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.

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Tafoni are pits formed by non-uniform weathering in otherwise uniform rock. Two equations have been proposed for the rate of development of tafoni, both based on 2000-year-old outcrops from the coast of Japan. We have taken tafoni measurements from the Meteor Crater, Arizona, and vicinity that extend the equations back at least 50 000 years. As reported in earlier studies, we found pit depth to be the best tafone parameter to measure. The size of the pit decreases significantly with increasing inclination of the rock surface; however, the size of the pit can vary greatly for other reasons. In some cases the measurements are statistically significantly different between two stations taken from contiguous areas of similar inclination and aspect in an apparently homogeneous bed. It is clear, however, that over tens of thousands of years tafoni enlarge significantly. Our data are generally log-normal and all are markedly heteroscedastic. The 1991 equation proposed by Matsukura and Matsuoka does not fit our data. The 1996 equation proposed by Sunamura provides a better fit. We propose a sigmoidal equation D D b1 C e b2C b3/t where D is the depth, t is the age, and b1, b2 and b3 vary with lithology. This new equation fits our data far better than the earlier published equations.

Undergraduate geography field courses are relatively rare at American universities, though they can exemplify well the fundamental tenets of learnercentered education. This article reports on an intensive three-week field analysis course taught at Northern Arizona University at Flagstaff, within the Department of Geography, Planning, and Recreation. Co-taught by the authors since 1999, Field Analysis (GGR 480) serves as the department's capstone course in geography, required of all undergraduate majors. Nine years of experiences, lessons learned, teaching and assessment strategies, assigned projects, and course development are described herein. Educators with little field experience are encouraged to develop their own field-based course in geography that applies learner-centered education and assessment techniques.

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