HR: 1340h
AN: H53C-1408    [Abstracts]
TI: Testing a new method for measuring micro-morphological change using embedded magnets in a travertine depositional environment, Fossil Creek, Arizona
AU: * Fuller, B M
EM: fuller@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, United States
AU: Compson, Z G
EM: zacchaeus.compson@nau.edu
AF: Dept. of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, United States
AU: Adams, K J
EM: Kenneth.adams@nau.edu
AF: Dept. of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, United States
AU: Marks, J C
EM: jane.marks@nau.edu
AF: Dept. of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, United States
AB: Measuring micro-morphological change of bed topography in a fluvial setting is often done by installing erosion pins or drilling holes that serve as fixed reference points. A shortcoming to these methods is that protruding pins and open holes can influence the local processes that alter the channel bed. Here we report on the development of a new method for documenting micro-topographic change, using magnets embedded within actively forming travertine along Fossil Creek, Arizona. These natural travertine structures occur as channel-spanning dams that create a step-pool morphology that provides important aquatic habitat. Biotic processes in turn, such as microbial and algal growth and the trapping of floating leaves and branches, can catalyze travertine deposition and vertical growth in travertine dams. The goal of this overarching study is to document travertine growth rates, downstream of a recently decommissioned diversion dam, to determine the relative influence of various biotic and abiotic processes. The method consists of gluing individual magnets (1cm in diameter and thickness; 1gauss magnetic intensity) onto the end of 10cm lengths of PVC pipe with the magnetic poles parallel to the pipe. We then drill a vertical hole in either bedrock or travertine, place the magnet-pipe assembly into the hole, and back-fill the hole with crushed travertine, to approximate the pre-existing surface as best as possible. Prior to installing the magnets we measure the background magnetic field, using a Schonstedt GA-72CD magnetic locator, to ensure there are no nearby magnetic anomalies. Because the strength of the magnetic field decreases geometrically with distance, we created a calibration curve specific to these magnets, which is valid over a range of up to 40cm. We use repeat measurements of magnetic field intensity to document vertical travertine growth over time. We use repeat surveys with a total station as an independent check on the change in dam elevations, at and adjacent to the magnet locations. Although the basic method is simple, a number of challenges have arisen. There are differences in magnetic permeability of travertine, bedrock and air or water, which we are addressing with calibration curves for specific materials and dam geometries. Although we commonly installed magnets at local topographic high points along dam crests, non-uniform travertine growth can shift the location of dam crests, complicating efforts to re-occupy the exact magnet location for later measurements. We address this problem by searching for the highest intensity location in a horizontal plane above the dam crest, and by careful photo-documentation of the evolving travertine structures. So far, the method provides individual measurements of micro-topographic vertical position with a precision of 1 to 3mm, while elevation differences due to travertine growth are precise to within 1 to 2cm, depending on the distance to the embedded magnet and the material composition of the new increment of dam growth.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting