of the boundary segment. It can be readily adapted to match the scale of error-boundary interactions responsible for error generation on a given overlay. The area of error generated by overlay depends on the sinuosity of polygon boundaries, as well as the magnitude of the coordinate errors on the input layers. Asymmetry in boundary shape has relatively little effect on error generation. Overlay errors are affected by real differences in boundary positions on the input layers, as well as errors in the boundary positions. Real differences between input layers tend to compensate for much of the error generated by coordinate errors. Thus, the area of change measured on an overlay layer produced by the XOR overlay operation will be more accurate if the area of real change depicted on the overlay is large. The model presented here considers these interactions, making it especially useful for estimating errors studies of landscape change over time. ?? 2005 The Ohio State University.

Geographical Analysis↗
USGS2005

Application of wavelet analysis for monitoring the hydrologic effects of dam operation: Glen canyon dam and the Colorado River at lees ferry, Arizona

Wavelet analysis is a powerful tool with which to analyse the hydrologic effects of dam construction and operation on river systems. Using continuous records of instantaneous discharge from the Lees Ferry gauging station and records of daily mean discharge from upstream tributaries, we conducted wavelet analyses of the hydrologic structure of the Colorado River in Grand Canyon. The wavelet power spectrum (WPS) of daily mean discharge provided a highly compressed and integrative picture of the post-dam elimination of pronounced annual and sub-annual flow features. The WPS of the continuous record showed the influence of diurnal and weekly power generation cycles, shifts in discharge management, and the 1996 experimental flood in the post-dam period. Normalization of the WPS by local wavelet spectra revealed the fine structure of modulation in discharge scale and amplitude and provides an extremely efficient tool with which to assess the relationships among hydrologic cycles and ecological and geomorphic systems. We extended our analysis to sections of the Snake River and showed how wavelet analysis can be used as a data mining technique. The wavelet approach is an especially promising tool with which to assess dam operation in less well-studied regions and to evaluate management attempts to reconstruct desired flow characteristics. Copyright ?? 2005 John Wiley & Sons, Ltd.

River Research and Applications↗
USGS2005

Complex channel responses to changes in stream flow and sediment supply on the lower Duchesne River, Utah

Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events. A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply. ?? 2004 Elsevier B.V. All rights reserved.

Geomorphology↗
USGS1988

Aggradation and degradation of alluvial sand deposits, 1965 to 1986, Colorado River, Grand Canyon National Park, Arizona

High discharges occurring between 1983-1985 resulted in redistribution of sand stored in zones of recirculating current in the Colorado River in Grand Canyon National Park. Redistribution resulted in net loss in the number of reattachment deposits in narrow reaches and aggradation of some separation deposits. Separation deposits were more stable than other types of deposits. Alluvial sand deposits that are large enough and of sufficient size for use as campsites were more stable than smaller lower-elevation deposits. Fluctuating flows between October 1985 and January 1986 caused erosion throughout the Grand Canyon, and caused erosion of some deposits created by the high flows of 1983-1985. Data collected for this study included measurements of flow velocity, scour-and-fill of sand deposits, topographic and bathymetric surveys, mapping of surface-flow patterns, water-surface slope surveys, sedimentological analysis, and replication of photographs. A classification system of alluvial sand deposits was developed on the basis of morphometric characteristics and the location of these deposits in relation to parts of recirculation zones. (Author 's abstract)

Open-File Report↗
USGS1988

Aggradation and degradation of alluvial sand deposits, 1965 to 1986, Colorado River, Grand Canyon National Park, Arizona; executive summary

High discharges that occurred in 1983-85 resulted in redistribution of sand stored in zones of recirculating current in the Colorado River in Grand Canyon National Park. Redistribution resulted in net loss in the number of reattachment deposits in narrow reaches and aggradation of some separation deposits. Separation deposits were more stable than other types of deposits. Alluvial sand deposits that are large enough and of sufficient areal extent for use as campsites were more stable than smaller lower-elevation deposits. Fluctuating flows between October 1985 and January 1986 caused erosion throughout the Grand Canyon and caused erosion of some deposits created by the high flows of 1983-85. (Author 's abstract)

Open-File Report↗