Cyclic Erosional Instability of Sandbars along the Colorado River, Grand Canyon, Arizona

Abstract

The study and management of regulated rivers have become important issues. A prime example is Glen Canyon Dam and its operational impacts on the downstream environment in Grand Canyon National Park, Arizona. We present an overview of the Glen Canyon Dam environmental issue, a novel methodology for monitoring dam impacts on alluvial sediments, and three years of study relating to the stability of alluvial deposits along the Colorado River. This research uses oblique photographs taken daily, automatically, from twenty-one fixed-position programmable cameras. Digital image-processing techniques created planimetric models of sandbar area from the photos for the period of March 1993 through May 1995. The technique allowed daily tracking of sandbar areas for detection of rapid erosional events. We demonstrated that rapid erosional events occur commonly on Grand Canyon sandbars. Sandbars are unstable over the study period, especially the first two years. Most of the rapid erosional events are associated with weekend or seasonal reduction in flow. Sandbar area frequency is bimodal with negative kurtosis, indicating that measurements taken at long time-steps are not likely to document mean area but rather minima or maxima. Time-series analysis suggests that periods of relative stability occur between rapid area-reducing events. Sandbars appear to adjust in two modes, a short-term adjustment mode occurring over hours and a long-term adjustment mode over days to weeks. The understanding and minimization of rapid-failure events should be increased, and the phenomenon needs to be addressed in any comprehensive sediment management plan.

The Use of Repeat Photography in Contemporary Geomorphic Studies: An Evolving Approach to Understanding Landscape Change

“…The period of time between measurements was critical in determining that the greatest volumes of change occurred during short intervals and that the events are cyclical. Cluer (1995) and Dexter and Cluer (1999) used a high oblique camera angle to take daily photographs with a programmable camera to capture changing sediment conditions along the Colorado River. Dexter and Cluer (1999), specifically studied rapid, erosional events of sandbars in the same area also using high oblique photographs.…”

“…Cluer (1995) and Dexter and Cluer (1999) used a high oblique camera angle to take daily photographs with a programmable camera to capture changing sediment conditions along the Colorado River. Dexter and Cluer (1999), specifically studied rapid, erosional events of sandbars in the same area also using high oblique photographs. Daily photographs of erosion events during periods of reduced flow showed that stability occurred between reducing events.…”

“…A specific attribute was assigned to individual cells and the attributes between cells were compared. Dexter and Cluer (1999) used high oblique programmable cameras housed in retrofitted military ammunition cans to document changes in sandbars. Exposures were made once every 24 h during low-flow river stages.…”

ADJUSTMENT OF STREAM CHANNEL CAPACITY FOLLOWING DAM CLOSURE, YEGUA CREEK, TEXAS1

“…Ongoing and future work will attempt to identify changes in sediment regime below Somerville Dam and to document the nature and efficiency of sediment delivery through the system. It may be that Yegua Creek could benefit from flushing flows (Reiser et al, 1989) and periodic flood releases (Dexter and Cluer, 1999) to remove sediments downstream of Somerville Dam, as has been successful in California and elsewhere (Wilcock et. al., 1996;Kondolf, 1997Kondolf, , 1998.…”

Measuring snow accumulation and ablation dynamics during rain‐on‐snow events: innovative measurement techniques

“…Fluvial geomorphologists used time-lapse photography to monitor changes in channel characteristics (e.g. Dexter and Cluer, 1999;Hancock and Willgoose, 2001;Sheets et al, 2002;Lawler, 2005). Cloud cover was measured using time-lapse sky view cameras (Holle et al, 1979;Ackerman, 1981).…”